Motor neuron diseases

Motor Neuron Diseases

  • Definition: Group of degenerative motor neuron diseases characterized by progressive muscle weakness and disability.

    • Causes: Mainly genetic.

Pathology & Causes

  • Muscle weakness and fatigue lead to disability.

Signs & Symptoms

  • General Symptoms:

    • History and physical examination reveal upper and lower motor neuron signs.

    • Muscle biopsy and electromyography (EMG) are essential diagnostics.

Diagnosis

  • Key Points:

    • Diagnosis relies on clinical history, physical examination, EMG results, and possibly muscle biopsy.

    • Tests confirm motor neuron disease, indicating both upper and lower motor neuron involvement.

Amyotrophic Lateral Sclerosis (ALS)

  • Overview:

    • ALS is a progressive, degenerative motor neuron disease known as Lou Gehrig’s disease.

    • Genetic associations in familial ALS provide insights into its pathogenesis.

Pathology & Causes

  • Mechanisms of Injury:

    • Protein aggregation leads to neuronal injury and death.

    • Mutations in SOD1 (20% of familial ALS) lead to misfolding, causing neuronal injury and protein aggregation.

    • C9orf72 mutation accounts for 40% of familial cases, signaling an unknown significance in ALS.

Signs & Symptoms

  • Early Symptoms:

    • Asymmetric hand weakness, dropping objects, and cramping in upper extremities.

    • Dysarthria, dysphagia, and dysphonia develop later.

  • Late Symptoms:

    • Respiratory weakness introducing dyspnea and infections like pneumonia.

Diagnosis

  • Laboratory Findings:

    • Elevated creatinine kinase due to muscle atrophy.

  • Diagnostic Criteria (El Escorial):

    • Evidence of lower and upper motor neuron disease.

    • Progressive spread of signs/symptoms based on clinical findings.

Treatments

  • Medications:

    • Disease-modifying agents have limited efficacy.

    • Riluzole reduces excitotoxicity from glutamate, slowing neuron degeneration.

    • Edaravone acts as a free radical scavenger to decrease oxidative stress.

  • Supportive Care:

    • Multidisciplinary approach for symptom management, respiratory care, and regular evaluations.

Spinal Muscular Atrophy (SMA)

  • Overview:

    • Genetic degenerative neurological disease primarily affecting children, characterized by lower motor neuron weakness and muscular atrophy.

  • Causes:

    • Loss-of-function mutation in the SMN1 gene leads to reduced motor neuron survival and degeneration of anterior horn cells.

  • Pathology:

    • Mutations lead to poor production of sufficient SMN protein, attributing to motor neuron death.

Symptoms & Diagnosis

  • Signs:

    • Common signs include proximal limb weakness, reduced muscle strength, hypotonia, and fasciculations.

  • Diagnostics:

    • EMG and muscle biopsies reveal characteristic abnormalities in muscle fiber size and activity.

Treatment

  • Medications:

    • Nusinersen, an experimental therapy, modifies mRNA splicing to increase functional SMN protein levels.

  • Supportive Interventions:

    • Focus on pulmonary care, nutritional management, and physical therapy to promote mobility and manage complications.

Types of Spinal Muscular Atrophy

  • Classification:

    • Type 0: Prenatal onset with severe congenital defects.

    • Type 1: Onset in infancy with a poor prognosis.

    • Type 2: Onset between 6-18 months with better outcomes.

    • Type 3: Adolescent onset with variable symptom severity.

    • Type 4: Adult-onset with mild symptoms.