GPAtlas🧭 When to suspect
Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by an expanded CAG (cytosine-adenine-guanine) trinucleotide repeat in the HTT gene on chromosome 4. It produces a characteristic triad of a movement disorder (chorea), progressive cognitive decline, and psychiatric and behavioural disturbance. Onset is usually between 30 and 50 years (mean around 40), with progression over roughly 15–20 years.
A family history of HD is the single most important clue: because inheritance is autosomal dominant, each child of an affected parent has a 50% chance of inheriting the expansion. Where the family history is unknown or unspoken, early HD is readily mistaken for Parkinson's disease or Alzheimer's disease, and psychiatric features often dominate for years before the movement disorder appears.
Diagnosis is genetic, confirmed by the CAG repeat count. The role of primary care is to recognise the triad against the family history, refer through the correct genetic and neurological pathway, and – once a diagnosis is made – manage the high-burden but treatable psychiatric symptoms while remaining alert to the two major causes of premature death: suicide and aspiration.
| CAG repeats | Classification | Interpretation |
|---|---|---|
| ≤ 26 | Normal | Will not develop HD; no risk to offspring. |
| 27–35 | Intermediate allele | Individual will not develop HD, but the repeat is unstable and may expand in offspring (particularly with paternal transmission). |
| 36–39 | Reduced penetrance | At risk of HD; may or may not develop symptoms within their lifetime. |
| ≥ 40 | Full penetrance | Will develop HD if they live long enough. |
Source: Huntington's Disease Association · EHDN
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