Ophthalmological findings in patients with spinocerebellar ataxia type 1 are not correlated with neurological anticipation

Macular Morpho-Functional and Visual Pathways Functional Assessment in Patients with Spinocerebellar Type 1 Ataxia with or without Neurological Signs

Spinocerebellar ataxia type 1 (SCA-ATXN1) is an autosomal dominant, neurodegenerative disease, caused by CAG repeat expansion in the ataxin-1 gene (ATXN1). In isolated reports of patients with neurological signs [symptomatic patients (SP)], macular abnormalities have been described. However, no reports exist about macular anomalies in SCA1 subjects carrying the ATXN1 mutation without neurological signs [not symptomatic carriers (NSC)]. Therefore, the main aim of our work was to evaluate whether the macular functional and morphological abnormalities could be detectable in SP, genetically confirmed and with neurological signs, as well as in SCA-ATXN1-NSC, harboring pathogenic CAG expansion in ATXN1. In addition, we investigated whether the macular involvement could be associated or not to an impairment of RGCs and of their fibers and of the neural conduction along the visual pathways. Herein, nine SCA-ATXN1 subjects (6 SP and 3 NSC) underwent the following examinations: visual acuity and chromatic test assessments, fundus oculi (FO) examination, macular and peripapillary retinal nerve fiber layer thickness (RNFL-T) analysis by Spectral domain-Optical Coherence Tomography (Sd-OCT) acquisition, multifocal electroretinogram (mfERG), pattern reversal electroretinogram (PERG) and visual evoked potentials (VEP) recordings. In four eyes of two SP, visual acuity reduction and chromatic abnormalities were observed; in three of them FO changes associated with macular thinning and outer retinal defects were also detected. In three NSC eyes, slight FO abnormalities were associated with qualitative macular morphological changes. By contrast, abnormal mfERG responses (exclusively from foveal and parafoveal areas) were detected in all SP and NSC (18 eyes). No abnormalities of PERG values, RNFL-T, and VEP responses were found, but in one SP, presenting abnormal papillo-macular bundle neural conduction. Results from our SCA-ATXN1 cohort suggest that a macular dysfunction, detectable by mfERG recordings, may occur in the overt disorder, and unexpectedly in the stage of the disease in which there is still an absence of neurological signs. In NSC, an exclusive dysfunction of preganglionic macular elements can be observed, and this is associated with both normal RGCs function and neural conduction along the visual pathways.

Ophthalmic Manifestations and Genetics of the Polyglutamine Autosomal Dominant Spinocerebellar Ataxias: A Review

Spinocerebellar ataxia (SCA) is a part of the cerebellar neurodegenerative disease group that is diverse in genetics and phenotypes. It usually shows autosomal dominant inheritance. SCAs, always together with the cerebellar degeneration, may exhibit clinical deficits in brainstem or eye, especially retina or optic nerve. Interestingly, autosomal dominant SCAs share a common genetic mechanism; the length of the glutamine chain is abnormally expanded due to the increase in the cytosine–adenine–guanine (CAG) repeats of the disease causing gene. Studies have suggested that the mutant ataxin induces alteration of protein conformation and abnormal aggregation resulting in nuclear inclusions, and causes cellular loss of photoreceptors through a toxic effect. As a result, these pathologic changes induce a downregulation of genes involved in the phototransduction, development, and differentiation of photoreceptors such as CRX, one of the photoreceptor transcription factors. However, the exact mechanism of neuronal degeneration by mutant ataxin restricted to only certain type of neuronal cell including cerebellar Purkinje neurons and photoreceptor is still unclear. The most common SCAs are types 1, 2, 3, 6, 7, and 17 which contain about 80% of autosomal dominant SCA cases. Various aspects of eye movement abnormalities are evident depending on the degree of cerebellar and brainstem degeneration in SCAs. In addition, certain types of SCAs such as SCA7 are characterized by both cerebellar ataxia and visual loss mainly due to retinal degeneration. The severity of the retinopathy can vary from occult macular photoreceptor disruption to extensive retinal atrophy and is correlated with the number of CAG repeats. The value of using optical coherence tomography in conjunction with electrodiagnostic and genetic testing is emphasized as the combination of these tests can provide critical information regarding the etiology, morphological evaluation, and functional significances. Therefore, ophthalmologists need to recognize and differentiate SCAs in order to properly diagnose and evaluate the disease. In this review, we have described and discussed SCAs showing ophthalmic abnormalities with particular attention to their ophthalmic features, neurodegenerative mechanisms, genetics, and future perspectives.

Temporal retinal nerve fiber loss in patients with spinocerebellar ataxia type 1

Background

Autosomal dominant spinocerebellar ataxia type 1 is an adult onset progressive disorder with well characterized neurodegeneration in the cerebellum and brainstem. Beyond brain atrophy, few data exist concerning retinal and optic nerve involvement.

Objective

To evaluate retinal changes in SCA1 patients compared to age and gender matched healthy controls.

Methodology/Principal Findings

Nine patients with SCA1 were prospectively recruited from the ataxia clinic and were compared to nine age and gender matched healthy controls. Both cohorts received assessment of visually evoked potentials and eye examination by optical coherence tomography to determine retinal nerve fiber layer thickness and total macular volume. While no differences were found in visually evoked potentials, SCA1 patients showed a significant reduction of mean retinal nerve fiber layer thickness (RNFLT) compared to healthy controls (84±13 µm vs. 97±8 µm, p = 0.004). Temporal areas showed the most prominent RNFLT reduction with high statistical significances (temporal-inferior: p<0.001, temporal: p<0.001, temporal-superior: p = 0.005) whereas RNFLT in nasal areas was in the range of the control group. From six SCA1 patients an additional macular scan was obtained. The comparison to the corresponding healthy control showed a slight but not significant reduction in TMV (8.22±0.68 mm³ vs. 8.61±0.41 mm³, p = 0.15).

Conclusion

In SCA1 patients, we found evidence for degeneration of retinal nerve fibers. The temporal focus of the observed retinal nerve fiber layer reduction suggests an involvement of the papillo-macular bundle which resembles pathology found in toxic or mitochondrial optic nerve disease such as Leber's hereditary optic neuropathy (LHON) or dominant optic atrophy (DOA).

Spinocerebellar degenerations

The spinocerebellar ataxias (SCA) are a large group of inherited disorders affecting the cerebellum and its afferent and efferent pathways. Their hallmark symptom is slowly progressive, symmetrical, midline, and appendicular ataxia. Some may also have associated hyperkinetic movements (chorea, dystonia, myoclonus, postural/action tremor, restless legs, rubral tremor, tics), which may aid in differential diagnosis and provide treatable targets to improve performance and quality of life in these progressive, incurable conditions. The typical dominant ataxias with associated hyperkinetic movements are SCA1-3, 6-8, 12, 14, 15, 17, 19-21, and 27. The common recessive ataxias with associated hyperkinetic movements are ataxia telangiectasia and Friedreich's ataxia. Fragile X tremor-ataxia syndrome (FXTAS) and multiple-system atrophy (a sporadic ataxia which is felt to have a genetic substrate) also have hyperkinetic features. A careful work-up should be done in all apparently sporadic cases, to rule out acquired causes of ataxia, some of which can cause hyperkinetic movements in addition to ataxia. Some testing should be done even in individuals with a confirmed genetic cause, as the presence of a secondary factor (nutritional deficiency, thyroid dysfunction) can contribute to the phenotype.

Ophthalmic features of spinocerebellar ataxia type 7

BACKGROUND

Patients with spinocerebellar ataxia 7 (SCA7) are known to develop ocular abnormalities. The purpose of this study was to characterize these abnormalities in greater detail and with the aid of newer quantitative technologies.

METHODS

Seven patients with SCA7 diagnosed by genetic analysis at La Paz Hospital (Madrid, Spain), a country-wide referral center for ataxias, were included in the study. Demographic data and ocular features were recorded from a complete ophthalmologic examination, specular microscopy, corneal topography (Pentacam), and optical coherence tomography (OCT).

RESULTS

All 7 patients had decreased visual acuity associated with varying degrees of macular pigmentary changes on ophthalmoscopy. All 7 had lower corneal endothelial cell densities than expected for their age, and 5 had increased corneal volume, although none had corneal edema. Patients with mild disease showed retinal thinning at the fovea. In patients with more advanced disease, retinal thinning was present also in the outer zone of the macula. Mean peripapillary retinal nerve fiber layer thickness was decreased in all patients; however, the temporal quadrant was spared except in advanced disease.

CONCLUSIONS

This study of 7 patients with SCA7 amplifies previous reports of ophthalmic abnormalities in this condition by providing data from specular microscopy, corneal topography, and OCT. Abnormalities were present in the anterior and posterior ocular segments, as well as in eye movements and pupillary reactions. Visual dysfunction, present in all patients, was associated with retinal thinning. Decreased endothelial cell density and increased corneal thickness were common.