Task specific focal dystonia: a presentation of spinocerebellar ataxia type 6.

Autosomal dominant cerebellar ataxias (ADCA) are characterised by clinical and genetic heterogeneity with a substantial overlap of clinical features and a variable degree of adherence to three distinct phenotypes according to Harding's clinical classification: ADCA type I, II, and III. 1 The availability of molecular genetic testing has provided increasing appreciation of a wider clinical spectrum than previously thought for each ADCA subgroup. In addition there is an increasing list of genes harbouring disease causing mutations which, to date, include spinocerebellar ataxia 2 SCA 6 is caused by unstable CAG expansion in the a-1A calcium channel gene (CACNA1A). 4 We present a confirmed case of SCA 6 presenting with a task specific focal dystonia (writer's cramp) predated the onset of progressive gait ataxia by five years, which further widens the clinical spectrum of SCA 6. A 57 year old female pharmacist first presented at the age of 42 with a two month history of progressive difficulty in writing prescriptions as a result of writing induced cramp affecting the right hand. This task specific focal dystonia was aggravated by stress and fatigue. Her father was reported to have had Parkinson's disease on the basis of developing tremor and gait disturbance at age 40. On examination, the patient's gait was normal and no focal neurological deficits were identified. Her writing, after a normal start, was interrupted by a semiflexed posture of the fingers to maintain the grasp of the pen, with the index finger and thumb appeared hyperextended and the wrist hyper-flexed. This was accompanied by an ache in the hand. A working diagnosis of writer's cramp with a suspicion of familial dystonic disorders was made and she was started on the anticholinergic drug, trihexyphenidyl. On annual follow up, she reported a satisfactory improvement in her symptoms. During this period and on more than one occasion, no cerebellar signs were identified. Five years after the onset of focal dystonia, she developed an intention tremor in her right hand and upper limb ataxia but with a normal gait. A diagnosis of autoimmune hypothyroidism was made following a con-firmatory test results and thyroxine was started concurrently with propranolol to control the tremor. Two years later, at the age of 49, she began to experience difficulties with maintaining her balance when walking downhill, which later progressed to an increasing tendency to sway or trip. The writer's cramp had persisted with progressive tremor and clumsiness in …