IM19.1-2 | Movement Disorder Foundations — Summary & Reflection

KEY TAKEAWAYS

The basal ganglia circuit operates through opposing direct (pro-movement, D1-mediated) and indirect (anti-movement, D2-mediated) pathways. Net output from GPi/SNr inhibits the thalamus; dopamine from SNc biases toward direct pathway activity and movement facilitation. Loss of dopaminergic SNc neurons (Parkinson's) → excess GPi inhibition → hypokinesia (TRAP features). Loss of STN or striatal indirect pathway neurons (Huntington's, STN lesion) → reduced GPi output → thalamic disinhibition → hyperkinesia.

Classification of movement disorders by phenomenology:
- Tremor: rest (PD, 4–6 Hz) vs postural (essential, 6–12 Hz) vs intention (cerebellar); activation pattern is the key discriminator
- Chorea: random, irregular, flowing, non-suppressible; causes: Huntington's (dominant, CAG repeat), Sydenham's (post-streptococcal, rheumatic), drug-induced (dopaminergic, OCP), metabolic
- Dystonia: sustained/repetitive twisting posture; action-worsened; sensory trick relief; treat focal with botulinum toxin
- Hemiballismus: STN lesion → proximal flinging; contralateral
- Tics: stereotyped, suppressible, with premonitory urge; Tourette = motor + vocal >1 year
- Myoclonus: sudden shock-like jerks; metabolic, post-anoxic, epileptic
- Wilson's disease: young + movement + psychiatric + liver → KF rings + low ceruloplasmin + high 24-h urinary copper; treat with chelation (trientine preferred in neurological disease) or zinc

Treatment is phenomenology- and aetiology-specific: dopamine replacement for PD; copper chelation for Wilson's; tetrabenazine/VMAT2 inhibitors for Huntington's chorea and tardive dyskinesia; propranolol/primidone for essential tremor; botulinum toxin for focal dystonia; secondary penicillin prophylaxis for Sydenham's chorea.

REFLECT

Reflect on Mr Rajan (rest tremor, rigidity, bradykinesia) from the opening case. Using the basal ganglia circuit model you have now built, trace exactly why the loss of SNc neurons produces his clinical features: how does reduced dopamine affect both the direct and indirect pathways, and why does this produce bradykinesia rather than weakness? Now consider Mrs Meena's dance-like chorea after a streptococcal throat infection. What immune mechanism explains her involuntary movements, and why is her heart the critical examination target? Finally, think about a common clinical scenario in your training: a patient on a long-term antipsychotic who develops abnormal involuntary movements — what are the different drug-induced syndromes you must distinguish (acute dystonia, akathisia, drug-induced parkinsonism, tardive dyskinesia), and how does the timing of onset after drug initiation help you differentiate them?