A 38-year-old man presents with progressive gait ataxia, dysarthria, and intention tremor for 6 months. Examination reveals nystagmus and past-pointing. MRI brain shows cerebellar atrophy with preserved brainstem. Which investigation is most appropriate to confirm the underlying etiology?

Explanation

## Clinical Presentation Analysis The patient presents with the classic triad of cerebellar dysfunction: 1. Gait ataxia (coordination disorder) 2. Dysarthria (speech disorder) 3. Intention tremor with nystagmus and past-pointing (dysmetria) The progressive course over 6 months with isolated cerebellar atrophy on MRI strongly suggests a hereditary cerebellar ataxia. ## Investigation of Choice **Key Point:** Spinocerebellar ataxias (SCAs) are the most common inherited cerebellar disorders in India and worldwide. Genetic testing of the SCA panel is the gold standard confirmatory investigation for progressive cerebellar ataxia with imaging evidence of cerebellar atrophy. **High-Yield:** SCA1, SCA2, SCA3 (Machado-Joseph disease), and SCA6 account for >90% of autosomal dominant cerebellar ataxias. SCA2 is particularly common in Indian populations. ## Why SCA Genetic Testing? - **Diagnostic accuracy:** Direct genetic confirmation with >95% sensitivity for common SCAs - **Prognostic value:** Identifies CAG repeat expansion number, which correlates with age of onset and disease progression - **Genetic counseling:** Enables family screening and reproductive planning - **Management implications:** Informs symptomatic treatment strategies and enrollment in clinical trials ## Differential Investigations Considered | Investigation | Role | Why Not First-Line Here | | --- | --- | --- | | Vitamin E levels | Rule out AVED (ataxia with vitamin E deficiency) | Normal MRI appearance differs; AVED typically shows spinal cord atrophy; less common than SCA | | CSF analysis | Detect inflammatory/demyelinating causes | Normal imaging argues against MS; oligoclonal bands not expected in SCA | | PET brain | Assess metabolic activity | Supportive but non-specific; does not confirm etiology; more useful in atypical cases | **Clinical Pearl:** Vitamin E deficiency should be screened in all ataxia patients, but the imaging pattern (cerebellar atrophy without spinal cord involvement) and progressive inheritance pattern favor SCA over AVED. **Mnemonic — AVED vs SCA:** **AVED** = **A**taxia with **V**itamin E **D**eficiency (treatable, recessive, spinal cord atrophy); **SCA** = **S**pinocerebellar **A**taxia (genetic, dominant, cerebellar atrophy).