Huntington Disease

Detailed Overview

HD is caused by a CAG trinucleotide repeat expansion in the HTT gene on chromosome 4, resulting in a mutant huntingtin protein with a polyglutamine tract. This toxic protein primarily targets the medium spiny neurons of the striatum (caudate and putamen), leading to profound brain atrophy. The disease demonstrates genetic anticipation, where paternal transmission often results in larger repeat numbers and earlier onset in subsequent generations. Symptoms typically begin in mid-life (30-50 years). HD follows an inexorable, progressive course over 15 to 20 years, stripping patients of their motor control, intellect, and personality, ultimately ending in death, usually from aspiration pneumonia. There is currently no disease-modifying treatment.

Epidemiology & Demographics

Prevalence: 2.7 to 5.7 per 100,000 in populations of European descent (lower in Asian/African populations). Age distribution: Onset typically between 30 and 50 years. Juvenile HD (onset <20 years) occurs with very high CAG repeats (>60). Gender ratio: Equal (1:1).

Etiological Mechanism

Autosomal dominant mutation causing a CAG trinucleotide repeat expansion (>39 repeats is fully penetrant) in the Huntingtin (HTT) gene.

Primary Causes

Primary: Mutant huntingtin (mHTT) protein aggregation causing striatal neuronal death

• Genetics: Having a parent with HD confers a 50% risk of inheriting the expanded gene.
• Paternal Transmission: Sperm exhibit instability in CAG repeats, leading to expansion and earlier disease onset in offspring (Anticipation).

The normal huntingtin protein is essential for embryonic development and neuronal function. In HD, the expanded CAG repeats translate into an abnormally long chain of glutamine (polyQ) in the HTT protein. This mutant HTT protein misfolds, escapes normal cellular degradation via the ubiquitin-proteasome system, and forms toxic intracellular inclusions within the nucleus and cytoplasm. The primary target of this toxicity is the GABAergic medium spiny neurons (MSNs) in the striatum (the caudate nucleus and putamen). Loss of these inhibitory MSNs disrupts the basal ganglia's indirect pathway, resulting in excessive excitatory output to the cortex, clinically manifesting as hyperkinetic chorea. As the disease progresses, widespread cortical atrophy occurs, leading to dementia. The exact mechanism of mHTT toxicity includes transcriptional dysregulation, mitochondrial dysfunction, and impaired axonal transport.

Characteristic Clinical Presentation

• Chorea: Rapid, involuntary, non-repetitive, dance-like movements affecting the face, trunk, and limbs.
• Psychiatric Changes: Depression, apathy, irritability, severe anxiety, and high risk of suicidal ideation; often precede motor symptoms.
• Cognitive Decline: Executive dysfunction, poor judgment, and slowed processing speed, progressing to global dementia.
• Dysphagia and Dysarthria: Difficulty swallowing and slurred speech, increasing the risk of aspiration and malnutrition.

Physical Examination Signs

• Motor impersistence (Milkmaid's grip during handshakes, inability to keep tongue protruded - 'trombone tongue')
• Choreiform movements at rest
• Abnormal eye movements (delayed initiation of voluntary saccades)
• Cachexia in later stages despite adequate caloric intake

• Aspiration Pneumonia: The leading cause of death due to severe dysphagia and loss of gag reflex.
• Suicide: Patients with HD have a suicide rate up to 10 times higher than the general population, particularly around the time of diagnosis or loss of independence.
• Severe Malnutrition: Due to constant hyperkinetic movement burning calories, combined with inability to safely swallow food.

Diagnostic Criteria & Guidelines

Clinical diagnosis requires the onset of an otherwise unexplained extrapyramidal movement disorder (typically chorea) in a patient with a family history of HD. Definitive diagnosis requires molecular genetic testing showing >35 CAG repeats in the HTT gene (fully penetrant if >=40).

Differential Diagnosis

• Sydenham Chorea (post-streptococcal, in children)
• Tardive Dyskinesia (medication-induced)
• Neuroacanthocytosis
• Spinocerebellar Ataxias

Laboratory Tests & Biomarkers

• HTT Gene PCR Analysis: Normal: <26 repeats. Intermediate (not affected but offspring at risk): 27-35. Reduced penetrance: 36-39. Full penetrance: >=40 repeats.

Imaging Modalities & Findings

• Brain MRI or CT:

• Early Stage
  Subtle changes in mood/cognition, minor involuntary movements (fidgeting). Can still work and drive.
• Middle Stage
  Prominent chorea, noticeable cognitive decline, requires assistance with ADLs. Swallowing becomes difficult.
• Late Stage
  Chorea may be replaced by severe rigidity/bradykinesia. Patient is non-verbal, bedbound, and entirely dependent on care.

Second-Line & Adjunctive Therapy

For chorea with concurrent psychosis/aggression: Atypical antipsychotics like Olanzapine 2.5-5 mg PO daily or Risperidone. For depression: SSRIs (e.g., Sertraline 50 mg PO daily).

Surgical & Procedural Management

Gastrostomy tube (PEG) placement in advanced stages to prevent aspiration pneumonia and manage severe malnutrition.

Recommended Lifestyle Changes

• High-calorie diet (up to 5000 kcal/day) to maintain weight due to constant choreiform movements
• Removal of environmental hazards (rugs, sharp furniture) to prevent falls
• Establish advance directives and a durable power of attorney early in the disease course

Patient Counseling & Advice

Deliver the diagnosis with extreme care alongside genetic counseling. Discuss the grim prognosis openly but supportively. Focus heavily on suicide risk screening. Advise at-risk family members that predictive genetic testing is available but requires rigorous pre-test psychological counseling.

Follow-Up & Monitoring Schedule

Multidisciplinary care involves a neurologist, psychiatrist, speech therapist, occupational therapist, and social worker. Frequent screening for depression and suicidal ideation is mandatory.

Preventive Strategies

In vitro fertilization (IVF) with preimplantation genetic diagnosis (PGD) can be used by affected individuals to ensure they do not pass the mutated gene to their children.

Relentlessly progressive and fatal. Average survival is 15 to 20 years after the onset of motor symptoms. Death is most commonly caused by aspiration pneumonia, subsequent sepsis, or suicide.