Definition and epidemiology
Huntington's disease is a progressive, fatal, autosomal dominant neurodegenerative disorder characterized by the triad of motor dysfunction (particularly chorea), cognitive impairment, and psychiatric disturbances. The disease results from an expanded CAG trinucleotide repeat in the huntingtin (HTT) gene located on chromosome 4p16.3.
HD affects approximately 5-10 per 100,000 individuals in populations of Western European descent, with lower prevalence in Asian and African populations. The typical age of onset is 35-45 years, though juvenile-onset HD (before age 20, accounting for 5-10% of cases) and late-onset forms also occur. Men and women are equally affected. The disease demonstrates complete penetrance when CAG repeats exceed 40, and shows genetic anticipation, particularly with paternal transmission.
- HD is autosomal dominant with complete penetrance when CAG repeats ≥40; shows anticipation with paternal transmission
- Classic triad: chorea, cognitive decline (executive dysfunction early), psychiatric symptoms (depression most common)
- Caudate atrophy with enlarged frontal horns ("boxcar ventricles") is the characteristic MRI finding
- Genetic testing showing ≥36 CAG repeats in HTT gene is diagnostic; 36-39 has reduced penetrance
- Juvenile-onset HD (Westphal variant) presents with rigidity and seizures rather than chorea
- Tetrabenazine/deutetrabenazine are first-line for chorea; SSRIs for depression
- Suicide risk is significantly elevated, especially around time of diagnosis
- No disease-modifying treatment exists; median survival from onset is 15-20 years; death usually from aspiration pneumonia
Etiology and risk factors
HD is caused by an abnormal expansion of CAG trinucleotide repeats in exon 1 of the HTT gene. Normal individuals have 10-35 CAG repeats, while affected individuals have 36 or more repeats. The relationship between repeat length and disease is:
- 36-39 repeats: Reduced penetrance, may or may not develop disease
- 40 or more repeats: Full penetrance, will develop disease if they live long enough
- 60 or more repeats: Typically associated with juvenile-onset HD
The expanded CAG sequence encodes an abnormally long polyglutamine tract in the huntingtin protein, resulting in protein misfolding and aggregation. The length of the CAG repeat correlates inversely with age of onset—longer repeats lead to earlier symptom onset.
Key risk factors:
- Having an affected parent (50% risk of inheritance)
- Paternal transmission carries higher risk of repeat expansion and earlier onset (anticipation)
- Longer CAG repeat length predicts earlier onset and more severe disease
HD is not caused by environmental factors, trauma, or lifestyle choices—it is purely genetic.
Pathophysiology
The mutant huntingtin protein (mHTT) containing an expanded polyglutamine tract undergoes abnormal folding and forms intracellular aggregates. These aggregates are toxic to neurons, particularly in the striatum (caudate nucleus and putamen), leading to progressive neuronal loss.
Mechanisms of neurodegeneration
The pathogenic mechanisms include:
- Transcriptional dysregulation: mHTT interferes with transcription factors, particularly affecting BDNF (brain-derived neurotrophic factor) expression
- Mitochondrial dysfunction: Impaired energy metabolism and increased oxidative stress
- Excitotoxicity: Enhanced sensitivity to glutamate-mediated excitotoxicity
- Impaired protein degradation: Dysfunction of the ubiquitin-proteasome system and autophagy
- Disrupted axonal transport: Interference with intracellular trafficking
Neuropathologic features
The characteristic pathologic changes include:
- Striatal atrophy: Particularly affecting the caudate nucleus and putamen, with relative sparing of the globus pallidus initially
- Cortical atrophy: Especially frontal and parietal cortex in advanced disease
- Neuronal loss: Preferential loss of GABAergic medium spiny neurons in the striatum
- Ventricular enlargement: Secondary to caudate atrophy
The striatal pathology disrupts the indirect pathway of the basal ganglia, leading to disinhibition of thalamocortical circuits and resulting in the characteristic hyperkinetic movements (chorea). As the disease progresses and the direct pathway becomes involved, bradykinesia and rigidity may emerge, particularly in juvenile-onset cases.
Clinical features
HD presents with a progressive triad of motor, cognitive, and psychiatric symptoms. The classic presentation varies by age of onset.
Motor symptoms
Chorea is the hallmark motor feature—involuntary, irregular, rapid, flowing movements affecting the face, trunk, and limbs. Early chorea may be subtle, appearing as fidgetiness or restlessness. As the disease progresses:
- Movements become more pronounced and disabling
- Dysarthria and dysphagia develop
- Gait becomes unsteady with risk of falls
- In late stages, chorea may decrease as bradykinesia and rigidity emerge
Other motor features include:
- Impaired voluntary movement initiation and coordination
- Abnormal eye movements (saccadic pursuit, slow saccades)
- Dystonia (particularly in juvenile-onset HD)
- Motor impersistence (inability to maintain tongue protrusion or grip)
Cognitive symptoms
Cognitive decline is progressive and eventually affects all patients:
- Executive dysfunction: Impaired planning, organization, multitasking (earliest feature)
- Psychomotor slowing: Bradyphrenia
- Memory impairment: Particularly retrieval deficits; recognition often preserved
- Impaired attention and concentration
- Dementia: Subcortical pattern, develops in advanced stages
Psychiatric symptoms
Psychiatric manifestations often precede motor symptoms by years:
- Depression: Most common, present in 40-50% of patients
- Irritability and aggression: Frequent behavioral changes
- Apathy: Loss of motivation and initiative
- Anxiety disorders: Including obsessive-compulsive features
- Psychosis: Delusions and hallucinations in 10-25%
- Suicidal ideation: Significantly elevated suicide risk, particularly around diagnosis
Juvenile-onset Huntington's disease
When onset occurs before age 20 (Westphal variant):
- Rigidity and bradykinesia predominate over chorea
- Seizures occur in 30-50%
- Rapid progression with more severe cognitive decline
- Cerebellar signs may be prominent
Diagnosis
Diagnosis is based on clinical features combined with genetic testing and neuroimaging.
Clinical diagnosis
A clinical diagnosis requires:
- Progressive motor disorder (typically chorea) with characteristic distribution
- Family history consistent with autosomal dominant inheritance
- Cognitive and/or psychiatric symptoms
- Exclusion of other causes
Genetic testing
Genetic analysis is confirmatory: Detection of ≥36 CAG repeats in the HTT gene establishes the diagnosis with certainty. Testing is indicated when:
- Clinical features are suggestive and family history is positive
- An at-risk individual requests predictive testing (requires careful genetic counseling)
Genetic counseling is mandatory before and after testing, addressing implications for family members, reproductive decisions, and psychosocial impact.
Neuroimaging
MRI brain typically shows:
- Atrophy of the caudate nucleus (most specific finding)
- Enlarged frontal horns of lateral ventricles ("boxcar ventricles")
- Increased bicaudate ratio (intercaudate distance / brain width)
- Putaminal atrophy
- Cortical atrophy in advanced disease
FDG-PET or SPECT may show reduced glucose metabolism in the caudate and putamen, even before structural changes are evident.
Additional testing
No specific laboratory tests are diagnostic, but workup may include:
- Exclusion of Wilson's disease (ceruloplasmin, 24-hour urinary copper) in younger patients
- Thyroid function tests
- Neuropsychological testing to document cognitive deficits
- Psychiatric evaluation
Predictive testing
Asymptomatic at-risk individuals may request predictive genetic testing. This requires:
- Extensive pre-test counseling
- Psychological evaluation
- Discussion of implications (insurance, employment, family planning)
- Post-test support regardless of result
Differential diagnosis
The differential diagnosis of HD includes other causes of chorea and neurodegenerative disorders:
Inherited choreas
- Benign hereditary chorea: Childhood onset, non-progressive, no cognitive decline
- Huntington's disease-like syndromes (HDL1-4): Phenotypically similar but different genetic mutations
- Neuroacanthocytosis syndromes: Chorea with acanthocytes on blood smear, elevated CK
- Wilson's disease: Younger onset, Kayser-Fleischer rings, abnormal copper studies
- Spinocerebellar ataxias: Ataxia more prominent than chorea
Acquired choreas
- Sydenham's chorea: Post-streptococcal, self-limited, childhood/adolescence
- Drug-induced chorea: Neuroleptics, oral contraceptives, anticonvulsants, stimulants
- Chorea gravidarum: During pregnancy, resolves post-partum
- Vascular chorea: Stroke involving basal ganglia, acute onset
- Systemic lupus erythematosus: With antiphospholipid antibodies
- Hyperthyroidism: Thyrotoxicosis with choreiform movements
- Polycythemia vera: Rarely presents with chorea
Other neurodegenerative disorders
- Alzheimer's disease: Memory impairment predominates, no chorea
- Frontotemporal dementia: Behavioral/language changes, no chorea
- Parkinson's disease: Bradykinesia, rigidity, tremor; opposite motor phenotype
Key distinguishing features of HD are the combination of autosomal dominant inheritance, adult onset of progressive chorea, cognitive decline, psychiatric symptoms, and caudate atrophy on imaging. Genetic testing provides definitive diagnosis.
Management
There is no cure for HD and no treatment that alters disease progression. Management focuses on symptomatic relief, supportive care, and addressing complications. A multidisciplinary approach is essential.
Pharmacologic management
Motor symptoms
For chorea:
- Tetrabenazine or deutetrabenazine: VMAT2 inhibitors, first-line agents specifically approved for HD chorea
- Antipsychotics: Second-generation agents (olanzapine, risperidone) or haloperidol if VMAT2 inhibitors inadequate
- Amantadine: May provide modest benefit
Treatment is initiated when chorea becomes functionally impairing. In late-stage disease when rigidity predominates, these agents should be reduced or discontinued.
For dystonia or rigidity:
- Anticholinergics (trihexyphenidyl) may help dystonia
- Dopaminergic agents are generally avoided due to risk of worsening chorea or psychosis
Psychiatric symptoms
Depression:
- SSRIs (sertraline, citalopram) are first-line
- Mirtazapine if insomnia is prominent
- Close monitoring for suicidal ideation is essential
Irritability and aggression:
- SSRIs as initial therapy
- Atypical antipsychotics (quetiapine, olanzapine) for severe cases
- Mood stabilizers (valproate) occasionally used
Psychosis:
- Atypical antipsychotics (quetiapine, risperidone, olanzapine)
- Avoid typical antipsychotics when possible due to motor side effects
Apathy:
- May respond to SSRIs, bupropion, or psychostimulants
- Often challenging to treat
Cognitive symptoms
No pharmacologic agents have proven efficacy for cognitive decline in HD. Cholinesterase inhibitors and memantine have shown no consistent benefit.
Non-pharmacologic management
Physical therapy:
- Gait training and balance exercises to reduce fall risk
- Stretching and range-of-motion exercises
- Assistive devices as needed
Occupational therapy:
- Adaptive strategies for activities of daily living
- Home safety modifications
- Assistive devices for eating and self-care
Speech and language therapy:
- Dysarthria management
- Swallowing evaluation and dysphagia therapy
- Communication aids in advanced stages
Nutritional support:
- High-calorie diet to prevent weight loss (hypermetabolic state common)
- Modified diet consistency for dysphagia
- PEG tube placement in advanced disease if appropriate
Psychosocial support:
- Genetic counseling for patients and families
- Support groups
- Advance care planning
- Caregiver education and respite care
Emerging therapies
Multiple disease-modifying strategies are under investigation:
- Antisense oligonucleotides targeting HTT mRNA (huntingtin-lowering therapies)
- Gene therapy approaches
- Cell-based therapies
These remain experimental and are not yet part of standard clinical care.
Complications and prognosis
HD is a progressive and ultimately fatal disease. Complications arise from motor, cognitive, and psychiatric deterioration.
Complications
Motor complications:
- Falls and fractures due to chorea and gait instability
- Dysphagia leading to aspiration pneumonia (leading cause of death)
- Weight loss and malnutrition despite adequate intake
- Speech impairment and loss of communication
- Complete loss of mobility in end-stage disease
Cognitive and psychiatric complications:
- Progressive dementia requiring total care
- Depression with high suicide risk (rates 4-6 times higher than general population)
- Behavioral disturbances causing social isolation
- Loss of insight and decision-making capacity
General complications:
- Pneumonia (most common cause of death)
- Cardiovascular disease
- Urinary tract infections
- Pressure ulcers in bedridden patients
- Choking
Prognosis
HD follows a relentlessly progressive course. Key prognostic factors:
Survival:
- Median survival from symptom onset is 15-20 years
- CAG repeat length inversely correlates with age of onset but only weakly predicts survival duration once symptomatic
- Juvenile-onset HD has more rapid progression (death within 10-15 years)
Functional decline:
- Early stage (years 1-5): Subtle motor signs, mild functional impairment, able to work
- Middle stage (years 5-15): Obvious chorea, cognitive decline, loss of independence, unable to work or drive
- Advanced stage (years 15-20): Severe motor impairment, dementia, total care required, bedridden
Causes of death:
- Pneumonia (most common, often aspiration)
- Cardiovascular disease
- Suicide (particularly early in disease course)
- Complications of immobility
Quality of life
Quality of life progressively declines due to loss of independence, inability to work, social isolation, and caregiver burden. Early psychiatric symptoms and loss of insight may cause significant distress. Comprehensive supportive care, advance care planning, and family support are crucial to optimizing quality of life throughout the disease course.
End-of-life care focuses on comfort, dignity, and management of dysphagia, respiratory secretions, and pain. Palliative care consultation is appropriate in advanced stages.
CAG repeats for Huntington's penetrance: <36 normal, 36-39 reduced penetrance, ≥40 full penetrance, ≥60 juvenile-onset. CHOREA for features: Caudate atrophy, Hereditary (autosomal dominant), Onset age 35-45, Rigidity in juvenile form, Executive dysfunction, Affective/psychiatric symptoms
Practice questions
1. A 42-year-old man presents with 6 months of involuntary jerky movements of his arms and trunk. His wife reports he has become increasingly irritable and has difficulty organizing tasks at work. His father died in a nursing home at age 55 with similar symptoms. Examination shows irregular, flowing involuntary movements of the face and limbs. MRI brain shows atrophy of the caudate nuclei with enlarged frontal horns of the lateral ventricles. Which of the following is the most likely number of CAG repeats in this patient?
2. A 17-year-old girl is brought to the clinic by her parents for progressive stiffness and school failure over the past year. Her father was diagnosed with Huntington's disease at age 40. On examination, she has bradykinesia, rigidity, and mild dystonia, but no chorea. She has had two generalized tonic-clonic seizures in the past 3 months. MRI shows caudate atrophy. Which of the following additional features is most likely in this patient compared to typical adult-onset Huntington's disease?
3. A 38-year-old woman with confirmed Huntington's disease (CAG repeat 44) presents for management of worsening involuntary movements that interfere with eating and dressing. She has mild depression but is adherent to sertraline. Examination shows prominent chorea of the trunk, face, and all limbs, with motor impersistence. Which of the following is the most appropriate initial pharmacologic treatment for her motor symptoms?
Frequently asked questions
What causes Huntington's disease?
Huntington's disease is caused by an abnormal expansion of CAG trinucleotide repeats in the HTT gene on chromosome 4. Normal individuals have 10-35 CAG repeats, while affected individuals have 36 or more repeats. The expanded CAG sequence produces a mutant huntingtin protein with an abnormally long polyglutamine tract that misfolds and forms toxic aggregates in neurons, particularly in the striatum. HD is inherited in an autosomal dominant pattern, meaning a child of an affected parent has a 50% chance of inheriting the mutation. It is not caused by environmental factors or lifestyle.
What are the first symptoms of Huntington's disease?
The earliest symptoms are often psychiatric or subtle cognitive changes, which may precede motor symptoms by years. Depression, irritability, apathy, and personality changes are common initial features. Cognitively, executive dysfunction—difficulty with planning, organization, and multitasking—appears early. Motor symptoms typically begin with subtle chorea appearing as fidgetiness or restlessness that patients or families may not initially recognize as abnormal. Some patients first notice difficulty with fine motor tasks, slowed movements, or clumsiness. The specific initial presentation varies, but the combination of behavioral changes with subtle movement abnormalities in someone with family history should raise suspicion.
How is Huntington's disease diagnosed?
Diagnosis is confirmed by genetic testing that detects 36 or more CAG repeats in the HTT gene. Clinical diagnosis is based on the characteristic triad of progressive motor symptoms (especially chorea), cognitive decline, psychiatric symptoms, and a family history consistent with autosomal dominant inheritance. MRI brain typically shows caudate nucleus atrophy with enlarged frontal horns of the lateral ventricles. Genetic testing provides definitive diagnosis and should be accompanied by thorough genetic counseling, especially for predictive testing in asymptomatic at-risk individuals. Other causes of chorea should be excluded, particularly in younger patients.
Is Huntington's disease curable?
No, there is currently no cure for Huntington's disease and no treatment that slows or stops disease progression. Management is entirely symptomatic and supportive. Medications like tetrabenazine can reduce chorea severity, and psychiatric symptoms can be treated with antidepressants or antipsychotics. Physical, occupational, and speech therapy help maintain function and quality of life. Research into disease-modifying therapies, including huntingtin-lowering approaches using antisense oligonucleotides, is ongoing but remains experimental. The disease is progressive and ultimately fatal, with median survival of 15-20 years from symptom onset.
Huntington's disease vs Parkinson's disease — what is the difference?
These are distinct neurodegenerative disorders with opposite motor features. Huntington's disease causes hyperkinetic movements (chorea—excessive, involuntary movements) due to striatal degeneration affecting the indirect basal ganglia pathway, while Parkinson's disease causes hypokinetic features (bradykinesia, rigidity, tremor) from substantia nigra degeneration affecting the direct pathway. HD is autosomal dominant and typically presents in midlife with chorea, cognitive decline, and psychiatric symptoms; caudate atrophy is the key imaging finding. Parkinson's is usually sporadic, presents later with resting tremor, bradykinesia, and rigidity; imaging is typically normal. HD has a strong genetic basis, while most Parkinson's cases are not clearly hereditary.
Key drugs
Related conditions and differentials
Sources and further reading
StatPearls (NCBI Bookshelf)MedlinePlusWHO ICD-10: G10
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.