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Spinocerebellar ataxia type 7 without retinal degeneration: a case report.

A 60-yr-old man developed progressive gait disturbance and limb ataxia at the age of 52. Family history was absent for neurological disorders. Examinations showed pure cerebellar syndrome. There was no retinal degeneration for 7 yr. A brain MRI done at the age of 56 showed atrophy of the cerebellar hemispheres and vermis. Genetic test confirmed the spinocerebellar ataxia type 7 with CAG repeat number of 42.


INTRODUCTION
The autosomal dominant cerebellar ataxias (ADCA) are a heterogeneous group of neurodegenerative disorders with overlapping phenotypes that include progressive dysfunction of the cerebellum, basal ganglia, brainstem, cerebral cortex, spinal cord, and peripheral nerves. Based on the phenotypic characteristics, Harding (1) classified ADCAs into three types: ADCA I, ADCA II, and ADCA III. Disorder in ADCA II show cerebellar ataxia accompanied by macular degeneration.
SCA7 is caused by the expansion of CAG trinucleotide repeat in the chromosome 3p12-21.1. The affected gene region encodes for the ataxin-7. Ataxin-7 is an 892 amino acid nuclear protein of unknown function. The N-terminal segment of ataxin-7 contains a polyglutamine stretch, and is believed to exert a toxic effect when the polyglutamine stretch is ex-panded. Normal SCA7 alleles range from 6 to 17 CAG repeats (12). Pathological alleles range from 37 and above (13)(14)(15). As in other SCAs, SCA7 has a broad spectrum of phenotypic abnormalities depending on the number of CAG repeats (16). Ataxia is a universal feature and constant in all age groups. On the other hand spasticity and ocular symptoms are variable according to the onset age. Visual loss and electroretinographic changes may precede funduscopic abnormalities (17). Retinal degeneration constitutes an important component in juvenile and early adult onset (<25 yr), but is variable in late onset patients (>40 yr) (18). In late onset patients visual failure due to retinal degeneration is a late symptom, and in some patients, it is not found for periods varying between 17 and 27 yr after the onset of ataxia (18).
Herein, we describe a SCA7 patient without retinal degeneration for 7 yr.

CASE REPORT
The patient was a 60-yr-old man who had had progressive gait disturbance and ataxia since the age of 52. Unsteady gait was the first symptom especially during running. Unsteady gait progressed until he became in a wheelchair-bound state. Dysarthria began at the age of 56. He had no significant past medical history. Family history was absent for neurological disorders.
On admission, he was alert, oriented and dysarthric. Eye examination by ophthalmologist showed an uncorrected visual acuity of 20/40 in the right and 20/60 in the left with Snellen rating for distance vision, normal visual field, and normal color discrimination with Ishihara color test. There was neither ophthalmoparesis nor nystagmus. Funduscopic examination showed neither macular pigmentation nor disc pallor (Fig. 1). There was no muscular atrophy or weakness. These ophthalmologic data were the same results as those at the age of 56. Pathologic reflexes were absent. Sensation and autonomic functions were normal. He was able to walk only with one-person assistance. Routine blood count, urinalysis, serum chemistries, and thyroid functions were normal.
Visual evoked potential was normal. A brain MRI done at the age of 56 showed a global atrophy of the cerebellar hemispheres and vermis, whereas the pons, medulla, and upper spinal cord were spared (Fig. 2).
Genomic testing revealed a pathological expansion of CAG repeats in the SCA7 gene (repeat number=42/10).

DISCUSSION
We have previously reported that there are cases of SCA1, 2, 3, 6, and 7 in Korea with SCA2 being the most common (19). Genetically confirmed SCA7 with retinal degeneration was previously reported in Korea (20). Phenotypically, our patient had pure cerebellar syndrome without retinal degeneration, therefore was not thought to be ADCAII or SCA7. Our case again emphasizes the phenotypic variability of SCAs, and the need for genetic confirmation whenever possible. The degree of CAG expansion roughly determines the onset age and phenotype (12). There was a strong negative correlation between the age at onset and the size of the CAG repeat expansion in SCA7 patients. Larger expansions were associated with an earlier onset, a more severe and rapid clinical course, and higher frequencies of decreased vision, ophthalmoplegia, extensor plantar response, and scoliosis (14). CAG repeat number of 42 in the SCA7 gene is usually associated with the late adulthood onset and less frequent retinal involvement (14) as in our case.