BACKGROUND: Anti-glutamic acid decarboxylase antibody (GAD-ab)-associated 
cerebellar ataxia is a rare neurological disorder characterized by cerebellar 
symptoms concomitant with high GAD-ab levels in serum and cerebrospinal fluid 
(CSF).
CASE REPORT: We report on 2 female siblings (aged 74 and 76 years) presenting 
with gradual progression of rotational vertigo, gait ataxia and vertical 
diplopia, continuously progressing for 6 months and 6 years, respectively. 
Autoimmune laboratory examinations showed remarkably increased serum and CSF 
GAD-ab levels. Their medical histories revealed late-onset type 1 diabetes 
mellitus (T1DM) and other concomitant autoimmune disorders (Grave's disease, 
Hashimoto's thyroiditis). Cerebral MRI and laboratory examinations were 
unremarkable. The diagnosis of GAD-ab-associated cerebellar ataxia with 
particular brainstem involvement was established in both women. After the 
exclusion of an underlying malignancy, immunosuppressive therapy has been 
initiated in both patients, which resulted in stabilization in one and in 
clinical improvement in the other patient.
DISCUSSION: The unique association of autoantibody-mediated cerebellar ataxia 
and late-onset T1DM in 2 siblings with similar clinical and paraclinical 
phenotypes strengthens the concept that hereditary factors might play a relevant 
role also in autoimmune diseases so far considered to be sporadic. Moreover, the 
occurrence of continuous vertical diplopia broadens the clinical spectrum of 
GAD-ab-associated neurological syndromes.