Huntington Disease

• Mutant Huntingtin Protein Formation and Aggregation: The expanded CAG repeat translates into a polyglutamine tract (polyQ) within the N-terminal region of the huntingtin protein, creating an abnormal protein with approximately 36-120+ glutamines (normal: 6-35). This polyQ expansion causes the protein to adopt an abnormal conformation, leading to misfolding and formation of insoluble aggregates. These aggregates accumulate progressively in neuronal nuclei and cytoplasm, initiating a cascade of cellular dysfunction. The aggregates recruit other proteins and interfere with normal cellular processes. Repeat length shows an inverse correlation with age of onset (anticipation phenomenon)—longer repeats (>60) cause juvenile-onset disease with rapid progression, while shorter pathogenic repeats (36-40) cause later onset with slower decline. The proteolytic cleavage of mutant huntingtin by caspases and other proteases generates N-terminal fragments that are more prone to aggregation and more neurotoxic than full-length protein.

• Transcriptional Dysregulation and Chromatin Remodeling: Mutant huntingtin protein interacts abnormally with transcription factors and co-regulators, including CREB-binding protein (CBP), which possesses histone acetyltransferase activity. This interaction impairs CBP function and reduces histone acetylation, leading to altered chromatin condensation and aberrant gene expression patterns. Transcriptional changes include downregulation of genes critical for neuronal survival (such as brain-derived neurotrophic factor, BDNF) and upregulation of pro-apoptotic genes. The selective vulnerability of medium spiny neurons (MSNs) in the striatum may relate to their dependence on BDNF-mediated signaling via the CREB pathway. Mutant huntingtin also disrupts normal RNA processing, including alternative splicing, further compromising neuronal function.

• Mitochondrial Dysfunction and Bioenergetic Failure: Mutant huntingtin localizes to mitochondria and causes progressive impairment of oxidative phosphorylation and ATP production. The protein interferes with calcium handling by mitochondria, leading to excessive calcium influx and activation of calpains and caspases. Impaired mitochondrial dynamics (fission-fusion balance) and defective mitophagy result in accumulation of dysfunctional mitochondria. This bioenergetic catastrophe is particularly devastating for medium spiny neurons, which have high energy demands due to their extensive dendritic arbors and sustained synaptic activity. The resulting energy depletion drives excitotoxicity and neuronal death.

• Excitotoxicity and Glutamate Dysfunction: Medium spiny neurons are particularly vulnerable to excitotoxic injury, likely due to their heavy innervation by corticostriatal glutamatergic neurons and their reliance on NMDA receptor-mediated signaling for trophic support. Mutant huntingtin impairs glutamate reuptake mechanisms and alters NMDA/AMPA receptor trafficking, leading to dysregulation of calcium homeostasis. Enhanced extrasynaptic NMDA receptor signaling promotes pro-death pathways. Additionally, mutant huntingtin affects the balance of direct (D1-containing) and indirect (D2-containing) medium spiny neuron populations, with preferential early degeneration of indirect pathway neurons. This circuit imbalance explains the hyperkinetic movement disorder seen in manifest disease.

• Protein Quality Control Failure: The proteasome system and autophagy-lysosomal pathway become overwhelmed by the burden of misfolded huntingtin. Sequestration of chaperone proteins (Hsp70, Hsp90) by mutant huntingtin aggregates diverts their function away from normal cellular proteins. Impaired protein clearance mechanisms allow progressive accumulation of aggregates. Neuroinflammation, including microglial activation and astrocytic changes, develops secondarily and contributes to further neuronal injury through release of pro-inflammatory cytokines and excitotoxins.

• CAG Trinucleotide Repeat Expansion in HTT Gene (Primary Cause): HD is caused exclusively by abnormal expansion of CAG repeats in exon 1 of the huntingtin gene on chromosome 4p16.3. Normal individuals carry 6-35 CAG repeats; 36-39 repeats confer reduced penetrance with variable expressivity; ≥40 repeats cause fully penetrant disease. The expansion arises through dynamic mutation mechanisms, with paternal transmission showing greater propensity for larger expansions (particularly when transmitted from an affected father in the reproductive years), explaining the phenomenon of anticipation in paternal lineages. No environmental triggers have been definitively established for disease manifestation, though stress, trauma, and infection may modulate symptom severity and progression rate. The mutation arose independently in multiple populations but is most common in those of European ancestry.

• Family History and Inheritance Pattern (Required Risk Factor): As an autosomal dominant disorder with complete penetrance (when repeat length ≥40), every child of an affected parent has a 50% risk of inheriting the expanded allele. Genetic anticipation is a hallmark feature, particularly with paternal transmission, meaning descendants may develop earlier-onset, more severe disease than the parent. De novo mutations, while rare in HD, can occur through expansion during spermatogenesis. Compound heterozygotes with two mutant alleles are extraordinarily rare but may exhibit more severe phenotypes.

• CAG Repeat Length (Primary Modifier of Disease Biology): Repeat length is the strongest predictor of age at onset and rate of progression. CAG repeats >60 predict juvenile-onset disease (before age 20) characterized by rigidity, dystonia, and rapid cognitive decline rather than classical chorea. Repeats of 40-50 typically cause onset in the 4th-5th decade with classic phenotype. Repeats <40 (but ≥36) show variable or late-onset disease. This correlation is robust and has been formalized in mathematical models predicting age at onset from repeat length.

• Chorea (Hyperkinetic Movement Disorder): Involuntary, rapid, irregular, unpredictable movements representing the hallmark motor sign of manifest HD. Chorea typically begins distally in the hands and feet before progressing proximally. Early chorea may be subtle—manifesting as restlessness, fidgetiness, or difficulty maintaining posture—and may not be recognized as pathological. The movements worsen with stress and emotional arousal and diminish with relaxation and sleep, distinguishing them from other hyperkinetic movement disorders. Pathophysiologically, chorea reflects selective degeneration of indirect pathway medium spiny neurons (expressing D2 dopamine receptors and enkephalin), leading to unopposed direct pathway activity and excessive movement initiation. As disease progresses and more neurons degenerate, motor features evolve toward rigidity and bradykinesia in later stages.

• Cognitive Decline: Early cognitive dysfunction typically manifests as executive dysfunction (impaired planning, organization, mental flexibility, working memory) before generalized dementia develops. Frontal-subcortical circuits are preferentially affected early, explaining the prominent dysexecutive features. Processing speed slows markedly. Memory loss emerges later and is often less prominent than frontal cognitive changes. By late stages, dementia is profound with global cognitive deterioration. The cognitive decline progresses relatively predictably and correlates with disease duration rather than age, distinguishing it from age-related cognitive decline.

• Psychiatric and Behavioral Manifestations: Psychiatric symptoms frequently precede motor manifestations by years, making them clinically important for diagnosis in presymptomatic individuals. Depression occurs in 40-50% of patients and may manifest as vegetative symptoms, anhedonia, or suicidality; it is highly treatment-responsive. Irritability and aggression are common, sometimes presenting as dangerous behavior. Obsessive-compulsive symptoms, including intrusive thoughts and repetitive behaviors, affect 10-15% of patients. Personality changes, apathy, and loss of initiative are prominent. Psychosis and hallucinations occur in up to 10% and may resemble primary psychiatric disorders, creating diagnostic confusion. These psychiatric symptoms likely reflect dorsolateral and ventromedial prefrontal cortex involvement and alterations in reward circuitry.

• Juvenile-Onset Huntington Disease (Westphal Variant): This aggressive form occurs when CAG repeats exceed 60 and typically manifests before age 20. Rather than classical chorea, juvenile-onset HD presents with rigidity, dystonia, bradykinesia, and prominent seizures (20-30% of cases). Cognitive decline is rapid and severe. Behavioral problems are often the first manifestation, presenting as school failure and conduct disturbances. Disease progression is rapid with shorter survival (10-15 years from onset). Paternal inheritance is common, reflecting the propensity for repeat expansion through paternal transmission.

• Late-Stage Manifestations: As disease progresses (typically after 10-15 years), motor features transition from chorea to rigidity, dystonia, bradykinesia, and postural instability. Speech becomes dysarthric and eventually incomprehensible. Swallowing dysfunction (dysphagia) develops, increasing aspiration risk. Patients become wheelchair-dependent and eventually bedbound. Contractures and muscle wasting develop. Sleep disturbances (insomnia, sleep fragmentation, REM behavior disorder) are prominent throughout the disease course.

• Physical Examination Findings: Early examination may reveal only subtle findings—mild chorea, slowed saccadic eye movements (impaired saccadic velocity and increased latency), or subtle cognitive slowing on bedside testing (months of year backward, serial 7 subtractions). Progressive disease shows obvious chorea affecting all limbs, trunk, and face, with inability to maintain tongue protrusion ("tongue quiver" sign) or maintain sustained hand grip. Increased tone may be present. Reflexes may be brisk. Gait becomes irregular and unsteady. Cognitive examination reveals executive dysfunction disproportionate to memory impairment early. Speech shows dysarthria and reduced fluency.

• Presymptomatic Stage: The period before clinical manifestation (which can last decades) is increasingly recognized as critical. Functional imaging shows subtle abnormalities in premanifest carriers, and recent biomarker studies (discussed in Diagnosis section) reveal evidence of neurodegeneration even 15+ years before symptom onset. Some premanifest carriers develop mood symptoms or subtle cognitive changes years before meeting diagnostic criteria for manifest disease.

• Clinical Diagnostic Criteria (Manifest Disease): The diagnostic criteria require the presence of involuntary chorea (the defining motor sign) combined with family history of HD in a first-degree relative, OR progressive chorea in the context of cognitive decline and/or behavioral changes, with age of onset typically between 30-50 years. The Unified Huntington's Disease Rating Scale (UHDRS) is the standard clinical assessment tool used in research and clinical practice, quantifying motor signs (including chorea score, dystonia, bradykinesia, rigidity, oculomotor findings, and postural stability), cognitive function (brief cognitive assessment with verbal fluency, symbol digit modalities test, and Stroop test), behavioral symptoms (using the Problem Behaviors Assessment), and functional capacity. A diagnostic confidence score (often used clinically) incorporates temporal profile, family history, and objective findings.

• Genetic Testing (Definitive Diagnosis): CAG repeat expansion analysis is the definitive diagnostic test, performed via PCR-based analysis of HTT gene repeats. The interpretation is straightforward: ≥40 CAG repeats confirm HD in the correct clinical context; 36-39 repeats indicate reduced penetrance with variable expressivity; <36 repeats exclude HD as a cause. Genetic testing should be preceded by genetic counseling in presymptomatic individuals and accompanied by counseling regarding the psychological implications of a positive result. Paternity should be established before testing family members. Repeat length should be assessed in both alleles, as rare individuals with two expanded alleles exist. Testing accuracy is >99% when performed by certified laboratories using validated methods.

• Imaging Findings: MRI of the brain is not required for diagnosis but shows characteristic findings that support the diagnosis. The hallmark finding is caudate nucleus atrophy—the caudate becomes shrunken with loss of its normal bulge into the lateral ventricle, and the lateral ventricles dilate proportionally. Striatal atrophy is evident on axial views. Cortical atrophy, particularly frontal, develops progressively. These findings correlate with disease burden. Functional neuroimaging (PET, fMRI) shows reduced striatal glucose metabolism and altered patterns of brain activation during cognitive and motor tasks, but these are research tools rather than clinical diagnostic tests. Structural MRI can help exclude other causes of chorea (e.g., structural lesions, vascular disease) in diagnostic uncertainty.

• Laboratory Studies: Standard laboratory studies are normal in HD and serve primarily to exclude other causes of chorea. CBC, metabolic panel, and thyroid function are normal. Serum ceruloplasmin and 24-hour urine copper are normal (exclude Wilson disease). Antinuclear antibodies and other autoimmune markers are negative (exclude autoimmune chorea). CSF analysis is normal.

• Biomarkers and Presymptomatic Detection: Plasma phosphorylated tau (p-tau181) and neurofilament light chain (NfL) have emerged as potential biomarkers of neurodegeneration in HD, with elevated levels correlating with mutant huntingtin burden and showing changes years before symptom onset. These blood-based biomarkers may eventually aid in monitoring disease progression and response to disease-modifying therapies but are not yet part of standard diagnostic criteria. CSF huntingtin levels are also being investigated as biomarkers. These biomarkers are likely to become increasingly important for early detection and therapeutic monitoring.

• Differential Diagnosis Considerations: The combination of chorea with cognitive decline and family history is highly specific for HD, but other diagnoses merit consideration: Wilson disease (autosomal recessive, young age, low ceruloplasmin, Kayser-Fleischer rings), neuroacanthocytosis syndromes (circulating acanthocytes, abnormal RBC morphology), systemic lupus erythematosus and other vasculitides (antinuclear antibodies, systemic features), tardive dyskinesia (history of antipsychotic exposure, less progressive), Sydenham chorea (acute presentation, recent streptococcal infection, murmur), hyperthyroidism (elevated TSH, hyperreflexia without dementia), and cerebral infarction or structural lesions (imaging reveals structural cause). The presence of family history, gradual insidious onset, progressive dementia, and psychiatric features make these alternatives unlikely.

• Disease-Modifying Therapy (First-Line for Manifest Disease): Tetrabenazine, a vesicular monoamine transporter 2 (VMAT2) inhibitor, was the first FDA-approved agent specifically for chorea in HD. Tetrabenazine depletes presynaptic monoamines (dopamine, serotonin) within nerve terminals, effectively reducing involuntary movements. The typical starting dose is 12.5 mg daily, titrated weekly by 12.5 mg increments to usual therapeutic doses of 37.5-75 mg/day in divided doses (usual range 25-100 mg/day). Its efficacy is significant—reducing chorea in 80% of patients and improving functional disability. Limitations include depression/suicidality risk (requiring careful psychiatric monitoring), akathisia, parkinsonism (bradykinesia and rigidity), orthostatic hypotension, and difficulty with dosing (requires multiple daily doses). Tetrabenazine should not

Disease-related complications

• Aspiration pneumonia: bulbar dysfunction and chorea of the pharyngeal/respiratory musculature produce incoordinated swallowing; the signaling findings are coughing with thin liquids, wet voice, and unexplained weight loss. This is the leading cause of death in HD and an emergency when it presents with fever, hypoxemia, and a new infiltrate. Formal swallow evaluation and diet modification are the standard preventive steps.
• Malnutrition and cachexia: constant involuntary movement raises resting energy expenditure while dysphagia and apathy reduce intake; progressive weight loss despite adequate-appearing meals is the tip-off and correlates with faster functional decline.
• Suicide and completed self-harm: frontostriatal disinhibition plus reactive depression makes suicide a major cause of death, with peak risk at the time of predictive testing and at loss of independence. Expressed hopelessness or a new plan is a psychiatric emergency requiring immediate safety assessment.
• Falls, fractures, and subdural hematoma: postural instability plus truncal chorea; any decline in mental status after a fall in an anticoagulated or atrophic-brain patient warrants urgent head CT.
• Seizures: largely confined to juvenile-onset (Westphal) disease; status epilepticus is an emergency.
• Immobility sequelae: contractures, pressure ulcers, and venous thromboembolism in the bedbound late stage.

Treatment-related complications

• VMAT2 inhibitor-induced depression and suicidality: tetrabenazine carries an FDA boxed warning; monoamine depletion unmasks or worsens depression. New anhedonia or suicidal ideation mandates dose reduction or discontinuation. The AAN's evidence-based guideline on pharmacologic treatment of chorea in HD supports tetrabenazine while emphasizing this psychiatric monitoring.
• Drug-induced parkinsonism and akathisia: dopamine depletion or D2 blockade converts a hyperkinetic patient into a rigid, bradykinetic, dysphagic one — a common cause of iatrogenic aspiration.
• Neuroleptic malignant syndrome: fever, rigidity, autonomic instability, and elevated CK after antipsychotic or VMAT2 inhibitor escalation — an emergency.
• QT prolongation with tetrabenazine and with antipsychotics used for chorea and irritability; obtain an ECG when combining agents.
• CYP2D6 poor metabolism: higher tetrabenazine doses require genotype-guided limits per labeling.

• The stem triad: middle-aged adult with chorea, executive-predominant dementia, and psychiatric change (depression, irritability, apathy) plus a parent who "died in a psychiatric hospital" or had a "movement disorder." Autosomal dominant, chromosome 4p16.3, CAG repeat.
• Single best next step: targeted CAG repeat length testing of HTT by PCR — it is definitive. In an asymptomatic at-risk adult, the best next step is pretest genetic counseling first; predictive testing is not performed on asymptomatic minors. Imaging is supportive, never diagnostic.
• Classic imaging buzzword: caudate atrophy with loss of the normal caudate convexity, giving "boxcar" dilation of the frontal horns of the lateral ventricles.
• The association examiners test: anticipation — repeat expansion is greatest in paternal transmission (spermatogenic instability), so juvenile-onset disease with rigidity, dystonia, bradykinesia, and seizures (Westphal variant) is inherited from the father.
• Mechanism trap: HD is a toxic gain of function from an expanded polyglutamine tract, not a loss-of-function/haploinsufficiency disease, and the repeat is in a coding exon (contrast with the noncoding expansions of fragile X, Friedreich ataxia, and myotonic dystrophy).
• Circuit trap: chorea comes from early loss of indirect-pathway (D2) medium spiny neurons, releasing the thalamus from inhibition. Do not attribute chorea to direct-pathway loss.
• Drug pearl: VMAT2 inhibitors (representative agent tetrabenazine) deplete presynaptic dopamine and are the AAN-supported agents for disabling chorea; the boxed warning for depression and suicidality is the tested adverse effect, and active untreated depression or suicidality is a contraindication.
• Common distractors: Sydenham chorea (child, post-streptococcal, self-limited), Wilson disease (young patient — check ceruloplasmin and slit-lamp for Kayser–Fleischer rings), tardive dyskinesia (antipsychotic exposure, orobuccolingual, nonprogressive), and neuroacanthocytosis (acanthocytes, prominent orolingual self-mutilation).