Movement disorders, dystonia and chorea: Viva

Branch 1: Naming and classifying the movement

A strong candidate names this dystonia and classifies it on the Albanese 2013 two axes. Dystonia is sustained or intermittent involuntary muscle contraction that causes twisting, repetitive movements, or abnormal postures. On axis I, the clinical characteristics, this is a childhood-onset, focal-to-generalising, action-induced lower-limb dystonia with a diurnal temporal pattern of evening worsening and morning improvement. On axis II, the aetiology, the diurnal pattern flags dopa-responsive dystonia, but the candidate keeps the inherited and acquired differentials open, including cerebral palsy, Wilson disease, and glutaric aciduria type 1, until the workup closes. The candidate also places the movement in the Sanger 2003 childhood hypertonia framework, which sorts hypertonia into spasticity, dystonia, and rigidity. [1][2]

Branch 2: The single diagnostic and therapeutic step

When the examiner presses on the next step, the candidate states that every dystonia of unknown cause in a child gets an empiric low-dose levodopa trial with a decarboxylase inhibitor, because dopa-responsive dystonia, or Segawa disease, is caused in its commonest form by a GCH1 mutation that reduces dopamine synthesis, and it responds dramatically and sustainedly. The diurnal lower-limb pattern makes dopa-responsive dystonia the leading diagnosis, and the trial is both diagnostic and therapeutic, so it precedes any invasive procedure. The candidate runs the metabolic and genetic workup in parallel, screening for Wilson disease with serum copper, caeruloplasmin, 24-hour urinary copper, and slit-lamp examination, and sending urine organic acids for glutaric aciduria type 1. [12]

Branch 3: The mechanism

If the examiner asks why dopamine is central, the candidate explains the basal ganglia motor loop. The cortex excites the striatum, and the striatum governs thalamocortical output through two parallel routes. The direct pathway facilitates wanted movement and the indirect pathway suppresses unwanted movement, and dopamine from the substantia nigra pars compacta tunes the balance by exciting the direct pathway through D1 receptors and braking the indirect pathway through D2 receptors. In dopa-responsive dystonia, reduced dopamine synthesis tips the balance, and replacing it with levodopa restores it. The same loop explains why a dopamine-blocking drug over-weights the indirect pathway and causes an acute dystonic reaction, and why loss of indirect-pathway braking produces chorea. [3]

Branch 4: The status dystonicus emergency

The examiner then offers a different child, an 8-year-old boy with dystonic cerebral palsy who arrives with escalating, painful, sustained dystonic spasms over 12 hours. The strong candidate recognises status dystonicus and triages to intensive care without delay. The immediate priorities are securing the airway and breathing, checking creatine kinase and urine myoglobin for rhabdomyolysis with aggressive hydration to protect the kidneys, and controlling pain. The candidate hunts for the precipitant, most often infection, a medication change or withdrawal, pain, or constipation, and in a non-verbal child with cerebral palsy has a low threshold for a hip and a pain problem. Escalation follows the ladder from a benzodiazepine and enteral anti-dystonia drugs through sedation and ventilation to intrathecal baclofen or deep brain stimulation for refractory disease. [6][7]

Branch 5: Counselling the family

Asked what to tell the family of the first child, the candidate explains that the diagnosis is likely dopa-responsive dystonia, that the levodopa trial will confirm it within days, and that the prognosis with levodopa is excellent, with a dramatic and sustained response that lets the child walk normally. The candidate counsels the family on the importance of adherence, the expected low maintenance dose, and the genetic nature of the condition, and arranges paediatric neurology follow-up. For the second child's family, the candidate is honest that status dystonicus is life-threatening, explains the escalation plan, and provides a written dystonia action plan with a clear safety-net for when to return. [12]

The examiner rewards a candidate who names and classifies the movement, who trials levodopa in every unexplained dystonia, who runs the status dystonicus ladder in order, who screens for Wilson disease, and who counsels the family honestly on prognosis and safety-netting. [1][7]