A 58-year-old woman from Mumbai presents with involuntary choreiform movements of her limbs and face that worsen with emotional stress, along with progressive cognitive decline and mood disturbances. Family history reveals her mother died at age 55 with similar symptoms. Genetic testing confirms a CAG trinucleotide repeat expansion. Which basal ganglia circuit abnormality best explains her hyperkinetic movement disorder?

Explanation

## Huntington's Disease and Basal Ganglia Circuit Dysfunction ### Clinical Recognition The patient presents with: - **Chorea** (involuntary, jerky movements) - **Cognitive decline** (dementia) - **Psychiatric symptoms** (mood disturbance) - **Positive family history** with early-onset death - **Genetic confirmation** of CAG repeat expansion (>36 repeats = Huntington's disease) This is the classic triad of Huntington's disease (HD). ### Pathology in Huntington's Disease **Key Point:** Huntington's disease selectively destroys **medium spiny neurons (MSNs) in the striatum**, particularly those expressing **D2 dopamine receptors** that form the **indirect pathway**. ### The Indirect Pathway Vulnerability | Feature | D1 Neurons (Direct) | D2 Neurons (Indirect) | |---------|-------------------|----------------------| | **Receptor type** | D1 dopamine receptors | D2 dopamine receptors | | **Pathway** | Striatum → GPi/SNr | Striatum → GPe → STN → GPi/SNr | | **Function** | Facilitates movement | Inhibits movement | | **Vulnerability in HD** | Relatively preserved | **Selectively degenerated** | | **Result** | Direct pathway hyperactive | Indirect pathway weakened | ### Circuit Imbalance in Huntington's Disease ```mermaid flowchart TD A["Striatum (MSNs with D2 receptors)<br/>DEGENERATED in HD"]:::urgent --> B["GPe<br/>(Reduced inhibition)"] B -->|Weakened GABA| C["STN<br/>(Hyperactive)"] C -->|Excessive glutamate| D["GPi/SNr<br/>(Hyperactive)"] E["Striatum (MSNs with D1)<br/>PRESERVED"] -->|Direct pathway<br/>unopposed| D D -->|Reduced inhibition| F["Thalamus"] F -->|Excessive excitation| G["Motor Cortex<br/>CHOREA & HYPERKINESIA"]:::outcome H["Cortical degeneration"]:::urgent --> I["Cognitive decline<br/>& Psychiatric symptoms"] ``` ### Why the Indirect Pathway Degenerates First **High-Yield:** The mutant huntingtin protein (mHTT) is particularly toxic to D2-expressing MSNs. The reasons are not fully understood but may involve: - Differential vulnerability to excitotoxicity - Selective mitochondrial dysfunction in D2 neurons - Impaired protein handling and autophagy ### Resulting Circuit Dysfunction 1. **Loss of D2 neurons** → Indirect pathway weakens 2. **D1 neurons relatively preserved** → Direct pathway remains active 3. **Net effect:** Unopposed direct pathway activity → Excessive thalamic disinhibition → **Hyperkinesia (chorea)** **Clinical Pearl:** This is the **opposite** of Parkinson's disease, where the direct pathway is weak and the indirect pathway is overactive, causing hypokinesia. ### Mnemonic: PD vs. HD Movement Disorders **INDIRECT = INhibition** - **Parkinson's:** Indirect pathway hyperactive → Movement INhibited → Bradykinesia - **Huntington's:** Indirect pathway DEgenerated → Movement DISinhibited → Chorea ### Why Levodopa Worsens Huntington's Disease In HD, dopamine strengthens the already-overactive direct pathway, exacerbating chorea. This is why dopamine antagonists (antipsychotics) are used to treat chorea in HD, not dopamine agonists.