Can Structural Grading of Foveal Hypoplasia Predict Future Vision in Infantile Nystagmus?: A Longitudinal Study

Purpose

To evaluate structural grading and quantitative segmentation of foveal hypoplasia using handheld OCT, versus preferential looking (PL), as predictors of future vision in preverbal children with infantile nystagmus.

Design

Longitudinal cohort study.

Participants

Forty-two patients with infantile nystagmus (19 with albinism, 17 with idiopathic infantile nystagmus, and 6 with achromatopsia) were examined.

Methods

Spectral-domain handheld OCT was performed in preverbal children up to 36 months of age. Foveal tomograms were graded using our 6-point grading system for foveal hypoplasia and were segmented for quantitative analysis: photoreceptor length, outer segment (OS) length, and foveal developmental index (FDI; a ratio of inner layers versus total foveal thickness). Patients were followed up until they could perform chart visual acuity (VA) testing. Data were analyzed using linear mixed regression models. Visual acuity predicted by foveal grading was compared with prediction by PL, the current gold standard for visual assessment in infants and young children.

Main Outcome Measures

Grade of foveal hypoplasia, quantitative parameters (photoreceptor length, OS length, FDI), and PL VA were obtained in preverbal children for comparison with future chart VA outcomes.

Results

We imaged 81 eyes from 42 patients with infantile nystagmus of mean age 19.8 months (range, 0.9–33.4 months; standard deviation [SD], 9.4 months) at the first handheld OCT scan. Mean follow-up was 44.1 months (range, 18.4–63.2 months; SD, 12.0 months). Structural grading was the strongest predictor of future VA (grading: r = 0.80, F = 67.49, P < 0.0001) compared with quantitative measures (FDI: r = 0.74, F = 28.81, P < 0.001; OS length: r = 0.65; F = 7.94, P < 0.008; photoreceptor length: r = 0.65; F = 7.94, P < 0.008). Preferential looking was inferior to VA prediction by foveal grading (PL: r = 0.42, F = 3.12, P < 0.03).

Conclusions

Handheld OCT can predict future VA in infantile nystagmus. Structural grading is a better predictor of future VA than quantitative segmentation and PL testing. Predicting future vision may avert parental anxiety and may optimize childhood development.

Visual Development During the Second Decade of Life in Albinism

PURPOSE

To evaluate change in best corrected visual acuity (BCVA) during the second decade of life and the effects of albinism type and extraocular muscle surgery on BCVA in children with albinism.

METHODS

In this retrospective longitudinal study, 41 patients with albinism with clinic visits recording binocular BCVA at least once between the ages of 10 and 13 years (visit A) and again between the ages of 17 and 20 years (visit B) were included. Type of albinism, age at each visit, and interval eye muscle surgeries were recorded for each patient.

RESULTS

Forty (98%) patients showed BCVA improvement or stability between visits A and B. There was no significant effect of interval extraocular muscle surgery on BCVA. Those carrying either a clinically presumed or moleculary confirmed diagnosis of oculocutaneous albinism types 1B and 2 had the best visual outcomes, consistent with previous studies.

CONCLUSIONS

In the majority of patients with albinism, significant improvement in BCVA occurs during the second decade of life. Extraocular muscle surgery was not a significant factor in BCVA improvement in albinism. Overall, the assessments support the finding of improvement of visual acuity in children with albinism at earlier ages and provide new information beneficial in predicting visual outcomes in the second decade of life. [J Pediatr Ophthalmol Strabismus. 2018;55(4):254-259.].

Nystagmus Does Not Limit Reading Ability in Albinism

Purpose

Subjects with albinism usually suffer from nystagmus and reduced visual acuity, which may impair reading performance. The contribution of nystagmus to decreased reading ability is not known. Low vision and nystagmus may have an additive effect. We aimed to address this question by motion compensation of the nystagmus in affected subjects and by simulating nystagmus in healthy controls.

Methods

Reading speed and eye movements were assessed in 9 subjects with nystagmus associated with albinism and in 12 healthy controls. We compared the reading ability with steady word presentation and with words presented on a gaze contingent display where words move in parallel to the nystagmus and thus correct for the nystagmus. As the control, healthy subjects were asked to read words and texts in steady reading conditions as well as text passages that moved in a pattern similar to nystagmus.

Results

Correcting nystagmus with a gaze contingent display neither improved nor reduced the reading speed for single words. Subjects with nystagmus and healthy participants achieved comparable reading speed when reading steady texts. However, movement of text in healthy controls caused a significantly reduced reading speed and more regressive saccades.

Conclusions

Our results argue against nystagmus as the rate limiting factor for reading speed when words were presented in high enough magnification and support the notion that other sensory visual impairments associated with albinism (for example reduced visual acuity) might be the primary causes for reading impairment.

PARTIAL OCULOCUTANEOUS ALBINISM AND IMMUNODEFICIENCY SYNDROMES: TEN YEARS EXPERIENCE FROM A SINGLE CENTER IN TURKEY

BACKGROUND AND AIM

Partial oculocutaneous albinism and immunodeficiency (OCA-ID) diseases are autosomal recessive syndromes characterized by partial hypopigmentation and recurrent infections. Moreover, some OCA-ID syndromes confer susceptibility to develop a life-threatening hyperinflammatory condition called hemophagocytic lymphohistiocytosis (HLH). We investigated the genetic, clinical and immunological characteristics of 20 OCA patients.

MATERIAL AND METHODS

Herein, we present the clinical and immunological characteristics of 20 OCA patients who referred to the Department of Pediatric Immunology, Erciyes University Medical Faculty in Kayseri, Turkey between 2004 and 2014.

RESULTS

Of the 20 OCA patients, 7 fulfilled diagnostic criteria for HLH, 9 showed defective functions of CD8 T cells and natural killer cells, and 8 received a definitive molecular diagnosis. Among the patients, we also report a patient diagnosed with two different genetic defects, in TYR and JAK3 genes, causing, respectively, OCA and ID.

CONCLUSION

Our results illustrate the variability of clinical presentations and disease severity in OCA-ID patients, with consequent challenges in diagnosing and treating these patients.

Clinical Insights Into Foveal Morphology in Albinism

PURPOSE

A hallmark of albinism is foveal hypoplasia. However, literature suggests variable foveal development. This study evaluates the association between ocular phenotype and foveal morphology to demonstrate the broad structural and functional spectrum.

METHODS

Best-corrected visual acuity (BCVA), nystagmus, angle kappa, stereoacuity, iris transillumination, macular melanin presence, foveal avascular zone, and annular reflex were recorded in 14 patients with albinism. Spectral-domain optical coherence tomography provided macular images.

RESULTS

The clinical phenotype was broad, with BCVA varying from 20/20 to 20/100. Better BCVA was associated with a preserved foveal avascular zone, annular macular reflex, stereoacuity, and macular melanin. Imaging demonstrated a continuum of foveal development correlating with BCVA. Individuals with a rudimentary pit had normal inner and outer segment lengthening and better BCVA.

CONCLUSIONS

The spectrum of ocular structure and visual function in albinism is broad, suggesting a possible diagnosis of albinism in a patient with an even more normal clinical presentation.

Albinism: particular attention to the ocular motor system

The purpose of this report is to summarize an understanding of the ocular motor system in patients with albinism. Other than the association of vertical eccentric gaze null positions and asymmetric, (a) periodic alternating nystagmus in a large percentage of patients, the ocular motor system in human albinism does not contain unique pathology, rather has "typical" types of infantile ocular oscillations and binocular disorders. Both the ocular motor and afferent visual system are affected to varying degrees in patients with albinism, thus, combined treatment of both systems will maximize visual function.

[Optical coherence tomography in patients with foveal hypoplasia and high visual acuity]

Two clinical cases of children with foveal hypoplasia and high visual acuity examined using optical coherence tomography are presented. Genetic examination found oculocutaneous albinism type 1 in one patient. Potential causes and functional value of foveal hypoplasia are discussed.

The functional significance of foveal abnormalities in albinism measured using spectral-domain optical coherence tomography

PURPOSE

The relationship between foveal abnormalities in albinism and best-corrected visual acuity (BCVA) is unclear. High-resolution spectral-domain optical coherence tomography (SD OCT) was used to quantify foveal retinal layer thicknesses and to assess the functional significance of foveal morphologic features in patients with albinism.

DESIGN

Cross-sectional study.

PARTICIPANTS

Forty-seven patients with albinism and 20 healthy control volunteers were recruited to the study.

METHODS

Using high-resolution SD OCT, 7×7×2-mm volumetric scans of the fovea were acquired (3-μm axial resolution). The B scan nearest the center of the fovea was identified using signs of foveal development. The thickness of each retinal layer at the fovea and foveal pit depth were quantified manually using ImageJ software and were compared with BCVA.

MAIN OUTCOME MEASURES

Total retinal thickness, foveal pit depth, photoreceptor layer thickness, and processing layer thickness in relation to BCVA.

RESULTS

Total photoreceptor layer thickness at the fovea was correlated highly to BCVA (P = 0.0008; r = -0.501). Of the photoreceptor layers, the outer segment length was correlated most strongly to BCVA (P<0.0001; r = -0.641). In contrast, there was no significant correlation between either total retinal thickness or pit depth and BCVA (P>0.05). This was because of an inverse correlation between total photoreceptor layer thickness and total processing layer thickness (P<0.0001; r = -0.696).

CONCLUSIONS

Neither the total retinal thickness nor the pit depth are reliable indicators of visual deficit, because patients with similar overall retinal thickness had widely varying foveal morphologic features. In albinism, the size of the photoreceptor outer segment was found to be the strongest predictor of BCVA. These results suggest that detailed SD OCT images of photoreceptor anatomic features provide a useful tool in assessing the visual potential in patients with albinism.

FINANCIAL DISCLOSURE(S)

The author(s) have no proprietary or commercial interest in any materials discussed in this article.

Reading acuity in albinism: evaluation with MNREAD charts

INTRODUCTION

The MNREAD reading acuity (RA) charts use continuous-text reading to measure (1) RA, ie, the smallest size of print that the patient can resolve; (2) maximum reading speed (MRS); and (3) critical print size (CPS), ie, the smallest print that the patient can read with maximum speed. This project used the MNREAD charts to evaluate reading performance in children and adults with albinism to determine the smallest print size recommended to facilitate effortless reading.

METHODS

A total of 63 subjects with albinism were administered the MNREAD test to determine the critical print size for effortless reading. The subjects read continuous, high-contrast text print until they could no longer discern the words. The MRS and CPS were determined with the MNREAD protocol. Presence or absence of nystagmus and strabismus was recorded. Distance visual acuity was measured with ETDRS charts.

RESULTS

The sample included 25 male and 38 female subjects with a median age of 16 years. Median MRS was 150 words per minute. The mean CPS was 0.84 logMAR, notably larger than the mean RA of 0.53 logMAR.

CONCLUSIONS

The MNREAD acuity charts can be used to determine functional reading ability and critical print size for effortless reading in individuals with albinism. The amount of enlargement in print size can be used in educational and vocational recommendations to facilitate reading.

Relationship of slow-phase velocity to visual acuity in infantile nystagmus associated with albinism

PURPOSE

To investigate the relationship between slow-phase velocity of nystagmus and visual acuity in children with albinism.

METHODS

Twelve children with infantile nystagmus associated with albinism had eye movements recorded by binocular video-oculography (10 patients) or by scleral search coil (2 patients). In children younger than 3 years of age visual acuities was assessed by means of Teller acuity cards and corrected for age. Histograms of horizontal eye velocities were fit by a gamma distribution in all patients (r2>0.85). The velocity at the peak of the gamma distribution was compared with the limiting velocity predicted by the dynamic visual acuity (DVA) model.

RESULTS

All histograms of slow-phase eye velocity were skewed toward lower velocities, with the peak distribution ranging from 5 to 20 degrees/second. The velocity at the peak of the gamma distribution for each subject was uniformly equal to or below the limiting velocity predicted by the DVA model. The average of the gamma distribution across all subjects corresponded to an upper limit to eye velocity of 17 degrees/second. At this velocity the DVA model predicted visual acuity of 20/50, which is lower than the average visual acuity reported in albinism. The distributions of eye velocity were lower than the limiting eye velocity predicted on the basis of DVA almost 50% of the time.

CONCLUSIONS

Visual acuity in albinism is limited by macular hypoplasia rather than by slow-phase eye velocity of nystagmus.

Visual acuity and nystagmus following strabismus surgery in patients with oculocutaneous albinism

OBJECTIVE

To evaluate the effect of strabismus surgery on nystagmus and visual acuity in patients with oculocutaneous albinism.

METHODS

We conducted a non-concurrent retrospective study of 13 Puerto Rican patients with all types of oculocutaneous albinism who underwent strabismus surgery. Patients underwent genetic linkage analysis to reach a final oculocutaneous albinism classification prior to surgery. Strabismus surgery was modified by under-correction of 0.5 mm in each muscle from the standard Marshall Parks' measurements in all patients.

RESULTS

Six of the 13 patients with oculocutaneous reported an improved best corrected visual acuity after surgery. Two of the 13 patients with oculocutaneous had no nystagmus following strabismus surgery. All patients were orthotropic following surgery.

CONCLUSION

Strabismus surgical undercorrection may be of benefit in patients with oculocutaneous albinism.

Acuity development in infantile nystagmus

PURPOSE

To compare development of acuity in patients with isolated infantile nystagmus and infantile nystagmus associated with a visual sensory defect.

METHODS

Visual acuities in 57 children (1 month to 4 years of age) with infantile nystagmus were assessed by using Teller acuity cards oriented vertically during binocular viewing. Twenty-two had isolated infantile nystagmus, 21 had albinism, 7 had aniridia, and 7 had mild or moderate bilateral optic nerve hypoplasia (BONH). Longitudinal acuity was measured in 40 of these patients (mean 1.8, 2.3, 3.1, and 3.3, measurements per patient group, respectively). The rate of acuity development across the study groups was quantified by linear regression of log acuity versus log age and compared to published normative data.

RESULTS

The rate of acuity development was similar across all groups and paralleled the normative data. The slope of log grating acuity versus log age (+/-SEM) was normal, 0.73; isolated infantile nystagmus, 0.80 +/- 0.11; albinism, 0.80 +/- 0.11; aniridia, 0.87 +/- 0.16; and BONH, 0.79 +/- 0.18. The slopes were not significantly different (ANCOVA, F(4,142) = 0.21, P = 0.93). Compared with published binocular normative data, mean acuity adjusted for age was reduced by 1.2 octaves in isolated infantile nystagmus and by 1.7 to 2.5 octaves in nystagmus with associated sensory defect.

CONCLUSIONS

The rate of acuity development in infantile nystagmus is largely independent of the gaze-holding instability or an associated visual sensory defect. Reduction of mean acuity in albinism, aniridia, and BONH is due to the visual sensory defect and exceeds the acuity reduction observed in isolated infantile nystagmus.

Albinism and developmental delay: the need to test for 15q11-q13 deletion

We report on a 17-month-old African girl with cutaneous and ophthalmologic features of oculocutaneous albinism type 2 as well as microcephaly, absent speech, and tremulous movements. Mutations of the P gene within the Angelman/Prader-Willi syndrome critical region at 15q11-q13 cause oculocutaneous albinism type 2. Comorbid oculocutaneous albinism and Angelman syndrome were suspected and confirmed by cytogenetics. Phenotypic features of Angelman syndrome or Prader-Willi syndrome in a patient with albinism should prompt further investigation.

Sweep Visual Evoked Potential Testing as a Predictor of Recognition Acuity in Albinism

OBJECTIVE

To determine if sweep visual evoked potential (VEP) acuity is predictive of recognition acuity in children with albinism.

METHODS

A retrospective review was performed in children with albinism who underwent sweep VEP testing from 1992 to 2003. All patients had a complete ophthalmologic examination with either binocular or monocular sweep VEP testing and at least 5 years of follow-up. Positive predictability of sweep VEP acuity was defined as final recognition acuity within 1 Snellen line of initial sweep VEP acuity.

RESULTS

Of the 13 patients included in the study, 11 had nystagmus, iris transillumination defects, and foveal hypoplasia at initial examination. The mean age at initial sweep VEP testing was 3.1 years (range, 0.1-10.0 years). Five of 13 patients had initial sweep VEP acuity that was predictive of final recognition acuity. Five additional patients had final recognition acuity, which surpassed initial sweep VEP acuity by 2 to 3 lines. Of these 10 patients, the mean duration for recognition acuity to reach VEP acuity was 5.4 years. There was no correlation between predictive VEP acuity and foveal pigmentation, refractive error, strabismus, nystagmus, or longer follow-up.

CONCLUSIONS

Sweep VEP testing can be used as a predictive tool for recognition acuity in children with albinism. Predictability was found in a clinical spectrum of albinism.

Topographical retinal function in oculocutaneous albinism

PURPOSE

Macular hypoplasia is the major ocular finding in albinism because it underlies reduced acuity and congenital nystagmus. Anatomic studies have shown a lack of foveal differentiation. However, functional anatomy of the macula in albinism is not known.

DESIGN

Observational case reports.

METHODS

We measured the topology of the retinal response in two patients with oculocutaneous albinism by using simultaneous fundus monitoring and multifocal electroretinographs (ERGs). Stimuli consisted of a 103-hexagon array centered on the macula. Recording conditions eliminated nystagmus artifact.

RESULTS

ERG amplitudes were reduced only in the central 5 to 10 degrees compared with adult norms. When the response was scaled by stimulation area, multifocal ERG amplitudes were constant across all retinal eccentricities.

CONCLUSIONS

The results suggest a homogeneous density of cone photoreceptors across the central retina in albinism, consistent with anatomic studies showing arrest of postnatal macular development. Further normative data are needed to fully quantify macular development in young children.

Correlation of grating acuity with letter recognition acuity in children with albinism

PURPOSE

This study was undertaken to determine whether grating acuity in early childhood can be used as a predictor of letter recognition acuity in patients with albinism.

METHODS

In this retrospective study, we compared the binocular grating acuities of children with albinism (30 at age 1, 29 at age 2, and 19 at age 3) to their letter recognition acuity at age 4-6 years.

RESULTS

Mean binocular grating acuity was 2.0, 1.9, and 1.5 octaves below age matched norms at ages 1, 2, and 3 years, respectively (P<0.001 at all ages). Mean grating acuity at ages 1, 2, and 3 correlated moderately (r=0.458, 0.502, and 0.471, respectively; all with P<0.05) with mean binocular letter recognition acuity of the same children at ages 4-6. A subgroup analysis of 9 patients followed longitudinally showed strong correlation of binocular grating acuity at ages 1 and 2 with letter acuity (r=0.745, P=0.021; r=0.930, P<0.001, respectively) and moderate correlation at age 3 (r=0.685, P=0.042). In the larger group and the longitudinal subgroup, mean binocular grating acuity at ages 1 and 2 was worse than mean binocular letter recognition acuity at age 4-6 (paired-samples t-test, P<0.001). Mean binocular grating acuity at age 3 in both groups was not significantly different than mean binocular letter recognition acuity at age 4-6 (paired-samples t-test, P=0.790, 0.215, respectively).

CONCLUSION

Parents should be informed that vision measured as grating acuity at age 3 provides an estimate of future letter recognition acuity in children with albinism.

Effects of extraocular muscle surgery on 15 patients with oculo-cutaneous albinism (OCA) and infantile nystagmus syndrome (INS)

PURPOSE

The purpose of this report is to characterize the clinical and electrophysiological effects of extraocular muscle surgery in 15 patients with oculo-cutaneous albinism (OCA) and infantile nystagmus syndrome (INS). Our hypothesis is that surgery on the extraocular muscles of patients with OCA and INS changes their nystagmus and their visual function.

DESIGN

Interventional, prospective, cohort, noncomparative case series.

METHODS

All 15 patients had surgery on all four virgin horizontal recti; three for strabismus alone, three for nystagmus alone, five for an eccentric gaze null zone alone, and four for an eccentric gaze null zone plus strabismus. All patients have been followed for at least six months. All 15 patients had the subjective outcome measure of pre- and postoperative binocular best optically corrected acuity (BBOCA). Objective outcome measures included anomalous head posture (AHP) in nine patients, eye movement recording measures of expanded nystagmus acuity function (NAFX) in 10 patients, null zone position (NUZP) and null zone width (NUZW) in 10 patients, and foveation time (FOV) in nine patients.

RESULTS

The results are summarized as follows; BBOCA increased 0.1 LogMar or greater in 14 of 15 patients. In those operated on for an AHP with or without associated strabismus the AHP improved significantly (P < .01 for all). The NAFX, NUZP, NUZW, and FOV measured from eye movement recordings showed persistent, significant increases in all patients (P < .01 for all).

CONCLUSIONS

This report adds to the evidence that surgery on the extraocular muscles in patients with INS has independent neurologic and visual results.

Positive angle kappa in albinism

BACKGROUND

Albinism is an inherited disorder of deficient melanin production. There is a high prevalence of strabismus in patients with albinism. We investigated the prevalence of a positive angle kappa in patients with albinism and report its effect on assessment of binocular alignment.

METHODS

We retrospectively reviewed the charts of 207 consecutive patients with the diagnosis of albinism seen at the University of Minnesota between 1984 and 2002. Angle kappa and alignment using prism and alternate-cover test (PACT) and Krimsky measurements were recorded.

RESULTS

In 99.6% of our patients with albinism, angle kappa was noted to be positive. The mean difference between PACT and Krimsky measurements was 17.11 prism diopters in the more exotropic (or less esotropic) direction per Krimsky test. Because of the high prevalence of a positive angle kappa, esodeviations often appeared less and exodeviations appeared greater than when measured using PACT.

CONCLUSIONS

Positive angle kappa can be considered another clinical feature of albinism. Consideration should be given to the effect of positive angle kappa on alignment as observed by Krimsky measurement when planning extraocular muscle surgery, particularly when preoperative sensory testing suggests binocular potential.

Oculocutaneous albinism type 4 is one of the most common types of albinism in Japan

Oculocutaneous albinism (OCA) is a complex genetic disease with great clinical heterogeneity. Four different types of OCA have been reported to date (OCA1, OCA2, OCA3, and OCA4). MATP was recently reported in a single Turkish OCA patient as the fourth pathological gene, but no other patients with OCA4 have been reported. Here, we report the mutational profile of OCA4, determined by genetic analysis of the MATP gene in a large Japanese population with OCA. Of 75 unrelated patients that were screened, 18 individuals (24%) were identified as having OCA4; they harbored seven novel mutations, including four missense mutations (P58S, D157N, G188V, and V507L) and three frameshift mutations (S90CGGCCA-->GC, V144insAAGT, and V469delG), showing that MATP is the most frequent locus for tyrosinase-positive OCA in Japanese patients. We discuss the functional melanogenic activity of each mutant allele, judging from the relationship between the phenotypes and genotypes of the patients. This is the first report on a large group of patients with OCA4.

Color vision in patients with the Hermansky-Pudlak syndrome

PURPOSE

To study color vision in patients with oculocutaneous albinism (OCA) METHODS: We evaluated color vision in 42 patients with OCA using the HRR color plates. Sixty seven percent of the patients had the Hermansky-Pudlak syndrome (HPS), diagnosed genetically or clinically. The remaining patients had unknown mutations leading to OCA.

RESULTS

47.6 % of patients of OCA of all types included had a color vision defect. Of these, 55% were female and 45% were male patients. 50% of patients with the HPS (all types) had a color vision deficit. 42.9% of patients with OCA of unknown type had color weakness. 57.1% had normal color vision.

CONCLUSIONS

Results suggest that many patients with OCA and the HPS have a mild red-green color perception deficiency that is not a sex linked trait. The prevalence of color vision deficits in our study population increased with decreasing visual acuity.

Tyrosinase gene mutations in oculocutaneous albinism�1 (OCA1): definition of the phenotype

Oculocutaneous albinism (OCA) is a common human genetic condition resulting from mutations in at least twelve different genes. OCA1 results from mutations of the tyrosinase gene and presents with the life-long absence of melanin pigment after birth (OCA1A) or with the development of minimal-to-moderate amounts of cutaneous and ocular pigment (OCA1B). Other types of OCA have variable amounts of cutaneous and ocular pigment. We hypothesized that white hair at birth indicates OCA1 and tested this in a sample of 120 probands with OCA and white hair at birth. We found that 102 (85%) of the probands had OCA1 with one or two identifiable tyrosinase gene mutations, with 169 (83%) of the 204 OCA1 tyrosinase gene alleles having identifiable mutations and 35 (17%) having no identifiable change in the coding, splice junction, or proximal promoter regions of the gene. The inability to identify the mutation was more common with OCA1B (24/35, 69%) than with OCA1A (11/35, 31%) alleles. Seven probands with no tyrosinase gene mutations were found to have OCA2 with one or two P gene mutations, and in eleven, no mutations were detected in either gene. We conclude that (1) the presence of white hair at birth is a useful clinical tool suggesting OCA1 in a child or adult with OCA, although OCA2 may also have this presentation; (2) the molecular analysis of the tyrosinase and P genes are necessary for precise diagnosis; and (3) the presence of alleles without identifiable mutations of the tyrosinase gene, particularly in OCA1B, suggests that more complex mutation mechanisms of this gene are common in OCA.

Tyrosinase processing and intracellular trafficking is disrupted in mouse primary melanocytes carrying the underwhite (uw) mutation. A model for oculocutaneous albinism (OCA) type 4

Oculocutaneous albinism (OCA) type 4 is a newly identified human autosomal recessive hypopigmentary disorder that disrupts pigmentation in the skin, hair and eyes. Three other forms of OCA have been previously characterized, each resulting from the aberrant processing and/or sorting of tyrosinase, the enzyme critical to pigment production in mammals. The disruption of tyrosinase trafficking occurs at the level of the endoplasmic reticulum (ER) in OCA1 and OCA3, but at the post-Golgi level in OCA2. The gene responsible for OCA4 is the human homologue of the mouse underwhite (uw) gene, which encodes the membrane-associated transporter protein (MATP). To characterize OCA4, we investigated the processing and sorting of melanogenic proteins in primary melanocytes derived from uw/uw mice and from wild-type mice. OCA4 melanocytes were found to be constantly secreted into the medium dark vesicles that contain tyrosinase and two other melanogenic enzymes, Tyrp1 (tyrosinase-related protein 1) and Dct (DOPAchrome tautomerase); this secretory process is not seen in wild-type melanocytes. Although tyrosinase was synthesized at comparable rates in wild-type and in uw-mutant melanocytes, tyrosinase activity in uw-mutant melanocytes was only about 20% of that found in wild-type melanocytes, and was enriched only about threefold in melanosomes compared with the ninefold enrichment in wild-type melanocytes. OCA4 melanocytes showed a marked difference from wild-type melanocytes in that tyrosinase was abnormally secreted from the cells, a process similar to that seen in OCA2 melanocytes, which results from a mutation of the pink-eyed dilution (P) gene. The P protein and MATP have 12 transmembrane regions and are predicted to function as transporters. Ultrastructural analysis shows that the vesicles secreted from OCA4 melanocytes are mostly early stage melanosomes. Taken together, our results show that in OCA4 melanocytes, tyrosinase processing and intracellular trafficking to the melanosome is disrupted and the enzyme is abnormally secreted from the cells in immature melanosomes, which disrupts the normal maturation process of those organelles. This mechanism explains the hypopigmentary phenotype of these cells and provides new insights into the involvement of transporters in the normal physiology of melanocytes.

Dark adaptation is faster in pigmented than albino rats

PURPOSE

Previous reports have raised the possibility that, compared to pigmented rats, albino rats might be night blind. The purpose of this study was to reinvestigate this issue by comparing the dark-adaptation process of the pigmented Long-Evans (LE) and albino Sprague Dawley (SD) rats.

METHODS

Scotopic ERGs obtained from LE and SD rats were recorded following periods of dark adaptation 0.5, 3 and 12 h. Intensity response functions were generated with flashes of white light spanning over a 7 log-unit range with a maximal intensity of 8 cd x s x m(-2) in energy.

RESULTS

SD rats showed a gradual increase in the amplitude of the scotopic b-wave Vmax (maximal 'saturated' rod b-wave amplitude) and retinal sensitivity (k) as the duration of the dark-adaptation period increased. In contrast, LE rats did not demonstrate any further significant gain in retinal function (Vmax or k) beyond 30 min of dark-adaptation. Thus for periods of dark-adaptation of 30 min or less, the rod function of the LE rats is superior to that of the SD rats while both strains have comparable retinal functions following 3 h or more of dark-adaptation.

CONCLUSIONS

Our results indicate that LE and SD rats differ in their rapidity to dark-adapt, a finding that could explain the previous claim that SD rats were night blind. The reduced bio-availability of calcium ions in eyes lacking melanin could explain this difference. Calcium was previously shown to play a key role in retinal adaptation processes.

Chiasmal coefficient of flash and pattern visual evoked potentials for detection of chiasmal misrouting in albinism

The diagnosis of albinism can be confirmed by electrophysiological examination, when chiasmal misrouting can be demonstrated. The present study describes a quantitative analysis method for this purpose. A chiasmal coefficient (CC) was calculated by correlating the differential potential over left and right hemisphere, when stimulating left versus right eye. This CC will be negative in albinism and positive in normal individuals. VEPs were recorded in 20 control subjects, four children with congenital motor nystagmus and six children with albinism. In up to 25% of the controls the CC was negative, when using flash VEP. However, with pattern VEP all had a positive CC. All children with albinism had a negative CC. Three of the four patients with congenital motor nystagmus had a positive CC, and one child had a small negative value with flash stimulation. In conclusion, determination of CC is a valuable and objective analysis method for electrophysiological determination of chiasmal misrouting. The method is relatively simple and only needs two electrode tracings. One should be aware of false-positive results when using flash stimulation. Whenever possible pattern stimulation should be used.

Grating acuity in albinism in the first three years of life

PURPOSE

To compare grating acuity of children 1 to 3 years old with albinism with that of age-matched normal children, and to determine whether ocular pigment and macular structure are predictive of grating visual acuity.

METHODS

In this retrospective cross-sectional review of 64 children with albinism, binocular grating acuity was measured with Teller acuity cards at 1, 2, or 3 years of age. Iris transillumination and macular transparency were graded according to published scales. The presence or absence of macular melanin pigment and a rudimentary foveal reflex were assessed with direct and indirect ophthalmoscopy, respectively.

RESULTS

Mean grating acuities of the children with albinism at ages 1 (2.48 cycles (cy)/degrees), 2 (3.62 cy/degrees), and 3 (5.87 cy/degrees) were 2.3, 2.1, and 1.7 octaves lower than published norms (all with P <.001). The studied variables did not correlate with visual acuity except for macular transparency at age 2 (r = 0.40; P =.030).

CONCLUSIONS

Grating acuity can be used to assess the reduction in visual function in albinism in the first 3 years of life. However, grading of iris transillumination and macular transparency and recognition of an annular reflex or macular melanin do not consistently correlate with the development of vision measured by grating acuity.

Abnormal visual projection in a human albino studied with functional magnetic resonance imaging and visual evoked potentials

The albino visual pathway is abnormal in that many fibres from the temporal retina project to the contralateral visual cortex. The visual projections in a human albino and a control have been investigated with fMRI and VEP during independent visual stimulation of both hemifields. Activity in the occipital cortex in the normal was contralateral to the stimulated visual field, whereas it was contralateral to the stimulated eye in the albino, independent of the stimulated visual field. Thus, the albino visual cortex is activated not only by stimulation in the contralateral visual field, but also by abnormal input representing the ipsilateral visual field. These novel findings help elucidate the nature of albino misrouting.

Tyrp1 and oculocutaneous albinism type 3

Tyrosinase-related protein 1 (Tyrp1) is a melanocyte-specific gene product involved in eumelanin synthesis. Mutations in the mouse Tyrp1 gene are associated with brown pelage, and in the human TYRP1 gene with oculocutaneous albinism type 3 (OCA3). In the murine system, Tyrp1 expresses significant dihydroxyindole carboxylic acid oxidase (i.e. DHICA oxidase) activity. However, in humans, TYRP1 is enigmatic in that despite extensive efforts focused on the study of its function, its actual role in the human melanocyte is still unclear. There is mounting evidence demonstrating that in addition to its role in eumelanin synthesis, Tyrp1 is involved in maintaining stability of tyrosinase protein and modulating its catalytic activity. Tyrp1 is also involved in maintenance of melanosome ultrastructure and affects melanocyte proliferation and melanocyte cell death. The current review is an attempt to consolidate our understanding of the role of Tyrp1 in the melanocyte.

[Characteristics and functions of melanin in retinal pigment epithelium]

Melanin granules in the retinal pigment epithelium (RPE) have many important functions which are not yet completely understood. Melanin in the RPE protects the cell from damage caused by oxidative stress. This pigment acts as a free radical sink and diminishes cytotoxic lipid peroxidation. Thus, melanin protects against light toxicity and against cytotoxic effects caused by ocular inflammation. Many enzymes, e.g. superoxide dismutase or carboanhydrase, are only activated in the presence of zinc. Melanin can store zinc and release it when required. The absence of melanin in patients with oculocutaneous albinism is accompanied by photophobia, poor visual acuity and nystagmus. Furthermore melanin is said to protect against damaging lipofuscin accumulation in the RPE. Melanosomes are involved in the lysosomal degradation pathways and possibly take part in the degradation of rod outer segments (ROS) in the RPE.

Mutations in the human orthologue of the mouse underwhite gene (uw) underlie a new form of oculocutaneous albinism, OCA4

Oculocutaneous albinism (OCA) affects approximately 1/20,000 people worldwide. All forms of OCA exhibit generalized hypopigmentation. Reduced pigmentation during eye development results in misrouting of the optic nerves, nystagmus, alternating strabismus, and reduced visual acuity. Loss of pigmentation in the skin leads to an increased risk for skin cancer. Two common forms and one infrequent form of OCA have been described. OCA1 (MIM 203100) is associated with mutations of the TYR gene encoding tyrosinase (the rate-limiting enzyme in the production of melanin pigment) and accounts for approximately 40% of OCA worldwide. OCA2 (MIM 203200), the most common form of OCA, is associated with mutations of the P gene and accounts for approximately 50% of OCA worldwide. OCA3 (MIM 203290), a rare form of OCA and also known as "rufous/red albinism," is associated with mutations in TYRP1 (encoding tyrosinase-related protein 1). Analysis of the TYR and P genes in patients with OCA suggests that other genes may be associated with OCA. We have identified the mouse underwhite gene (uw) and its human orthologue, which underlies a new form of human OCA, termed "OCA4." The encoded protein, MATP (for "membrane-associated transporter protein") is predicted to span the membrane 12 times and likely functions as a transporter.

Hermansky-Pudlak syndrome type 3 in Ashkenazi Jews and other non-Puerto Rican patients with hypopigmentation and platelet storage-pool deficiency

Hermansky-Pudlak syndrome (HPS), consisting of oculocutaneous albinism and a bleeding diathesis due to the absence of platelet dense granules, displays extensive locus heterogeneity. HPS1 mutations cause HPS-1 disease, and ADTB3A mutations cause HPS-2 disease, which is known to involve abnormal intracellular vesicle formation. A third HPS-causing gene, HPS3, was recently identified on the basis of homozygosity mapping of a genetic isolate of HPS in central Puerto Rico. We now describe the clinical and molecular characteristics of eight patients with HPS-3 who are of non-Puerto Rican heritage. Five are Ashkenazi Jews; three of these are homozygous for a 1303+1G-->A splice-site mutation that causes skipping of exon 5, deleting an RsaI restriction site and decreasing the amounts of mRNA found on northern blotting. The other two are heterozygous for the 1303+1G-->A mutation and for either an 1831+2T-->G or a 2621-2A-->G splicing mutation. Of 235 anonymous Ashkenazi Jewish DNA samples, one was heterozygous for the 1303+1G-->A mutation. One seven-year-old boy of German/Swiss extraction was compound heterozygous for a 2729+1G-->C mutation, causing skipping of exon 14, and resulting in a C1329T missense (R396W), with decreased mRNA production. A 15-year-old Irish/English boy was heterozygous for an 89-bp insertion between exons 16 and 17 resulting from abnormal splicing; his fibroblast HPS3 mRNA is normal in amount but is increased in size. A 12-year-old girl of Puerto Rican and Italian background has the 3,904-bp founder deletion from central Puerto Rico on one allele. All eight patients have mild symptoms of HPS; two Jewish patients had received the diagnosis of ocular, rather than oculocutaneous, albinism. These findings expand the molecular diagnosis of HPS, provide a screening method for a mutation common among Jews, and suggest that other patients with mild hypopigmentation and decreased vision should be examined for HPS.

Diurnal variation in synaptic ribbon length and visual threshold

Previous work suggests that photoreceptor synaptic ribbon length and absolute dark-adapted threshold may vary during a 24-h diurnal cycle. To test this hypothesis, we examined the length of photoreceptor synaptic ribbons and the dark-adapted threshold in black (+/+) and albino (c2J/c2J) C57BL/6J mice at six times over a 24-h period. Testing began 2 h after light onset (ZT 2:00) and continued at successive 4-h intervals (12 h:12 h light:dark). We determined the length of the synaptic ribbons in frozen sections by labeling them with an antibody specific for synaptic ribbons. Synaptic ribbons vary in length at different points in the diurnal cycle in both types of mice, but the synaptic ribbons in black mice are longer than those in albino mice by an average of 0.33 microm. The synaptic ribbons of black mice also have a larger response to changes in the light cycle. Ribbon length in black mice ranges from 1.66 microm to 1.4 microm, whereas ribbon length in albino mice ranges from 1.32 microm to 1.25 microm. The shortest ribbons are evident 6 h after light onset in both types of mice, whereas the longest ribbons appear within 2 h after light onset. These changes in synaptic ribbon length support the idea that photoreceptor synaptic ribbons are dynamic structures whose length changes over a 24-h diurnal cycle. Examining black and albino mice with a water-maze behavioral assay showed that visual thresholds in black and albino mice vary over the 24-h diurnal cycle. The visual thresholds of albino mice are elevated compared with black mice at all times tested. This is consistent with previous findings of visual thresholds in hypopigmented mice. The lowest threshold (greatest sensitivity) is present 2 h after light onset (ZT 2:00) and corresponds to the time when synaptic ribbons are the longest. The highest threshold is observed 6 h after light onset, the time when synaptic ribbons are shortest. These results show that synaptic ribbon length and visual sensitivity vary together in relation to the time.

Stereopsis in patients with albinism: clinical correlates

PURPOSE

A hallmark of albinism is excessive decussation of retinostriate projections at the optic chiasm. This misprojection might lead to abnormalities in the retinal correspondence and may account for the usual absence of stereovision. We report on 2 groups of patients with albinism who have either fine or gross stereopsis and compare the clinical findings of these groups to other patients with albinism with similar visual acuities but no stereopsis.

METHODS

A retrospective chart review of patients with albinism was used to segregate those with a letter visual acuity of 20/100 or better in one eye, assessment of stereopsis, and strabismus < or = 10 PD. Forty-five patients were identified. Albinism type, best-corrected visual acuity, motility, Titmus vectograph stereoacuity, iris and macula transparency grades, and the presence or absence of both melanin and an annular reflex in the macula were tabulated. A comparison of the clinical characteristics of the groups with and without stereopsis was made.

RESULTS

Those albino subjects who demonstrated stereopsis had better visual acuity, less iris transillumination, more frequent presence of melanin in the macula, less nystagmus, and less marked foveal hypoplasia than the albino subjects without stereopsis. No nystagmus was clinically detected in 5 patients with fine stereopsis. All these differences were statistically significant. Macular transparency grade was not significantly different between the groups.

CONCLUSIONS

Patients with albinism who demonstrate stereopsis tend to have better visual acuity, more iris pigment, and more melanin pigment in the macula than their counterparts without stereopsis.

Development of receptoral responses in pigmented and albino guinea-pigs (Cavia porcellus)

We describe the postnatal development of the electroretinogram (ERG) receptoral response in the guinea pig. In addition, the time course and nature of maturation was compared between albino and pigmented strains to consider the role that melanogenesis might have in this process. Electroretinograms were collected on groups of albino and pigmented animals from postnatal day (PD) PD1 to PD60. A-wave amplitudes and implicit times were extracted from filtered data (0-75 Hz). Receptoral components were modelled using the delayed gaussian model of Hood and Birch [1] fitted as an ensemble to the raw data. Guinea pigs show saturated amplitudes (RmP3) that are 50% of adult values at birth, these mature by PD12. Receptoral delay (t(d)) also undergoes some postnatal maturation, while phototransduction gain (log S) is adult-like at birth. Albino animals had significantly (p<0.05) larger RmP3 and log S across all ages. Guinea pigs have significant postnatal development in their receptoral response. Maturation of RmP3 implies a postnatal increase in rod outer segment length. Whereas the adult values of log S implies a mature phototransduction process at birth. We argue that the likely cause for the larger log S of albino eyes is compatible with theories of increased levels of internal light. Whereas the larger RmP3, even after allowing for increased light effectiveness, may reflect a lower ocular resistance in albino eyes due to their lower levels of melanin. Furthermore, decreased RmP3 and log S with age is observed in the pigmented group only and is consistent with increased ocular resistance due to melanin development in this strain.

Interstitial pneumonia in Hermansky-Pudlak syndrome: significance of florid foamy swelling/degeneration (giant lamellar body degeneration) of type-2 pneumocytes

Although usual interstitial pneumonia (UIP)-like IP has been known as the most serious complication of Hermansky-Pudlak syndrome (HPS), its pathologic features and pathogenesis are poorly understood. We investigated biopsied and autopsied lung tissues from five patients who died of UIP-like IP associated with HPS (HPSIP). The salient histopathologic features of HPSIP observed were: (1) alveolar septa displaying florid proliferation of type-2 pneumocytes (2PCs) with characteristic foamy swelling/degeneration; (2) patchy fibrosis with lymphocytic and histiocytic infiltration centered around respiratory bronchioles, occasionally showing constrictive bronchiolitis; and (3) honeycomb change without predilection for the lower lobes or subpleural area. Those peculiar 2PCs were histochemically characterized by the over accumulation of phospholipid, immunohistochemically by a weak positivity for surfactant protein, and ultrastructurally by the presence of numerous giant lamellar bodies that compressed the nucleus with occasional cytoplasmic disruption, together suggesting a form of cellular degeneration with an over accumulation of surfactant (giant lamellar body degeneration). The present study strongly indicates that there is a basic defect in the formation/secretion process of surfactant by the 2PCs in HPS, which may well be the triggering factor for the HPSIP development. Other factors, such as macrophage dysfunction, may be working synergistically for further acceleration of the inflammatory process.

Hermansky-Pudlak syndrome and pale ear: melanosome-making for the millennium

Hermansky-Pudlak syndrome (HPS) is a rare autosomal recessive disorder characterized principally by oculocutaneous albinism, a bleeding tendency, and a ceroid-lipofuscin lysosomal storage disease. These clinical manifestations of HPS are associated with defects of multiple cytoplasmic organelles--melanosomes, platelet granules, and lysosomes--suggesting that the HPS gene product is involved in some shared feature of the biogenesis or functions of these diverse organelles. The HPS gene has been cloned, and a number of pathologic mutations of the gene have been identified. Functional studies indicate that the HPS protein is part of a high-molecular weight complex involved in the biogenesis of early melanosomes. Additional disorders with similarities to HPS have been identified in man, mouse, flies, and yeast, and it is rapidly becoming clear that understanding these disorders will shed new light on the mechanisms by which cells traffic newly synthesized proteins through the cytoplasm to assemble functional organelles.

Pulmonary function and high-resolution CT findings in patients with an inherited form of pulmonary fibrosis, Hermansky-Pudlak syndrome, due to mutations in HPS-1

OBJECTIVE

To describe and correlate pulmonary function and high-resolution CT (HRCT) scan scores in individuals with a high risk for development of pulmonary fibrosis, ie, Hermansky-Pudlak syndrome (HPS) patients with mutations in the HPS-1 gene.

DESIGN

Cross-sectional analysis of consecutive, eligible patients.

PATIENTS

Thirty-eight HPS inpatients at the National Institutes of Health Clinical Center with HPS-1 mutations.

RESULTS

Thirty-seven patients were Puerto Rican and exhibited the typical 16-base pair (bp) duplication in exon 15 of HPS-1. One non-Puerto Rican was homozygous for a different mutation (intervening sequence 17 -2 A-->C) previously reported in the HPS-1 gene; he died at age 35 of pulmonary insufficiency. For the 23 patients who had pulmonary symptoms, the mean age of onset was 35 years. For all 38 patients (mean age, 37 +/- 2 years), the mean FVC was 71% of predicted; the mean FEV(1), 76%; mean total lung capacity (TLC), 72%; mean vital capacity (VC), 68%; and mean diffusing capacity of the lung for carbon monoxide (DLCO), 72%. When patients were grouped according to the extent of their reduction in FVC, the other four pulmonary function parameters followed the FVC. Seventeen patients had abnormal chest radiographs, and 31 (82%) had abnormal HRCT scans of the chest, for which a scoring system of 0 (normal) to 3 (severe fibrosis) is presented. The mean +/- SEM HRCT score for 38 patients was 1.30 +/- 0.17. HRCT scores correlated inversely with FVC and DLCO.

CONCLUSIONS

Mutations in the HPS-1 gene, whether or not they involve the typical 16-bp duplication seen in Puerto Rican patients, are associated with fatal pulmonary fibrosis. In affected patients, the FVC, FEV(1), TLC, VC, and DLCO fall in concert, and this functional deficit correlates with HRCT scan evidence of progression of interstitial lung disease.

Current understanding on the role of retinal pigment epithelium and its pigmentation

Retinal pigment epithelium (RPE) is a monolayer of cuboidal cells that is strategically placed between the rod and cone photoreceptors and the vascular bed of the choriocapillaris. It has many important functions, such as phagocytic uptake and breakdown of the shedded photoreceptor membranes, uptake, processing, transport and release of vitamin A (retinol), setting up the ion gradients within the interphotoreceptor matrix, building up the blood-retina barrier, and providing all transport from blood to the retina and back. This short review focuses on the role of the pigment granules in RPE. Although the biology of the pigment granules has been neglected in the past, they do seem to be involved in many important functions, such as protection from oxidative stress, detoxification of peroxides, and binding of zinc and drugs, and, therefore, serve as a versatile partner of the RPE cell. Melanin plays a role in the development of the fovea and routing of optic nerves. New findings show that the melanin granules are connected to the lysosomal degradation pathway. Most of these functions are not yet understood. Deficit of melanin pigment is associated with age-related macula degeneration, the leading cause of blindness.

Platelet von Willebrand factor in Hermansky-Pudlak syndrome

The Hermansky-Pudlak Syndrome (HPS) is an autosomal recessive inherited disorder characterized by oculocutaneous albinism, tissue accumulation of ceroid pigment, and a mild to moderate bleeding diathesis attributed to storage-pool deficient (SPD) platlets. Patients have platelet aggregation and release abnormalities. In addition, low levels of plasma von Willebrand factor (vWF) antigen in some HPS patients have been associated with a greater bleeding tendency than would be predicted from either condition alone. Other HPS patients have severe bleeding despite normal levels of plasma vWF, suggesting that at least one additional factor is responsible for their bleeding diathesis. Because platelet vWF levels have been well correlated with clinical bleeding times in patients with von Willebrand's disease, we have measured the platelet vWF activity and antigen levels in 30 HPS patients and have attempted to correlate their clinical bleeding with these values. The platelet vWF activity levels in patients was significantly lower than that of normal subjects (P < 0.0001). The patients as a group also had slightly lower values of plasma vWF activity when compared with normals (P-0.03). In 11 of the HPS patients, the multimeric structure of plasma vWF showed a decrease in the high molecular weight multimers and an increase in the low molecular weight multimers. In correlating the platelet and plasma vWF values with the bleeding histories, we were not able to show a predictable relationship in the majority of the patients.

Genetic defects and clinical characteristics of patients with a form of oculocutaneous albinism (Hermansky-Pudlak syndrome)

BACKGROUND

Hermansky-Pudlak syndrome is characterized by oculocutaneous albinism, a storage-pool deficiency, and lysosomal accumulation of ceroid lipofuscin, which causes pulmonary fibrosis and granulomatous colitis in some cases. All identified affected patients in northwest Puerto Rico are homozygous for a 16-bp duplication in exon 15 of a recently cloned gene, HPS. We compared the clinical and laboratory characteristics of these patients with those of patients without the 16-bp duplication.

METHODS

Forty-nine patients -- 27 Puerto Ricans and 22 patients from the mainland United States who were not of Puerto Rican descent -- were given a diagnosis on the basis of albinism and the absence of platelet dense bodies. We used the polymerase chain reaction to determine which patients carried the 16-bp duplication.

RESULTS

Twenty-five of the Puerto Rican patients were homozygous for the 16-bp duplication, whereas none of the non-Puerto Rican patients carried this mutation. Like the patients without the duplication, the patients with the 16-bp duplication had a broad variation in pigmentation. Nine of 16 adults with the duplication, but none of the 10 without it, had a diffusing capacity for carbon monoxide that was less than 80 percent of the predicted value. High-resolution computed tomography in 12 patients with the 16-bp duplication revealed minimal fibrosis in 8, moderate fibrosis in 1, severe fibrosis in 1, and no fibrosis in 2. Computed tomography in eight patients without the duplication revealed minimal fibrosis in three and no fibrosis in the rest. Inflammatory bowel disease developed in eight patients (four in each group) between 3 and 25 years of age.

CONCLUSIONS

The 16-bp duplication in exon 15 of HPS, which we found only in Puerto Rican patients, is associated with a broad range of pigmentation and an increased risk of restrictive lung disease in adults.

Mouse models of Hermansky Pudlak syndrome: a review

Hermansky Pudlak Syndrome (HPS) is a recessively inherited disease affecting the contents and/or the secretion of several related subcellular organelles including melanosomes, lysosomes, and platelet dense granules. It presents with disorders of pigmentation, prolonged bleeding, and ceroid deposition, often accompanied by severe fibrotic lung disease and colitis. In the mouse, the disorder is clearly multigenic, caused by at least 14 distinct mutations. Studies on the mouse mutants have defined the granule abnormalities of HPS and have shown that the disease is associated with a surprising variety of phenotypes affecting many tissues. This is an exciting time in HPS research because of the recent molecular identification of the gene causing a major form of human HPS and the expected identifications of several mouse HPS genes. Identifications of mouse HPS genes are expected to increase our understanding of intracellular vesicle trafficking, lead to discovery of new human HPS genes, and suggest diagnostic and therapeutic approaches toward the more severe clinical consequences of the disease.

Hereditary external ophthalmoplegia synergistic divergence, jaw winking, and oculocutaneous hypopigmentation: a congenital fibrosis syndrome caused by deficient innervation to extraocular muscles

BACKGROUND

The congenital fibrosis syndrome is a hereditary form of external ophthalmoplegia that is considered to be a primary myopathy.

PURPOSE

To document the coexistence of two distinct forms of ocular motor synkinesis in a subgroup of patients with congenital fibrosis syndrome.

METHODS

Clinical and intraoperative examination results and extraocular muscle biopsy specimens from four patients with congenital fibrosis syndrome were studied.

RESULTS

Three patients displayed a variant of synergistic divergence characterized by simultaneous abduction with intorsion and depression of the synkinetically abducting eye. Three patients had variant of Marcus Gunn jaw winking characterized by elevation of a ptotic eyelid during mouth opening. Three patients had oculocutaneous hypopigmentation.

CONCLUSIONS

A subgroup of patients with congenital fibrosis syndrome display two distinct synkinetic ocular movements in conjunction with oculocutaneous hypopigmentation. The patterns of neuronal misdirection implicate a regional innervational disturbance involving cranial nerves III through VI as the underlying cause of diffuse hereditary ophthalmoplegia in these patients.

Unilateral alpha reactivity: an electroencephalographic finding in albinism

Persons with various forms of albinism frequently have misrouting of their optic nerve fibers. Visual evoked potential (VEP) studies can confirm this because monocular stimulation tends to result in asymmetrical rather than symmetrical occipital response. This type of decussation abnormality is associated only with albinism. In the electroencephalogram (EEG) of normal subjects, unilateral eye opening or closing does not affect the frequency, reactivity, regulation, and symmetry of alpha rhythm. In contrast, in our study, 6 of 19 persons with oculocutaneous and ocular albinism responded to unilateral eye opening with contralateral disappearance or significant attenuation of their alpha rhythm. The most likely explanation of this phenomenon is that the visual pathway misrouting is complete or nearly complete in some children with albinism so that one rather than both occipital lobes receive visual information from each eye. This type of alpha behavior, like VEP studies, supports the diagnosis of albinism, and such testing can be easily performed during a routine EEG recording.

Poor stereopsis can support size constancy in albinism

PURPOSE

The size of a retinal image is inversely related to the distance to the object that generates the image. Normal subjects therefore exhibit size constancy, in which the perceived size of an image is scaled according to its perceived distance. Albinos usually have such poor binocular vision that they perform very poorly on clinical tests for stereopsis. To investigate the functional consequences of this poor stereopsis, we investigated whether stereopsis in these subjects could support size constancy.

METHODS

The stereothresholds of 10 albinos and 12 normal control subjects were measured. The presence of absence of size constancy was investigated by having subjects equate the subjective size of stereoscopically presented images whose image disparity indicated that they were at different distances.

RESULTS

Laboratory results indicated that eight albinos (including five whose clinical tests indicated a lack of stereopsis) had measurable stereopsis of several thousand are seconds or better. Of these, four also exhibited size constancy.

CONCLUSIONS

Albinos who do not demonstrate stereopsis on clinical tests can have stereoscopic perception that commonly used clinical tests do not detect. Moreover, some of these patients even use this poor stereopsis in judging the size of stereoscopically presented images.

Response of the primary auditory cortex to electrical stimulation of the auditory nerve in the congenitally deaf white cat

Neural activity plays an important role in the development and maintenance of sensory pathways. However, while there is considerable experience using cochlear implants in both congenitally deaf adults and children, little is known of the effects of a hearing loss on the development of the auditory cortex. In the present study, cortical evoked potentials, field potentials, and multi- and single-unit activity evoked by electrical stimulation of the auditory nerve were used to study the functional organisation of the auditory cortex in the adult congenitally deaf white cat. The absence of click-evoked auditory brainstem responses during the first weeks of life demonstrated that these animals had no auditory experience. Under barbiturate anaesthesia, cortical potentials could be recorded from the contralateral auditory cortex in response to bipolar electrical stimulation of the cochlea in spite of total auditory deprivation. Threshold, morphology and latency of the evoked potentials varied with the location of the recording electrode, with response latency varying from 10 to 20 ms. There was evidence of threshold shifts with site of the cochlear stimulation in accordance with the known cochleotopic organisation of AI. Thresholds also varied with the configuration of the stimulating electrodes in accordance with changes previously observed in normal hearing animals. Single-unit recordings exhibited properties similar to the evoked potentials. Increasing stimulus intensity resulted in an increase in spike rate and a decrease in latency to a minimum of approximately 8 ms, consistent with latencies recorded in AI of previously normal animals (Raggio and Schreiner, 1994). Single-unit thresholds also varied with the configuration of the stimulating electrodes. Strongly driven responses were followed by a suppression of spontaneous activity. Even at saturation intensities the degree of synchronisation was less than observed when recording from auditory brainstem nuclei. Taken together, in these auditory deprived animals basic response properties of the auditory cortex of the congenitally deaf white cat appear similar to those reported in normal hearing animals in response to electrical stimulation of the auditory nerve. In addition, it seems that the auditory cortex retains at least some rudimentary level of cochleotopic organisation.

Vision in albinism

PURPOSE

The purpose of this investigation was to study vision in albinism from 3 perspectives: first, to determine the characteristics of grating acuity development in children with albinism; second, to study the effect of illumination on grating acuity; and third, to define the effect of melanin pigment in the macula on visual acuity.

METHODS

I. Binocular and monocular grating acuity was measured with the acuity card procedure in 40 children with albinism during the first 3 years of life. Recognition acuity was eventually measured in 27 of these patients. Ocular pigment was documented by a previously established method of grading iris transillumination and macular transparency. II. Grating acuity under standard and increased illumination levels was measured in 20 adults with albinism (group I) compared with that in 20 adults with nystagmus due to conditions other than albinism (group II) and 20 adults without ocular abnormalities (group III). Recognition acuity measured with the ETDRS charts was also recorded for each group. III. Best-corrected binocular acuity was measured in 29 patients with albinism who were identified with melanin pigment in their maculas by direct ophthalmoscopy.

RESULTS

I. Both binocular and monocular grating acuity was reduced 2 to 3 octaves below the norm for ages 6 months to 3 years. Limited data available in the first 6 months of life did not show failure of vision to develop. Grating acuity measurements overestimated eventual recognition acuity. Mean recognition acuity was 20/111. A relationship between grating acuity development and presence or absence of ocular pigment was not found. II. Grating acuity was significantly better for groups I and II under the condition of increased illumination (P < .03). For patients with albinism, grating acuity under standard illumination was significantly better than recognition acuity (P < .001). For all groups, grating acuity under increased illumination was significantly better than recognition acuity (P < .01). III. Mean recognition acuity in patients with albinism and melanin pigment in their maculas (20/47) was significantly better than measured recognition acuity in Project I (P < .001). All had foveal hypoplasia, but 8 patients had an incompletely developed annular reflex in the macula, 6 patients showed stereoacuity, and 3 patients had no nystagmus.

CONCLUSIONS

I. Grating acuity development in albinism seems to progress along a curve that is asymptotic to visual development in a normal population. II. Increasing illumination does not reduce grating acuity in patients with albinism. Grating acuity overestimates recognition acuity in these patients. III. Ophthalmoscopic detection of melanin pigment in the macula in patients with albinism is associated with better vision.

Blue light-emitting diode built-in contact lens electrode can record human S-cone electroretinogram

PURPOSE

To develop a contact lens electrode with a built-in blue light-emitting diode (LED) to record the electroretinogram from short-wave sensitive-cone (S-cone) electroretinogram (ERG) in humans.

METHODS

The ERG was recorded using the blue LED (450 nm) built-in electrode under a yellow background illumination from a slide projector in five normal subjects, a patient with blue cone monochromatism, and a patient with fundus albipunctatus. For comparison, the ERG was also recorded using a yellow LED (566 nm) built-in electrode under the same background illumination in the normal subjects.

RESULTS

The amplitude and the peak time of the b-wave recorded with 3-Hz blue stimuli were 5.1 to 12.4 microV and 63 to 68 msec, respectively in normal subjects, 7.3 microV and 65 msec in the patient with blue cone monochromatism, and 11.4 microV and 65 msec in the patient with fundus albipunctatus. Normal subjects responded to neither 30-Hz blue stimuli nor 3-Hz yellow stimuli.

CONCLUSIONS

Results indicate that the b-wave recorded with 3-Hz blue stimuli is elicited from the S-cone system. This simple technique is useful for testing the human blue cone system.

Contour interaction in the presence of congenital nystagmus

Using a computer-generated acuity task, the resolution of vertical and horizontal Landolt Cs was assessed at three levels of contrast, each for three different crowding conditions. Eighteen adults participated; six had congenital idiopathic nystagmus, six were oculocutaneous albinos and six served as controls. Contour interaction was evident when bars were placed 1 gap-width from the Landolt C and was more marked as contrast increased. When scaled to the individual resolution limit, the magnitude of contour interaction displayed by the albinos was not significantly different from the controls, however the idiopaths did exhibit a greater crowding effect than the controls. These results imply that fixation instability is a contributory factor in contour interaction, but only when retinal image velocity is above a certain threshold.