A 42-year-old woman from Mumbai presents with involuntary choreiform movements of her limbs, emotional lability, and cognitive decline over the past 18 months. Her father died at age 55 with similar symptoms. Genetic testing confirms a CAG trinucleotide repeat expansion in the huntingtin gene. Which basal ganglia circuit dysfunction best explains her hyperkinetic movement disorder, and what is the underlying neurochemical imbalance?

Explanation

## Clinical Diagnosis This patient presents with **Huntington disease (HD)**: progressive chorea (involuntary, jerky movements), cognitive decline, and psychiatric symptoms. The family history and CAG repeat expansion confirm the diagnosis. ## Basal Ganglia Pathology in Huntington Disease **Key Point:** Huntington disease causes selective degeneration of **medium spiny neurons (MSNs) in the striatum that express D2 dopamine receptors**—these neurons form the **indirect pathway**. The direct pathway (D1-expressing neurons) is relatively spared early in the disease. ### Circuit Imbalance in Huntington Disease | Pathway | Status in HD | Effect on Thalamus | Clinical Result | |---------|--------------|-------------------|----------| | **Direct** | Relatively preserved | Disinhibition (unopposed) | Facilitates unwanted movement | | **Indirect** | Degenerates (D2 neurons lost) | Reduced inhibition | Cannot suppress movement | | **Net Effect** | Direct > Indirect | Thalamus overactive | **Hyperkinetic symptoms (chorea)** | ### Neurochemical Consequence 1. **Loss of D2-expressing MSNs** → indirect pathway degenerates 2. **Direct pathway becomes relatively overactive** → excessive thalamic disinhibition 3. **Relative dopamine excess effect** → the remaining dopamine preferentially activates the unopposed direct pathway 4. **Result:** Involuntary, excessive movement (chorea) **High-Yield:** The **indirect pathway degeneration** is the hallmark pathology. This is the **opposite** of Parkinson disease, where the indirect pathway is overactive but structurally intact. ## Why Dopamine Antagonists Help **Clinical Pearl:** Antipsychotics (e.g., haloperidol, risperidone) reduce chorea by blocking dopamine, thereby reducing the relative excess effect on the preserved direct pathway. This is a key clinical clue to the mechanism. **Mnemonic:** **INDIRECT = Involuntary Degeneration** — when the indirect pathway degenerates (as in HD), the direct pathway dominates, causing hyperkinetic symptoms (excessive, involuntary movement).