Neurology

Myasthenia Gravis

Definition

Myasthenia Gravis is a condition in neuromuscular transmission with autoimmune bases.

There are certain antibodies that act against the nicotinic acetylcholine receptor (Ach), preventing depolarization of the postsynaptic membrane, preventing muscle excitation.

Acetylcholine:

Acetylcholine is a neurotransmitter released by neurons of the peripheral nervous system and central nervous system. It behaves as an excitatory neurotransmitter of the neuromuscular plate acting through the opening of ion channels or as an inhibitory neurotransmitter where its effect on ion channels is indirectly exerted by means of G protein-bound receptors.

The acetylcholinetase enzyme inactivates acetylcholine, denaturing it in acetyl and choline.

Epidemiology

The incidence of Myasthenia Gravis is 1 case per 100,000 inhabitants. More common in women during the second and third decade of life. Although in men it is common to appear in the seventh and eighth decade of life.

Pathophysiology

The autoimmune form of Myasthenia Gravis is characterized by the presence of antibodies that attack the acetylcholine receptors located in the neuromuscular plate, as a response to this there is a decrease in the sensitivity to Ach in the postsynaptic portion. As this decrease in sensitivity occurs, although normal amounts of Ach are released, it is sometimes not able to depolarize enough ion channels to initiate muscle excitation.

Units of IgG, C3 and circulating anti-receptor antibodies have been identified in the neuromuscular receptors. These antibodies cause the destruction of Ach receptors by activation of lytic fractions of complement. In addition to the degradation of the receptors, the antibodies can alter the transmission of impulses, blocking the Ach receptors.

Clinic of Myasthenia Gravis

  • Muscle weakness that is exacerbated with exercise and improves with rest.
  • Start of symptoms can occur suddenly or gradually.
  • Weakness of the eye muscles.
  • Diplopia.
  • Palpebral ptosis.
  • Immobility of the face
  • Asphyxia in some cases, in which the thoracic muscles are affected.

Classification

For the management of Myasthenia Gravis, a classification has been established, called the Osserman classification (1958):

  • Grade Ia: Ocular Myasthenia:
  • Grade Ib Myasthenia generalized mildly
  • Grade II Myasthenia generalized moderately
  • Grade III Myasthenia generalized severely
  • Grade IV Myasthenic crisis

Patients may initially have diplopia and palpebral ptosis in the case of ocular myasthenia, also signs of bulbar involvement, with voice alterations such as dysphonia, dysphagia and in severe cases, grades III and IV of the classification, muscle involvement. respiratory Patients with generalized myasthenia present fatigue and weakness that worsens with repetitive muscle contraction and that the patient notices when performing activities and that the picture worsens as the day progresses, with greater fatigue in the afternoons. Generalized weakness occurs in 85% of patients with generalized myasthenia (Toro 1994).

Clinical features

  • Class I Weakness of any ocular muscle or ptosis without evidence of another commitment
  • Class II Ocular compromise and weakness of other mild muscles
  • Class III Ocular compromise and weakness of other muscles of moderate character
  • Class IV Ocular compromise and weakness of other muscles of severe character.
  • Class V Orotracheal intubation requirement to maintain the airway

(Clinical classification according to the American Foundation of Myasthenia Gravis.)

Differential diagnostics

The difficulties in its diagnosis and its clinical course are very variable and in some cases it allows 1 of every 3 patients not to be diagnosed in the first years of the pathology, since they confuse the symptoms with other pathologies:

  • Primary ocular disorders
  • Myopathies
  • Brain injuries
  • Cerebral palsy
  • Amyotrophic Lateral Sclerosis
  • Guillain Barre syndrome
  • Miller Fisher syndrome

Diagnostic tests

Antibody screening: this test has a variation in its sensitivity, since it varies according to the ocular or generalized commitment, to the time of onset that can be early before 40 years or late after 40 years and in the presence of thymoma. Being more sensitive to myasthenia gravis generalized than to ocular.

Edrophonium Test: This consists of the application of Tensinol or Camsilon in an initial dose of 1-2 mg to evaluate hypersensitivity and subsequently of 8 mg, in order to evaluate the motor response. During the performance of this test can occur cardiovascular complications so it is mandatory to practice continuous cardiac monitoring. A sensitivity of 97% has been described.

Elecodiagnostic techniques: Basic studies such as motor neuroconduction show a decrease in the action potential in very advanced cases.

Treatment

For ocular myasthenia, the treatment of choice are steroids.

In generalized cases of Myasthenia Gravis therapy are acetylcholinesterase inhibitors. The main medicine is Mestinón.

Another inhibitor of acetylcholinesterase is Neostigmine

When these medications fail, the use of steroids is prescribed.

Other therapies that are used are plasmapheresis that decreases the number of antibodies.

Human hyperimmune gammaglobulin that functions as an immunomodulator of the immune response.

Thymectomy is indicated in patients with thymoma and in whom a generalized myasthenia is evident.

Bibliography

  1. Anthony S. Fauci, Eugene Braunwald, Dennis L. Kasper. Principios de Medicina Interna. 17ª ed. México DF: Mc-Graw Hill Interamericana Editores; 2009.
  2. Stanley L. Robbins, Ramzi S. Cotran. Patología estructural y funcional. Octava edición. España. Elsevier Saunder ; 2010.
  3. Toro J, Vallejo E. Tratado de Neurología Clínica. Santa Fe de Bogotá: Lerner Ltda; 1994.
  4. Javier Triana, Fernando Ortiz; MIASTENIA GRAVIS, DE LA CLÍNICA A LA FIBRA única [Sede web]; Bogotá, Facultad de medicina Universidad Nacional de Colombia; Acceso 4 de Septiembre del 2014; Disponible en: http://www.medicina.unal.edu.co/Departamentos/Rehabilitacion/doc/miastenia%20Gravis%20_Tri.pdf

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