Bielschowsky head-tilt test--II. Quantitative mechanics of the Bielschowsky head-tilt test

How Lingering Fusional Adaptation Influences the Bielschowsky Head Tilt Test in Superior Oblique Paresis

Background The lack of a positive Bielschowsky head tilt test (BHTT) is commonly seen as an indicator that superior oblique paresis (SOP) is not present. This study investigated the influence of fusion on the BHTT in unilateral SOP.

Patients/Methods and Material We analyzed vertical fusional vergence using our eye-tracking haploscope and the value of BHTT difference (BHTTD) in 11 patients who were diagnosed with congenital unilateral SOP and able to fuse.

Results Patients used one of three different mechanisms of vertical vergence to achieve fusion. The three fusional mechanisms were associated with a significantly different BHTTD (p < 0.05). Seven of the eleven patients used vertical recti-mediated fusion and had a mean BHTTD ± SD of 21.7 ± 6.3 prism diopters (PD). Three of these patients whom we measured after a patch test for at least 30 min showed a decreased BHTTD (12.7 ± 3.8 PD). Three of the eleven patients used a mixed (oblique/rectus) fusional mechanism and had a mean BHTTD ± SD of 9.3 ± 8.6 PD. Of these patients, the one whom we measured after patching showed an increase of 11 PD in BHTTD. The remaining patient used oblique muscle-mediated fusion and had a BHTTD of only 3 PD that increased to 21 PD after patching. One explanation for this BHTT behavior in the latter patient involves lingering vergence adaptation of the “paretic” superior oblique muscle (SOM) and contralateral inferior oblique muscle, which makes these muscles more effective when activated, as is the case on ipsilateral head tilt (part of the ocular counter-roll mechanism), lessening the expected increase in hyperdeviation. Similarly, in our patients with mixed fusion, the vergence-adapted “paretic” SOM and contralateral superior rectus muscle are activated on ipsilateral and contralateral tilt, respectively, lessening the hyperdeviation in both directions. In the other seven patients, however, the vergence-adapted ipsilateral inferior rectus muscle and contralateral superior rectus muscle are activated on contralateral tilt, accentuating the BHTTD.

Conclusion Depending upon the specific muscles used for vertical fusion, the BHTTD is decreased or increased. The presence of a large BHTTD points to lingering or persisting fusional tonus involving the vertical rectus muscles. The lack of a positive BHTT does not rule out the diagnosis of SOP, but rather may be caused by lingering or persevering fusional tonus involving the oblique muscles. Performing the BHTT after a patch test for a minimum of 30 minutes may be necessary to reveal the BHTTD, supporting the diagnosis of SOP.

Can Binocular Alignment Distinguish Hypertropia in Sagging Eye Syndrome From Superior Oblique Palsy?

Purpose

Although the three-step test (3ST) is typically used to diagnose superior oblique palsy (SOP), sagging eye syndrome (SES) has clinical similarities. We sought to determine if alignment measurements can distinguish unilateral SOP from hypertropia in SES.

Methods

We studied hypertropic subjects who underwent surface-coil magnetic resonance imaging (MRI) demonstrating either SO cross-section reduction indicative of congenital or acquired palsy (SOP group) or lateral rectus muscle sag (SES group). Alignment was measured by Hess screen and prism-cover testing. Multiple supervised machine learning methods were employed to evaluate diagnostic accuracy. Rectus pulley coordinates were determined in SES cases fulfilling the 3ST.

Results

Twenty-three subjects had unilateral SOP manifested by SO atrophy. Eighteen others had normal SO size but MRI findings of SES. Maximum cross-section of the palsied SO was much smaller than contralaterally and in SES (P < 2 × 10-5). Inferior oblique cross-sections were similar in SOP and SES. In both SOP and SES, hypertropia increased in contralateral and decreased in ipsilateral gaze and was greater in ipsilateral than contralateral head tilt. In SES, nine subjects (50%) fulfilled the 3ST and had greater infraplacement of the lateral than medial rectus pulleys in the hypotropic orbit. Supervised machine learning of alignment data distinguished the diagnoses with areas under the receiver operating curves up to 0.93, representing excellent yet imperfect differential diagnosis.

Conclusions

Because the 3ST is often positive in SES, clinical alignment patterns may confound SES with unilateral SOP, particularly acquired SOP. Machine learning substantially but imperfectly improves classification accuracy.

Statics of plant mechanics

David A. Robinson took all that was known in his time about ocular anatomy, extraocular muscle force generation, and neural control of extraocular muscles, and integrated this information into a quantitative model of the static behavior of the ocular motor plant suitable for application to strabismus, the pathological misalignment of the eyes. Robinson's comprehensive mathematical descriptions of the essential details he knew to be the properties of the ocular motor plant highlighted the critical gaps in the state of knowledge that he very explicitly bridged by quantitative assumptions that later motivated focused research that ultimately revealed many missing pieces of the puzzle. Robinson suggested that it should be possible, in principle, to account (in a computational model) for all the mechanical factors and neural drives that regulate binocular alignment and strabismus, and that such modeling could assist in diagnosis and treatment of this common ophthalmic disorder.

Special features of superior oblique hypofunction due to tendon abnormalities

While most cases of superior oblique (SO) hypofunction represent contractile weakness due to denervation, sometimes the lesion is exclusively in the tendon. This study sought to distinguish the pattern of incomitant strabismus caused by deficiency of SO oculorotary force caused by tendon abnormalities versus that of neurogenic palsy. Clinical and magnetic resonance imaging (MRI) findings of 7 cases of unilateral SO tendon interruption or extirpation were compared with 11 cases of age matched unilateral SO palsy having intact tendons. We compared angles of misalignment with high-resolution MRI in central gaze and deorsumversion. Muscle bellies in neurogenic palsy were markedly atrophic with maximal cross sections averaging 6.5 ± 2.7 mm², in contrast with 13.5 ± 3.0 mm² contralesionally (P < .0001). In contrast, SO muscle bellies ipsilateral to tendon interruption had maximum cross sections averaging 15.1 ± 3.0 mm² occurring more posterior than on the contralesional side whose maximum averaged 12.1 ± 2.4 mm². While cross sections of SO bellies ipsilateral to tendon interruption exhibited normal contractile increase in infraduction (P < .0005), there was nevertheless strabismus with incomitance similar to that in SO atrophy. Binocular alignment was statistically similar (P > .5) in the two groups for all diagnostic positions, including head tilt, except in deorsumversion, where cases with SO tendon abnormalities averaged 20.5 ± 6.9Δ ipsilateral hypertropia, significantly more than 8.5 ± 6.6Δ in neurogenic SO atrophy (P = .001). The average difference in hypertropia Hypertropia averaged 9D greater in deorsumversion than central gaze in tendon abnormalities, but 4.1Δ less in SO atrophy (P< .019). In contralesional version, average overelevation in adduction was 1.7 (scale of 0-4) in tendon abnormalities, and 2.6 in SO atrophy (P = .23), while average underdepression in adduction was -2.3 in cases of tendon abnormalities and -1.6 in SO atrophy (P = .82). Repair of the SO tendon in three cases was effective, while alternative procedures were performed when repair was infeasible. While both denervation and tendon interruption impair SO oculorotary function, interruption causes greater hypertropia in infraversion. Surgical tightening of interrupted SO tendons may have particularly gratifying effects. Posterior SO thickening and large hypertropia in infraversion suggest SO tendon interruption that may guide a surgical strategy of tendon repair.

Quantitative analysis of structure-function relationship between ocular motility and superior oblique muscle hypoplasia in unilateral superior oblique palsy

AIMS

To determine the structure-function relationship between the degree of superior oblique (SO) hypoplasia and ocular motility in unilateral SO palsy.

METHODS

A total of 166 patients with unilateral SO palsy were divided into three groups based on their aetiology and high-resolution MRI findings by an in-plane resolution of 0.25 mm: (1) congenital SO palsy and unilateral trochlear nerve agenesis (absent group, n=79), (2) congenital SO palsy and symmetric trochlear nerves on both sides (present group, n=40) and (3) acquired SO palsy (acquired group, n=47) who all had symmetric trochlear nerves on both sides. The degree of SO hypoplasia was defined as the ratio of SO area between the paretic and nonparetic sides (SO_P/N) at the optic nerve-globe junction on MR images. Multivariate analysis was performed to investigate the relationship between SO hypoplasia and ocular motility parameters.

RESULTS

The degree of SO hypoplasia (SO_P/N) showed a weak negative correlation with bilateral head tilt differences in all groups (β=-0.009, p<0.001 in the absent group; β=-0.003, p=0.034 in the present group; β=-0.007, p=0.002 in the acquired group). There was only a weak positive correlation with SO_P/N and hypertropia differences between both gazes in the absent group (β=0.009, p<0.001) and the acquired group (β=0.007, p=0.001). In addition, none of the other ocular motility parameters were related to the degree of SO hypoplasia in all groups.

CONCLUSION

Regardless of the aetiology of unilateral SO palsy, the structure-function relationship of the paretic SO size and ocular motility examination was weak and almost negligible.

A neurologist and ataxia: using eye movements to learn about the cerebellum

The cerebellum, its normal functions and its diseases, and especially its relation to the control of eye movements, has been at the heart of my academic career. Here I review how this came about, with an emphasis on epiphanies, "tipping points" and the influences of mentors, colleagues and trainees. I set a path for young academicians, both clinicians and basic scientists, with some guidelines for developing a productive and rewarding career in neuroscience.

Rectus Pulley Displacements without Abnormal Oblique Contractility Explain Strabismus in Superior Oblique Palsy

PURPOSE

Using high-resolution magnetic resonance imaging (MRI), we investigated whether rectus pulleys are significantly displaced in superior oblique (SO) palsy and whether displacements account for strabismus patterns.

DESIGN

Prospective case-control study.

PARTICIPANTS

Twenty-four patients diagnosed with SO palsy based on atrophy of the SO muscle on MRI and 19 age-matched orthotropic control subjects.

METHODS

High-resolution, surface coil MRI scans were obtained in multiple, contiguous, quasicoronal planes during monocular central gaze fixation. Pulley locations in oculocentric coordinates in the following subgroups of patients with SO palsy were compared with normal results in subgroups of patients with SO palsy: unilateral versus bilateral, congenital versus acquired, and isotropic (round) versus anisotropic (elongated) SO atrophy. Expected effects of pulley displacements were modeled using Orbit 1.8 (Eidactics, San Francisco, CA) computational simulation.

MAIN OUTCOME MEASURES

Rectus pulley positions and ocular torsion.

RESULTS

Rectus pulleys typically were displaced in SO palsy. In unilateral SO palsy, on average the medial rectus (MR) pulley was displaced 1.1 mm superiorly, the superior rectus (SR) pulley was displaced 0.8 mm temporally, and the inferior rectus (IR) pulley was displaced 0.6 mm superiorly and 0.9 mm nasally from normal. Displacements were similar in bilateral SO palsy, with the SR pulley additionally displaced 0.9 mm superiorly. However, the lateral rectus pulley was not displaced in either unilateral or bilateral SO palsy. The SR and MR pulleys were displaced in congenital SO palsy, whereas the IR and MR pulleys were displaced in acquired palsy. Pulley positions did not differ between isotropic and anisotropic palsy or between patients with cyclotropia of less than 7° versus cyclotropia of 7° or more. Simulations predicted that the observed pulley displacements alone could cause patterns of incomitant strabismus typical of SO palsy, without requiring any abnormality of SO or inferior oblique strength.

CONCLUSIONS

Rectus pulley displacements alone, without abnormal oblique muscle contractility, can create the clinical patterns of incomitant strabismus in SO palsy. This finding supports accumulating evidence that clinical binocular misalignment patterns are not reliable indicators of contractile function of the SO muscle. Ocular torsion does not correlate with and thus cannot account for pulley displacements in SO palsy.

How to deal with diplopia

Diplopia is a frequent neuro-ophthalmologic symptom with diverse etiologies. This article describes elementary diagnostic tests and frequent causes of diplopia. Monocular diplopia persists when the other eye is closed and usually disappears when the patient looks through a pinhole. It is usually caused by errors in the optical media of the eye and has to be differentiated from spectacle-induced side effect and non-organic disorders. A sign of non-organic etiology is absence of change in image position when the head is tilted. Binocular diplopia disappears regardless of which eye is closed. Binocular diplopia occurs when the images of both eyes cannot be fused. The most frequent direct cause of diplopia is acquired strabismus. Knowledge of several specific types of strabismus enables efficient patient management. Congenital and decompensating strabismus like accommodative esotropia, pathophoria, strabismus surso- and deorsoadductorius, retraction syndrome, Brown's syndrome and esotropia in high myopia only need ophthalmologic treatment. Orbital injury, orbital tumor, ocular myositis, Graves orbitopathy and vascular disease usually require multidisciplinary management. Neurogenic paresis, superior oblique myokymia, ocular neuromyotonia, myasthenia, chronic progressive external ophthalmoplegia (CPEO), internuclear ophthalmoplegia (INO) and skew deviation require specific neurologic examination. Treatment of diplopia includes treatment of the fundamental disorder, monocular occlusion, prisms and strabismus surgery.

Vertical alignment in monkeys with unilateral IV section: effects of prolonged monocular patching and trigeminal deafferentation

We investigated monocular viewing and trigeminal (V) deafferentation on the vertical deviation (VD) in monkeys following intracranial IV section. Two monkeys wore a patch for four to six weeks, one over the paretic eye and the other over the normal eye following IV section. Two other monkeys had combined IV and V section with the paretic eye patched postlesion. In monkeys with IV section alone, the VD lessened within the first week postlesion but then increased gradually with the same eye still patched. Thus binocular viewing was unnecessary for the later VD increase. With combined IV and V section, the VD also transiently lessened postlesion. We have proposed that the decrease in VD after IV section is adaptive, driven by an error signal using ocular proprioception and efference copy. Since V section did not eliminate the early decrease in VD, we suggest some orbital afference is transmitted centrally via other cranial nerves. However, the later increase in VD suggests either that the proprioceptive effect cannot be sustained or that mechanical changes supervene to increase the VD.

Effects of intracranial trochlear neurectomy on the structure of the primate superior oblique muscle

PURPOSE. Although cyclovertical strabismus in humans is frequently attributed to superior oblique (SO) palsy, anatomic effects of SO denervation have not been studied. Magnetic resonance imaging (MRI) and orbital histology was used to study the effects of acute trochlear (CN4) denervation on the monkey SO. METHODS. Five juvenile macaque monkeys were perfused with formalin for 5 weeks: 15 months after unilateral or bilateral 10-mm intracranial trochlear neurectomy. Denervated and fellow orbits were imaged by MRI, embedded whole in paraffin, serially sectioned at 10-mum thickness, and stained with Masson trichrome. Whole muscle and individual fiber cross sections were quantified in SO muscles throughout the orbit and traced larger fibers in one specimen where they were present. RESULTS. MRI demonstrated marked reduction in midorbital cross section in denervated SO muscles, with anterior shift of SO mass preserving overall volume. Muscle fibers exhibited variable atrophy along their lengths. Denervated orbital layer (OL) fiber cross sections were slightly but significantly reduced from control at most anteroposterior locations, but this reduction was much more profound in global layer (GL) fibers. Intraorbital and intramuscular CN4 were uniformly fibrotic. In one animal, there were scattered clusters of markedly hypertrophic GL fibers that exhibited only sparse myomyous junctions only anteriorly. CONCLUSIONS. CN4 denervation produces predominantly SO GL atrophy with relative OL sparing. Overall midorbital SO atrophy was evident by MRI as early as 5 weeks after denervation, as denervated SO volume shifted anteriorly. Occasional GL fiber hypertrophy suggests that at least some SO fibers extend essentially the full muscle length after trochlear neurectomy.

Acute superior oblique palsy in the monkey: effects of viewing conditions on ocular alignment and modelling of the ocular motor plant

We investigated the immediate and long-term changes in static eye alignment with acute superior oblique palsy (SOP) in the monkey. When the paretic eye was patched immediately after the lesion for 6-9 days, vertical alignment slowly improved. When the patch was removed and binocular viewing was allowed, alignment slowly worsened. In contrast when a monkey was not patched immediately after the lesion vertical alignment did not improve. We also show that a model of the eye plant can reproduce the observed acute deficit induced by SOP, but only by abandoning Robinson's symmetric simplification of the reciprocal innervation relationship within pairs of agonist-antagonist muscles. The model also demonstrated that physiologic variability in orbital geometry can have a large impact on SOP deficits.

Effect of diagnostic occlusion in acquired trochlear nerve palsy

BACKGROUND

Monocular occlusion eliminates the stimulus for fusional vergence. Diagnostic occlusion may therefore be helpful in isolating the genuine profile of the fundamental ocular motility disorder, which may be an important finding regarding both differential diagnosis of strabismus and dosage of surgery. We investigated the effect of diagnostic occlusion on the motility pattern of acquired trochlear nerve palsy.

PATIENTS AND METHODS

Forty-eight patients aged between 6 and 78 years (median 49 years) with unilateral trochlear nerve palsy were first examined without patching, and then after 3 days of diagnostic occlusion. The onset of palsy was 1-35 years before (median 2 years). Squint angles localized with a dark red glass in front of the non-paretic eye were measured at a distance of 2.5 m, using the Harms tangent screen. Vertical and cyclotorsional angles in primary position (PP), 25 degrees abduction of the non-paretic eye (adduction of the paretic eye), and 25 degrees downgaze were measured.

RESULTS

The relation between hyperdeviation of the paretic eye and excyclodeviation (medians of the angles in degrees, ranges in brackets) before and after diagnostic occlusion was 5/5 and 4/6 (0;14/-1;10 and 0;19/2;13) in PP. In contralateral gaze, the relation was 8/5 and 8/6 (0;21/0;10 and 1;24/1;15), and in downgaze, 10/7 and 8/8 (0;21/1;14 and 0;23/3;18). The increase in excyclodeviation, though statistically significant (in PP, p = 0.0002) was small, with a median of 1 degree and large variability. The decrease in hyperdeviation was statistically significant in downgaze. The head-tilt phenomenon remained unchanged.

CONCLUSIONS

In patients with trochlear nerve palsy, diagnostic occlusion regularly causes an increase in excyclodeviation. In 25% of patients, this increase exceeds 3 degrees. The more variable change in vertical deviation, and the lack in change in the head-tilt phenomenon, can be explained by the fact that central gain-modulation causing an increase in both vertical deviation and the head-tilt phenomenon is not reversible within the relatively short time of 3 days. Diagnostic occlusion can eliminate compensatory innervation and may thereby release the genuine motility pattern, but the occlusion can also induce artificial squint angles.

Absence of relationship between oblique muscle size and bielschowsky head tilt phenomenon in clinically diagnosed superior oblique palsy

PURPOSE

To study whether the variation in maximum oblique muscle size accounts for individual variation in the Bielschowsky head tilt phenomenon (BHTP) in clinically diagnosed superior oblique (SO) palsy.

METHODS

Seventeen subjects with clinically diagnosed early-onset or idiopathic SO palsy and 14 normal subjects were enrolled in the study. Magnetic resonance imaging (MRI) in coronal and sagittal planes was used for quantitative morphometry of inferior oblique (IO) and SO muscles. Maximum cross-sectional area of the SO and IO cross section at the mid-inferior rectus crossing were determined in central gaze and compared with paretic eye hypertropia on ipsilesional versus contralesional head tilt.

RESULTS

Mean (+/-SD) maximum SO cross section was 18.1 +/- 3.2 mm(2) in normal subjects, 14.2 +/- 6.8 mm(2) ipsilesional to SO palsy, and 19.2 +/- 4.5 mm(2) contralesional to SO palsy. The ipsilesional SO cross section was significantly smaller than the contralesional (P = 0.004) and normal (P = 0.01) ones. The mean IO cross section was 18.3 +/- 3.5 mm(2) in normal subjects, 21.3 +/- 7.9 mm(2) ipsilesional to SO palsy (P = 0.43), and 22.0 +/- 6.7 mm(2) contralesional to SO palsy (P = 0.26). Hyperdeviation varied with head tilt by 20.1 +/- 5.5 degrees in subjects with SO atrophy, and 10.3 +/- 5.6 degrees in subjects without SO atrophy (P = 0.003). Although oblique muscle cross sections did not correlate with BHTP, subjects with clinically diagnosed SO palsy segregated into groups exhibiting normal versus atrophic SO size.

CONCLUSIONS

SO size does not account for the variation in BHTP in clinically diagnosed SO palsy, supporting the proposition that the BHTP is nonspecific for SO function.

Magnetic resonance imaging of the functional anatomy of the inferior rectus muscle in superior oblique muscle palsy

PURPOSE

Biomechanical modeling consistently indicates that superior oblique (SO) muscle weakness alone is insufficient to explain the large hypertropia often observed in SO muscle palsy. Magnetic resonance imaging (MRI) was used to investigate if any size or contractility changes in the inferior rectus (IR) muscle may contribute.

DESIGN

Prospective, case-control study.

PARTICIPANTS

Seventeen patients with unilateral SO muscle palsy and 18 orthotropic subjects.

METHODS

Surface coils were used to obtain sets of contiguous, 2-mm-thick, high-resolution, coronal MRI views in different gazes. Cross-sectional areas of the IR and SO muscles were determined in supraduction and infraduction for evaluation of size and contractility. Diagnosis of SO muscle palsy was based on clinical presentations, subnormal contractility, and SO muscle size less than the normal 95% confidence limit.

MAIN OUTCOME MEASURES

Cross-sectional areas of the IR and SO muscles.

RESULTS

Patients had 15.9+/-7.2 prism diopters (Delta; mean+/-standard deviation) of central gaze hypertropia and exhibited ipsilesional SO muscle atrophy and subnormal contractility. Mean ipsilesional, contralesional, and normal IR muscle cross-sections were 28.5+/-3.5 mm(2), 31.9+/-3.8 mm(2), and 31.8+/-5.8 mm(2), whereas mean contractility was 16.5+/-3.8 mm(2), 20.5+/-4.1 mm(2), and 16.6+/-4.8 mm(2), respectively. Ipsilesional IR muscle cross-section and contractility was significantly less than contralesional cross-section and contractility (P<0.01).

CONCLUSIONS

In SO muscle palsy, the contralesional IR muscle is larger and more contractile than the ipsilesional IR muscle, reflecting likely neurally mediated changes that augment the relatively small hypertropia resulting from SO muscle weakness alone. Recession of the hyperfunctioning contralesional IR muscle recession in SO muscle palsy is a physiologic therapy.

Gillies Lecture: ocular motility in a time of paradigm shift

Abstract Recent progress in understanding of the structure and function of extraocular muscles, and our ability to image them clinically, allows prediction of revolutionary progress in diagnosis and treatment of strabismus in the coming decades. This perspective memorializes a lecture given in honour of Dr William Gillies, who has for decades been the paternal leader of strabismology in southern Australia.

Magnetic Resonance Imaging of Human Extraocular Muscles During Static Ocular Counter-Rolling

The rectus extraocular muscle (EOM) pulleys constrain EOM paths. During visual fixation with head immobile, actively controlled pulleys are known to maintain positions causing EOM pulling directions to change by one-half the change in eye position. This pulley behavior is consistent with Listing's law (LL) of ocular torsion as observed during fixation, saccades, and pursuit. However, pulley behavior during the vestibulo-ocular reflex (VOR) has been unstudied. This experiment studied ocular counter-rolling (OCR), a static torsional VOR that violates LL but can be evoked during MRI. Tri-planar MRI was performed in 10 adult humans during central target fixation while positioned in right and left side down positions known to evoke static OCR. EOM cross-sections and paths were determined from area centroids. Paths were used to locate pulleys in three dimensions. Significant ( P < 0.025) counter-rotational repositioning of the rectus pulley arrays of both orbits was observed in the coronal plane averaging 4.1° (maximum, 8.7°) from right to left side down positions for the inferior, medial, and superior rectus pulleys. There was a trend for the lateral rectus averaging 1.4°. Torsional shift of the rectus pulley array was associated with significant contractile cross-section changes in the superior and inferior oblique muscles. Torsional rectus pulley shift during OCR, which changes pulling directions of the rectus EOMs, correlates with known insertions of the oblique EOM orbital layers on rectus pulleys. The amount of pulley reconfiguration is roughly one-half of published values of ocular torsion during static OCR, an arrangement that would cause rectus pulling directions to change by less than one-half the amount of ocular torsion.

Magnetic resonance imaging of the functional anatomy of the inferior oblique muscle in superior oblique palsy

PURPOSE

To study size and contractility of the normal inferior oblique (IO) muscle using high-resolution magnetic resonance imaging (MRI) and to evaluate abnormalities of the superior oblique (SO) and IO muscles in chronic SO palsy.

DESIGN

Prospective, case control study.

PARTICIPANTS

Thirteen patients with SO palsy and 17 orthotropic subjects.

METHODS

High-resolution, surface coil MRI was used to obtain sets of contiguous, 2-mm thick coronal and sagittal images repeated in multiple gaze directions. Digital image analysis was used to measure IO and SO muscle cross-sectional areas for evaluation of size and contractility. Diagnosis of SO palsy in one bilateral and 12 unilateral cases was based on subnormal contractility and SO size less than the normal 95% confidence limit. Ipsilesional and contralesional oblique muscles were compared with controls and correlated with clinical characteristics.

RESULTS

In all subjects, anterior movement and contractile thickening of the IO were observed in supraduction, with posterior movement and relaxational thinning in infraduction. The mean (+/- standard deviation) cross-sectional area of 15 normal control IO muscles was 13.4 +/- 3.9 mm(2), with mean contractile increase from infraduction to supraduction of 5.7 +/- 2.6 mm(2). Subjects with SO palsy had incomitant hypertropia with a wide range of overelevation and underelevation in adduction (i.e., upshoot, downshoot). SO atrophy correlated with underdepression in adduction (P < 0.0001). Contralesional SO cross-section was slightly greater than normal (P = 0.004). The IO cross-section ipsilesional and contralesional to SO palsy did not, however, differ significantly from normal and did not correlate with elevation in adduction (P > 0.2).

CONCLUSIONS

Quantitative morphometry by MRI can demonstrate IO size and contractility. Even in cases of unequivocal SO palsy associated with ipsilesional SO atrophy and deficient contractility, the degree of elevation in adduction was not correlated with IO size. This finding suggests that the associated overelevation in adduction, commonly termed "inferior oblique overaction," actually arises from some other mechanism than IO hypertrophy or excess contractility. Revision of clinical terminology seems warranted.

Large Bielschowsky head-tilt phenomenon and inconspicuous vertical deviation in the diagnostic positions in congenital superior oblique palsy

PURPOSE

To report a case of congenital superior oblique palsy with an unusually large Bielschowsky head-tilt phenomenon (BHP) and disproportional inconspicuous vertical deviation.

METHODS

Case report.

RESULTS

An 18-year-old woman presented with slight compensatory head tilting and a Bielschowsky head-tilt phenomenon of 50 Delta on left tilting. Magnetic resonance imaging revealed atrophy of the left superior oblique muscle. A Hess screen test showed a slight underaction of the left superior oblique muscle, but neither an obvious overaction of the ipsilateral inferior oblique muscle nor inhibitory palsy of the contralateral superior rectus muscle was found. With a 3-mm recession of the ipsilateral superior rectus muscle, Bielschowsky head-tilt phenomenon decreased to 25 Delta.

CONCLUSION

A large Bielschowsky head-tilt phenomenon was possibly caused by an increased gain of the otolith-ocular reflex affecting the vertical rectus muscle.

Head-position-dependent adaptation of nonconcomitant vertical skew

Vertical phoria can be trained to vary with either head position or orbital eye position. The present experiments show that subjects can simultaneously adapt their eye-position-specific (nonconcomitant) vertical phorias in different directions at different head positions. Eye-position-dependent and head-position-dependent adaptive pathways, therefore, are not independent. Rather, the adaptation of vertical skew takes into account both eye and head position. In additional experiments, the magnitude of the nonconcomitant adaptive response was shown to be related to otolith output, increasing with head tilt ipsilateral to the tilt position at which training was received and decreasing in the contralateral direction.

Regulation of static and dynamic ocular alignment in patients with trochlear nerve pareses

Ocular alignment and saccades were studied in seven patients with trochlear nerve pareses, before and after strabismus surgery. Prior to surgery, a position-dependent vertical ocular misalignment was present, and downward saccades were hypometric in the paretic eye. Strabismus surgery reduced the magnitude and position-dependence of the static misalignment. Saccade conjugacy improved in the patients with congenital pareses, and in the patient with a gradual-onset acquired paresis, but less improvement occurred in subjects with traumatic pareses. The post-operative change in saccade conjugacy relative to the change in static alignment correlated with pre-operative vertical vergence, suggesting that changes in saccade yoking depend on an interaction between saccades and vertical vergence.

Functional anatomy of normal human rectus muscles

The actions of extraocular muscles depend on their positions as a function of gaze. These positions vary with muscle forces, which are normal only in alert subjects making voluntary fixations. Magnetic Resonance Imaging (MRI) was used to view normal human orbits, with voluntary gaze varied over a circular field 77 deg in dia, centered on the orbital axis. Computer-aided reconstructions, reflecting the data of four normal adult subjects, produced clear pictures of the rectus muscles and optic nerve, and yielded data on muscle paths and cross-sections. From their origins in the orbital apex to their points of tangency with the globe, rectus muscle side-slip, relative to the orbit, is approximately zero; consequently, their "muscle planes" (though not necessarily their axes of rotation) are approximately fixed in the orbit. As the rectus muscles contract they draw in towards the orbital axis, and as they relax they bow outwards; this excursion is as large as 3.7 mm. Contraction also tends to cause the planes of maximum cross-section to move posteriorly.

A four-step test for diagnosis of pseudo superior oblique palsy

An 8-year-old boy presented with an ocular torticollis that had appeared many months after a ptosis repair. The three-step test was positive for a superior oblique palsy. However, at the time of surgery the forced duction test showed a marked restriction in depression of the eye. These findings were duplicated before the second procedure normalized the ocular movements. The surgical microscope was of great help for the extensive dissection necessary to correct the condition. The fourth step, the forced duction test, was essential for accurate differential diagnosis between true superior oblique palsy and a mechanical hypertropia.