A 52-year-old woman with a 2-year history of progressive unsteadiness, truncal ataxia, and hyperreflexia is evaluated. Examination shows nystagmus, dysmetria, and brisk tendon reflexes with extensor plantar responses. MRI brain reveals cerebellar atrophy with T2 hyperintensity in the spinal cord. Which investigation is most appropriate to establish the diagnosis?

Explanation

## Clinical Presentation Synthesis The combination of findings in this patient is characteristic of a **hereditary spinocerebellar ataxia (SCA)**: - **Cerebellar signs:** Progressive truncal ataxia, dysmetria, nystagmus - **Upper motor neuron signs:** Hyperreflexia, extensor plantar responses (Babinski sign) - **Imaging:** Cerebellar atrophy + spinal cord T2 hyperintensity - **Course:** Slowly progressive over 2 years This constellation — cerebellar degeneration combined with corticospinal tract involvement — is the hallmark of **spinocerebellar ataxias (SCAs)**, particularly those with spinal cord involvement (e.g., SCA1, SCA2, SCA3/Machado-Joseph disease, Friedreich's ataxia). ## Why Genetic Testing Is the Most Appropriate Investigation? **Key Point:** The SCAs are a heterogeneous group of autosomal dominant (and some recessive) neurodegenerative disorders caused by trinucleotide repeat expansions or other mutations. Genetic testing (PCR-based repeat sizing or next-generation sequencing panels) is the **definitive diagnostic tool** for establishing the specific SCA subtype. **High-Yield:** The coexistence of: 1. Progressive cerebellar atrophy (Purkinje cell degeneration) 2. Spinal cord T2 hyperintensity (spinocerebellar and corticospinal tract involvement) 3. Hyperreflexia + Babinski sign (upper motor neuron involvement) 4. Slowly progressive 2-year course in a middle-aged adult ...is the classic phenotype of hereditary SCA with spinal cord involvement, NOT subacute combined degeneration (SCD) from B12 deficiency. ## Why NOT Vitamin B12 / MMA (Option D)? | Feature | SCA | B12 Deficiency (SCD) | |---|---|---| | Course | Slowly progressive (years) | Subacute (weeks–months) | | Reflexes | **Hyperreflexia** (UMN) | **Hyporeflexia** (PNS involvement) | | Cerebellar atrophy on MRI | Prominent | Mild/absent | | Spinal cord T2 | Spinocerebellar + corticospinal tracts | Dorsal + lateral columns | | Definitive test | **Genetic testing** | Serum B12 + MMA | **Clinical Pearl:** In B12 deficiency–induced SCD, the predominant spinal cord finding is **dorsal column** (posterior column) demyelination causing **loss of vibration and proprioception with hyporeflexia**, not hyperreflexia. The presence of **brisk reflexes and Babinski sign** in this stem points away from B12 deficiency and toward a hereditary SCA with corticospinal tract involvement. (Harrison's Principles of Internal Medicine, 21st ed., Chapter on Ataxic Disorders) ## Investigation Hierarchy for SCA | Investigation | Role | |---|---| | **Genetic testing (SCA panel / trinucleotide repeat analysis)** | **Definitive diagnosis** | | MRI brain + spine | Structural characterization | | Nerve conduction studies | Assess peripheral involvement | | Serum B12, copper | Rule out treatable mimics | **Mnemonic — SCA Clues:** **H**yperreflexia + **A**taxia + **C**erebellar atrophy + **S**pinal cord T2 = **HACS** → Think **Hereditary SCA** → **Genetic Testing** ## Why Other Options Are Incorrect? - **Option B (Brain biopsy with electron microscopy):** Invasive and not indicated when a non-invasive genetic diagnosis is available and the clinical picture is consistent with hereditary SCA. - **Option C (Serum copper and ceruloplasmin):** Copper deficiency myelopathy can mimic SCD but presents with hyporeflexia and sensory ataxia, not the full SCA phenotype with prominent cerebellar atrophy and UMN signs. - **Option D (Serum vitamin B12 and methylmalonic acid):** B12 deficiency causes hyporeflexia (peripheral neuropathy predominates), not hyperreflexia; prominent cerebellar atrophy is not a hallmark of SCD. The UMN signs and cerebellar atrophy here are inconsistent with B12 deficiency.