Parkinson's vs Huntington's Disease: Two Sides of the Basal Ganglia
What's the difference between Parkinson's and Huntington's disease?
Side view
Midline view
The Short Answer
Both diseases disrupt the basal ganglia, the circuit that decides which movements to allow. In Parkinson's, the loss of dopamine-producing cells in the brain stem makes movement hard to start, so movements become slow, stiff, and small. In Huntington's, cells in the basal ganglia that normally hold back unwanted movements die first, so involuntary jerking movements break through.
At a Glance
| Aspect | Parkinson's disease | Huntington's disease |
|---|---|---|
| Main movement problem | Too little movement: slowness, stiffness, resting tremor | Too much movement: involuntary jerks and writhing (chorea) |
| Where damage starts | Dopamine-producing cells in the substantia nigra (brain stem) | Neurons in the striatum, part of the basal ganglia |
| Cause | Usually unknown; a minority of cases are linked to genes | Always genetic: an expanded CAG repeat in the HTT gene |
| Inheritance | Usually not directly inherited | Dominant: each child of a parent with it has a 50% chance |
| Typical age at onset | Usually over 60 | Usually 30 to 50 |
| Effect of dopamine medicines | Boosting dopamine (levodopa) eases symptoms | Reducing dopamine signalling can ease chorea |
One Circuit, Two Opposite Failures
The basal ganglia act like a gate for movement. At any moment your brain has many possible actions ready; the basal ganglia let the chosen one through and hold back the rest. In an influential 1989 model, Roger Albin, Anne Young, and John Penney described two routes through the basal ganglia: a "direct pathway" that helps release movement, and an "indirect pathway" that suppresses it. Healthy movement depends on a balance between the two, and dopamine helps keep that balance.
The model, since refined, neatly explains why these two diseases look like mirror images.
Parkinson's: The Gate Stays Shut
In Parkinson's disease, dopamine-producing neurons in the substantia nigra, in the midbrain part of the brain stem, gradually die. With less dopamine reaching the basal ganglia, the balance tips toward suppression. Movements become slow and small, the muscles stiffen, and starting to move - getting up from a chair, taking a first step - becomes hard.
Huntington's: The Brakes Fail
In Huntington's disease, a faulty gene causes neurons in the striatum to die. In the early stages, the neurons of the suppressing "indirect" pathway are hit hardest, so the brakes on unwanted movement weaken. The result is chorea: brief, involuntary, dance-like movements that flow from one body part to another. Later in the disease, as more neurons are lost, people with Huntington's can also become slow and stiff.
The Dopamine Seesaw
The opposite nature of the two diseases shows up in their treatment. Parkinson's symptoms improve with levodopa, which the brain converts into dopamine. Too much dopamine stimulation, though, can tip the balance the other way: after years of treatment, many people with Parkinson's develop involuntary movements called dyskinesias that resemble chorea. In Huntington's, medicines that reduce dopamine signalling, such as tetrabenazine, can ease chorea.
Beyond Movement
Both diseases affect much more than movement. The basal ganglia work in loops with the frontal lobe for thinking and mood as well, so both conditions can cause slowed thinking, difficulty planning, depression, and anxiety. In Huntington's, changes in thinking and behaviour are often the most disabling part of the disease, and can come years before the movement symptoms. In Parkinson's, loss of smell, constipation, and acting out dreams during sleep can come years before the tremor.
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Key Terms
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Sources & Further Reading
- The functional anatomy of basal ganglia disorders - Albin, Young & Penney (1989), Trends in Neurosciences
- The basal ganglia: focused selection and inhibition of competing motor programs - Mink (1996), Progress in Neurobiology
- Parkinson's Disease - National Institute of Neurological Disorders and Stroke
- Huntington's Disease - National Institute of Neurological Disorders and Stroke
- A novel gene containing a trinucleotide repeat that is expanded and unstable on Huntington's disease chromosomes - The Huntington's Disease Collaborative Research Group (1993), Cell