Dance-like involuntary movements, psychiatric storm, and a family history. The caudate is melting away. A CAG repeat got longer than it should have. Here is how to spot it, why the movements look like that, and what to do.
A 42-year-old man is brought in by his wife for jerky, dance-like movements of his face and limbs that he cannot control. Over the past year his mood has darkened, he has become impulsive, and he has trouble keeping his job. His father died at age 55 in a psychiatric facility with similar movements. Brain MRI shows symmetric loss of the bumps in the floor of the lateral ventricles.
What is the most likely diagnosis?
Wilson disease
Parkinson disease
Huntington disease
Tardive dyskinesia
Sydenham chorea
Chorea + family history + caudate-atrophy MRI = Huntington until proven otherwise.
Wilson disease also gives a young patient a movement disorder, but it brings liver disease and corneal copper rings, not a parent who died in a psychiatric ward. Parkinson is the opposite picture: too little movement, rest tremor, no family pattern like this. Tardive dyskinesia requires years on a dopamine blocker. Sydenham chorea is a child with a recent strep infection.
The missing caudate bumps are the giveaway. The caudate normally bulges into the floor of the lateral ventricle. When it atrophies the floor flattens out and the ventricle looks square (boxcar).
Pattern Recognition
Four Clues That Lock It In
Spot any two of these together and you have your answer. The fifth panel of the brain is on fire and the family knows it.
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Dance-Like Movements
Chorea: quick, jerky, unpredictable movements that flow from one body part to the next. Looks like the patient is fidgeting in fast-forward. They cannot suppress it; they often try to hide it as a purposeful gesture.
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Parent Had The Same Thing · Earlier
Autosomal dominant with anticipation: one affected parent, and the kid gets sick younger and harder than the parent did. The CAG tract physically lengthens during transmission, especially through dad\'s sperm.
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Caudate Gone, Ventricles Look Square
Boxcar ventricles: the caudate normally bulges into the floor of each lateral ventricle. When it atrophies the floor flattens, the ventricle widens, and you get the pathognomonic square outline.
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CAG Over 40
Trinucleotide repeat expansion in the HTT gene on chromosome 4. Normal is under 27. Over 40 confirms Huntington. The higher the count, the earlier and harder the disease hits.
Anatomy Toggle
Where The Disease Lives: The Striatum
The striatum is caudate plus putamen. Add the globus pallidus and the whole thing becomes the basal ganglia. Huntington picks one structure and burns it down first: the caudate. Flip the toggle and watch the bump disappear while the ventricle widens to fill the empty space.
Coronal · Anterior
Healthy brain. The caudate head pushes up into the floor of each lateral ventricle, giving it the classic curved C-shape.
Generations
The CAG Repeat Gets Longer Each Generation
The HTT gene normally carries fewer than 27 CAG repeats. The repeat is unstable when copied. Each time it passes through sperm it tends to stretch a little. Cross 40 and you have disease. Cross 60 and the disease shows up before adulthood. Tap to walk through a family.
Generation 1 · Grandparent
18 CAG
Normal
10 to 26 · Normal27 to 35 · Carrier36 to 39 · Reduced40+ · Disease
Why The Movements Look Like That
The Indirect Brake Gets Cut
The basal ganglia have two pathways pointing at the thalamus. The direct path is the gas pedal: it tells the thalamus to go, which tells the cortex to move. The indirect path is the brake: it tells the thalamus to hush. Huntington kills the GABA neurons in the caudate that feed the brake. The brake fails. Movement floods out. Parkinson does the opposite damage and gets the opposite picture.
Direct path says go, indirect path says stop. They balance. Dopamine from the substantia nigra fine-tunes both. Result: smooth, controlled movement.
Net Output · Smooth Movement
Imaging
Boxcar Ventricles On MRI
This is the picture. Coronal slice through the basal ganglia. In a healthy brain the caudate head curves up into the floor of each lateral ventricle. In Huntington that bump is gone, the floor flattens, and the ventricle widens into a square (boxcar) shape. Tap the image to enlarge.
What to look for: the lateral ventricles look square-ish at the level of the basal ganglia. The smooth curved bump where the caudate head should be has been replaced by a flat floor. The entire striatum looks thinned. Brain volume overall is reduced.
Source: Wikimedia Commons · CC BY-SA 3.0
Huntington Disease · Caudate Atrophy MRI
Bilateral caudate head atrophy. The lateral ventricles widen and lose the curved floor (boxcar appearance). The diagnostic signature on imaging.
Wikimedia Commons · CC BY-SA 3.0
Quiz · 5 Cases
Lock It In
Five vignettes. One right answer each. Tap your pick, then read the breakdown. Every wrong choice gets explained so you walk away smarter.
0 / 5
Try again to lock the pattern.
clinical Walkthrough
clinical Walkthrough
Original clinical vignettes. Shuffled, never-repeat, full explanations for every choice.
Medically reviewed by Kaitlyn Cocuzzo, MD and Fatima Ali, DO · Last updated June 30, 2026 at 1:10 AM ET
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