USMLE Guide: Huntington's Disease

Overview

Huntington's Disease (HD), also known as Huntington's chorea, is a hereditary neurodegenerative disorder characterized by the progressive deterioration of nerve cells in the brain. This article provides a comprehensive overview of the etiology, clinical features, diagnosis, and management of Huntington's Disease, aimed at medical students preparing for the United States Medical Licensing Examination (USMLE) Step 1.

Etiology

• HD is caused by a mutation in the huntingtin (HTT) gene on chromosome 4.
• The mutation involves an expansion of the CAG trinucleotide repeat, resulting in an abnormally long polyglutamine segment in the huntingtin protein.
• The size of the CAG repeat determines the age of onset and severity of the disease.
• The inheritance pattern is autosomal dominant, with each child of an affected parent having a 50% chance of inheriting the mutation.

Clinical Features

1. Motor Symptoms:

   • Involuntary movements (chorea) are the hallmark feature of HD, initially involving the face, neck, and upper extremities.
   • As the disease progresses, chorea becomes more severe and is accompanied by dystonia and parkinsonism.
   • Impaired fine motor skills, gait instability, and difficulties with coordination and balance are common.

2. Cognitive Symptoms:

   • HD leads to progressive cognitive decline, affecting executive functions, memory, and judgment.
   • Psychiatric symptoms, such as depression, anxiety, irritability, and apathy, are also prevalent.

3. Other Features:

   • Weight loss and muscle wasting occur due to increased energy expenditure.
   • Dysphagia and dysarthria may develop, leading to difficulties in swallowing and speaking.
   • Sleep disturbances and seizures can be observed in advanced stages of the disease.

Diagnosis

• Genetic Testing:
  • Molecular genetic testing confirms the presence of the HTT gene mutation.
  • Southern blot analysis, polymerase chain reaction (PCR), or direct DNA sequencing can be used.
• Clinical Examination:
  • Characteristic motor symptoms (chorea) and a positive family history are suggestive of HD.
  • Neuropsychiatric evaluation helps identify cognitive and psychiatric manifestations.
• Imaging Studies:
  • Magnetic resonance imaging (MRI) reveals cortical atrophy, particularly in the caudate nucleus.
  • Positron emission tomography (PET) scans may show reduced glucose metabolism in affected areas.

Management

1. Symptomatic Treatment:

   • Tetrabenazine and other dopamine-depleting agents help reduce chorea and improve motor symptoms.
   • Benzodiazepines and atypical antipsychotics can be used to manage psychiatric symptoms.
   • Physical, occupational, and speech therapies assist in improving functional abilities and quality of life.

2. Supportive Care:

   • Psychosocial support, counseling, and patient education play a crucial role in managing HD.
   • Nutrition and hydration should be optimized to prevent weight loss and malnutrition.
   • Regular follow-up visits are necessary to monitor disease progression and adjust treatment plans.

3. Genetic Counseling:

   • Genetic counseling should be offered to affected individuals and their families to discuss the inheritance pattern, risks, and reproductive options.

Prognosis

• HD is a progressive disease with no cure.
• The age of onset varies, but symptoms usually appear between the ages of 30 and 50 years.
• Overall life expectancy is reduced, with death typically occurring 15-20 years after symptom onset.
• Complications, such as pneumonia, sepsis, and injuries from falls, contribute to mortality.

Conclusion

Huntington's Disease is a devastating neurodegenerative disorder characterized by motor, cognitive, and psychiatric symptoms. This USMLE guide has provided an overview of the etiology, clinical features, diagnosis, and management of HD. Understanding the pathophysiology and clinical manifestations is crucial for medical students preparing for the USMLE, as it helps in recognizing and managing this complex condition effectively.