Huntington Disease
An inherited, fatal neurodegenerative disease causing chorea, dementia, and psychiatric issues, caused by CAG repeat expansions in the HTT gene.
Emergency Management: Acute suicidal ideation requiring inpatient psychiatric admission. Aspiration leading to acute hypoxemic respiratory failure.
Huntington Disease (HD) is a devastating, fully penetrant autosomal dominant neurodegenerative disorder. It is characterized by the progressive triad of chorea (involuntary, jerky movements), cognitive decline leading to dementia, and severe psychiatric disturbances.
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
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.
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).
No cure exists. Management is purely symptomatic. For chorea: Vesicular monoamine transporter 2 (VMAT2) inhibitors like Tetrabenazine 12.5 mg PO daily to start, or Deutetrabenazine 6 mg PO daily. These deplete dopamine to reduce hyperkinesia.
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.
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.