A 42-year-old woman from Bangalore presents with involuntary, jerky, purposeless movements of her limbs and trunk that worsen with stress and improve during sleep. She has a family history of similar illness in her mother and maternal uncle. Cognitive decline and emotional lability are noted. Genetic testing confirms a CAG trinucleotide repeat expansion. On functional neuroimaging, there is selective loss of medium spiny neurons in the striatum, particularly affecting neurons that express D2 dopamine receptors. Which basal ganglia circuit dysfunction best explains her hyperkinetic movement disorder?

Explanation

## Clinical Diagnosis: Huntington's Disease The patient presents with **chorea** (involuntary, jerky movements), **cognitive decline**, **emotional instability**, and a **positive family history** with **CAG trinucleotide repeat expansion** — pathognomonic for **Huntington's disease (HD)**. ## Pathophysiology of Huntington's Disease **Key Point:** Huntington's disease causes selective degeneration of **medium spiny neurons (MSNs) expressing D2 dopamine receptors**, which are the primary neurons of the **indirect pathway**. This results in **loss of the indirect pathway** and relative **overactivity of the direct pathway**. ### Basal Ganglia Circuit Imbalance in HD | Circuit | Status in HD | Effect on Thalamus | Clinical Result | |---------|-------------|--------------------|-----------------| | **Direct pathway** | **Relatively overactive** (due to loss of indirect pathway) | Disinhibition (excessive facilitation) | **Hyperkinesia — chorea, involuntary movements** | | **Indirect pathway** | **Selectively degenerated** (loss of D2+ MSNs) | Loss of inhibitory tone | Unopposed direct pathway activity | ### Mechanism of Chorea in Huntington's Disease 1. **Selective neuronal loss:** The huntingtin protein mutation causes preferential death of **D2-receptor-expressing MSNs** in the striatum 2. **Indirect pathway collapse:** These D2+ neurons form the indirect pathway (Striatum → GPe → STN → GPi/SNpr) 3. **Loss of inhibitory control:** Without the indirect pathway, the direct pathway (Striatum → GPi/SNpr) becomes unopposed 4. **Net effect:** **Excessive disinhibition of the thalamus** → **hyperkinetic movement disorder (chorea)** **Clinical Pearl:** Chorea is the **opposite** of Parkinson's disease. In Parkinson's, the direct pathway is suppressed (hypokinesia); in Huntington's, the indirect pathway is lost (hyperkinesia). This is why **levodopa worsens chorea** in HD patients — it further enhances the already-overactive direct pathway. **High-Yield:** The **D2-receptor selectivity** is crucial: D2+ MSNs die in HD, while D1+ MSNs (which form the direct pathway) are relatively spared. This explains why the direct pathway becomes relatively overactive. **Mnemonic:** **HUNT = Huntington's Unopposed direct pathway Neuronal loss (D2+) Thalamic disinhibition** → Chorea and hyperkinesia. ### Contrast with Parkinson's Disease ```mermaid flowchart TD A[Basal Ganglia Pathology]:::outcome --> B{Which neurons die?}:::decision B -->|SNpc dopamine neurons| C[Parkinson's Disease]:::outcome C --> D[Direct pathway suppressed]:::action D --> E[Hypokinesia: bradykinesia, rigidity, tremor]:::outcome B -->|Striatal D2+ MSNs| F[Huntington's Disease]:::outcome F --> G[Indirect pathway lost]:::action G --> H[Hyperkinesia: chorea, involuntary movements]:::outcome ``` [cite:Kandel & Schwartz Principles of Neural Science 6e Ch 41]