Huntington Chorea

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1- Introduction

2- Anatomical Overview

3- Causes

4- Treatment 

Introduction

Huntington’s disease (HD), also known as Huntington’s chorea, is a genetic neurodegenerative disorder characterized by involuntary movements (chorea), cognitive decline, and psychiatric symptoms.
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Anatomical Overview

Basal Ganglia

  1. Caudate Nucleus and Putamen:
    • Primary Sites of Degeneration: HD prominently affects the striatum, which comprises the caudate nucleus and putamen. These structures play critical roles in motor control, cognitive processing, and emotional regulation.
    • Neuronal Loss: Progressive loss of medium spiny neurons (MSNs), which are inhibitory neurons that utilize gamma-aminobutyric acid (GABA) as a neurotransmitter.
    • Impact on Circuitry: Disruption of cortico-striatal circuits, which normally coordinate movement and behavior.

Cerebral Cortex

  1. Frontal Cortex and Beyond:
    • Widespread Cortical Involvement: As HD advances, cortical areas beyond the striatum are affected, including the frontal cortex.
    • Cognitive Decline: Degeneration in cortical regions leads to impairments in executive functions, memory, and decision-making abilities.
    • Psychiatric Symptoms: Changes in mood, personality, and behavior due to cortical involvement.

Thalamus

  1. Thalamic Nuclei:
    • Connection with Basal Ganglia and Cortex: The thalamus acts as a relay station, integrating sensory and motor signals between the basal ganglia and cortex.
    • Impact on Motor Function: Degeneration in thalamic nuclei contributes to motor symptoms such as chorea (involuntary movements) and dystonia (sustained muscle contractions).

White Matter Tracts

  1. Corpus Callosum and White Matter Pathways:
    • Disruption of Connectivity: HD affects white matter integrity, including the corpus callosum, which connects the left and right cerebral hemispheres.
    • Impaired Communication: Loss of myelinated fibers and disruption of axonal pathways contribute to cognitive decline and overall brain dysfunction.

Ventricular Enlargement

  1. Enlarged Ventricles:
    • Hydrocephalus Ex Vacuo: As neuronal loss progresses, there is relative preservation of cerebrospinal fluid (CSF), leading to dilation of the lateral ventricles.
    • MRI Findings: Imaging studies often show enlarged ventricles, which indicate widespread brain atrophy and loss of tissue volume.

Causes

  • Huntington’s disease (HD) is caused by a genetic mutation that leads to neurodegeneration in certain areas of the brain. Here are the key causes and factors involved in Huntington’s disease:

    Genetic Mutation

    1. HTT Gene Mutation: Huntington’s disease is primarily caused by a mutation in the HTT gene, located on chromosome 4. This gene provides instructions for making a protein called huntingtin. In HD, there is an abnormal expansion of a CAG trinucleotide repeat within the gene. The greater the number of repeats, the earlier the onset and more severe the symptoms tend to be.

    2. Autosomal Dominant Inheritance: HD follows an autosomal dominant pattern of inheritance. This means that a person who inherits one copy of the mutated gene from either parent will eventually develop the disease. Each child of an affected parent has a 50% chance of inheriting the mutated gene.

    Pathophysiology

    1. Neurodegeneration: The mutated huntingtin protein interferes with normal cellular processes, leading to neuronal dysfunction and eventual death in specific regions of the brain, particularly the basal ganglia (including the caudate nucleus and putamen) and cerebral cortex.

    2. Impact on Neurotransmitters: The abnormal huntingtin protein disrupts the balance of neurotransmitters, such as dopamine and glutamate, which are crucial for communication between neurons. This imbalance contributes to the motor, cognitive, and psychiatric symptoms observed in HD.

    Risk Factors

    1. Genetic Testing: Individuals with a family history of HD can undergo genetic testing to determine if they carry the mutated HTT gene. Predictive genetic testing can identify whether someone will develop the disease before symptoms appear.

    2. Age of Onset: The age at which symptoms of Huntington’s disease appear can vary widely, but it tends to be earlier in individuals with a larger number of CAG repeats in the HTT gene.

    Other Considerations

    1. Anticipation: HD exhibits a phenomenon called anticipation, where the disorder may start at an earlier age and worsen more rapidly with each successive generation due to an increase in the number of CAG repeats.

Treatment

    • Symptomatic Management: Focuses on alleviating symptoms and improving quality of life.
    • Medications: Such as dopamine-depleting agents (tetrabenazine) to reduce chorea or antidepressants for psychiatric symptoms.
    • Therapies: Physical therapy, speech therapy, and occupational therapy to maintain function and independence.
    • Supportive Care: Providing psychological support and assistance with daily activities.
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