Objective: To describe a case of spinocerebellar ataxia type 12 (SCA-12) with an unusual phenotype.

Background: SCA-12 is a rare autosomal dominant cerebellar ataxia which occurs secondary to mutations in the PPP2R2B gene, and has been commonly reported among the Agarwal community in India.  The disease is typically characterized by a fourth decade onset of action tremor of the upper extremities.  As the disease progresses patients inevitably develop ataxia.  In later stages of the illness, hyperreflexia, parkinsonism, psychiatric symptoms and cognitive impairment may develop.  To the best of our knowledge myoclonus has never been reported in SCA-12.

Method: Case report

Results: A 54-year-old man from an Agarwal community presented with a 36-year history (age at onset-18) of myoclonic jerks, behavioral disturbances – anxiety, anger outbursts, features of obsessive compulsive disorder for 30 years, tremor of bilateral upper limbs for 20 years, imbalance while walking for 2 years, and 2 month history of cognitive decline and urinary incontinence.  There was a strong family history of tremor, ataxia, dementia, seizures and features suggestive of myoclonus.
On examination, he was bed bound, cognitively impairment had emotional lability.  He had a voice tremor, upper limb tremor and myoclonus, limb rigidity, incoordination, and hyperreflexia.  Neuroimaging revealed diffuse cerebral and cerebellar atrophy, EEG was normal.  Based on the positive family history and ethnicity, SCA-12 was considered as a possibility.  Genetic testing revealed the presence of a pathogenic compound heterozygous CAG repeat of 38±5/52±5 on the PPP2R2B gene which confirmed the diagnosis SCA-12.  Based on the unusual presentation of the patient and family history of seizure and myoclonus, a possibility of a second co-existent disease was considered.  No pathogenic mutations were observed on whole exome sequencing, following which a diagnosis of SCA-12 with an unusual phenotype was arrived upon.

Conclusion: SCA-12 may have variable presentations beyond tremor and ataxia.  This should be investigated as a probable diagnosis in patients when clinical suspicion is strong and is supported by family history and ethnicity.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/an-unusual-phenotype-of-spinocerebellar-ataxia-type-12/