Myasthenia

Myasthenia gravis ^{[39, 41, 212]}, from the Latin ^[219] (meaning "grave muscle weakness"), is a relatively rare chronic autoimmune disease that preferentially affects women ^[178].

Myasthenia is linked to a defect in the transmission of nerve impulses between the nerve endings of motor neurons and muscle fibers ^[219]. It manifests as muscle weakness that worsens with activity and improves with rest. Its severity lies in the risk of respiratory complications ^[69] that can be life-threatening for the patient.

Pathophysiology :

The neuromuscular transmission defect results from the blocking of acetylcholine receptors (AChR) at the motor end plate by anti-acetylcholine receptor autoantibodies (anti-AChR antibodies) ^[218].

Alongside this humoral factor, it is established that the thymus plays an important role in the pathogenesis of myasthenia ^[187]. Approximately 10% of patients with myasthenia have a thymoma, and two out of three patients have thymic hyperplasia ^[185].

These immune abnormalities may have a genetic basis ^[220]; myasthenia appears to be linked to a different HLA phenotype depending on whether it is an early-onset form in young women or a late-onset form.

Diagnosis :

In half of the cases, the first signs are ocular, with diplopia (double vision) or ptosis (drooping of the upper eyelids) ^[107]. In other cases, the onset is marked by difficulty with phonation, chewing, or weakness in the limb muscles.

In myasthenia, muscle weakness increases with exertion ^[187] and can lead to partial paralysis of the affected muscle. Rest improves muscle strength ^[75]. However, in severe forms of the disease, muscle strength is permanently reduced and does not improve, even after prolonged rest.

The clinical examination must be supplemented by an anticholinesterase test (Prostigmin), for which a positive result is a significant diagnostic argument in favor of myasthenia, although a negative result does not rule out the diagnosis ^[207].

Treatment :

Myasthenia can be treated with cholinesterase inhibitor medications. These drugs prevent the breakdown of acetylcholine and thus prolong its action on the receptors of the motor end plate ^[212].

In the case of a thymoma, a thymectomy is performed. Immunomodulators are sometimes used, such as intravenous polyvalent immunoglobulin infusions and corticosteroids.