Myasthenia Gravis Clinical Features

Myasthenia Gravis (MG)

• A chronic disease due to disorder of transmission at the myoneural junction as a result of formation of circulating autoantibodies against acetylcholine (Ach) receptors.
• Characterized by muscular fatigability, may be restricted to a group of muscles, with descending march course and tendency to relapse and remission.
• In later stages, associated with permanent weakness.
• Thymus gland is usually hyperplastic or site of neoplasm.

Clinical Features of Myasthenia Gravis

• Fatigability appears on repeated or sustained movements, e.g., counting, talking, or upgaze.
• Involvement of:
  • Ocular muscles: diplopia, drooping eyelids.
  • Facial and bulbar muscles: dysphonia, dysarthria, dysphagia, and nasality of voice.
  • Limbs: proximal weakness.
• No wasting of muscles and reflexes are normal or brisk.

Diagnosis of Myasthenia Gravis

• Clinical diagnosis: ask patient to count up to 50, talk continuously for 5 minutes, or do upgaze for 5 minutes to demonstrate fatigability.
• Therapeutic test: inject 2mg of tensilon (anticholine esterase) IV to observe rapid improvement of fatigue.
• Detection of serum anti-Ach receptor antibodies.
• X-ray chest and CT chest to detect thymoma.
• EMG study: decreased amplitude and decrementing response in the amplitude of CMAP in response to repeated stimulation.

Myotonic Muscles Disorders

• Primary muscle disease characterized by myotonic phenomena: delayed relaxation of a skeletal muscle after contraction.
• Clinically:
  • Slowness in relaxation of the hand grip.
  • Persisting dimpling after sharp blow on a muscle belly (e.g., tongue, thenar muscles, or deltoid muscle).
• Hereditary syndromes of myotonia:
  • Genetically determined AD inheritance.
  • Low chloride conductance, alteration of K+ and Na+ channels.

Inflammatory Muscle Diseases

• Clinical diagnosis.
• EMG study:
  • Decreased duration and amplitude of action potential.
  • Increased incidence of polyphasic potential.
  • Detection of fibrillation potentials.
• Histopathologically:
  • Muscle fiber necrosis, phagocytosis, and regeneration.
  • Interstitial, perivascular, and perifascicular inflammatory cell infiltration.
• Laboratory: increased levels of CPK and transaminase.

Treatment of Inflammatory Muscle Diseases

• Prednisolone orally 60 mg/day or 120 mg every other day.
• Immune suppression (azathioprine 2-2.5 mg/kg/day) combined with prednisolone.
• IV methylprednisolone in severe cases.
• IV immunoglobulin and plasmapheresis.

Muscular Dystrophy

• Definition: Lack of dystrophin in muscle fibers.
• EMG study:
  • Increased polyphasic action potential.
  • Short duration of action potential.
  • Decreased amplitude of mean action potential.
• DNA studies and genetic counseling.
• Treatment:
  • No specific drug treatment.
  • Supportive treatment (e.g., vitamins, physiotherapy, wearing light spinal supports, ventilatory support, and psychological support).
  • Infrequent course of methylprednisolone (1mg/kg/day) for transient improvement of power.
  • Dealing with complications (e.g., bed sores, pneumonia).
  • Genetic counseling before marriage.