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Myasthenia gravis

Miastènia gravis

Myasthenia gravis (MG) is a neuromuscular disease characterized by fatigue and weakness which worsens with exercise and improves with rest. Etymologically, the word myasthenia comes from the Greek mys" (muscle) and “asteneia" (weakness) while the word “gravis" (serious) is derived from the Latin.

MG belongs to the group of rare diseases, i.e. it is infrequent. Its estimated prevalence is 50 to 200 cases per million people. Its presentation is bimodal: onset occurs before age 40 in women, while in men onset is at around 50 years old. It is uncommon in children. Recent epidemiological studies have described an increase in the incidence of new late onset cases (at over 60 years old) with no clear predominance in terms of gender.

 

Description

Signs and symptoms

The main initial symptom principal in MG is muscular weakness and fatigue, with daily and hourly variations. The muscular weakness gradually becomes worse during periods of physical activity and improves after periods of rest. Many patients also refer fluctuations related to the seasons of the year, workplace/psychological stress, menstruation and concurrent infections. In general, the weakness and fatigue worsen as the day ends. The fluctuations and deterioration according to the intensity of physical activity are the sign that distinguishes MG from other diseases such as mental fatigue and chronic fatigue syndrome, in which tiredness appears when the patient gets up in the morning and is at rest.

MG generally begins with ocular symptoms, in the form of double vision (diplopia) and/or a drooping eyelid (ptosis) which may be unilateral or bilateral. The ocular symptoms tend to worsen when watching television, reading or driving, and especially under conditions with high levels of light. Symptoms resulting from involvement of the bulbar musculature are also frequent: problems swallowing, chewing and speaking. Approximately 16% of patients present one of these bulbar symptoms at onset. Other manifestations that patients may present are difficulty in keeping the head erect, inability to keep the mouth closed, dyspnea, premature fatigue when walking, turning over in bed, raising the arms, and opening bottles or cans.

Approximately 20% of patients may present a myasthenic crisis during their lives, which involves serious respiratory or swallowing problems, which may require hospital admission. These crises may be triggered by various stress-related factors, infections, medication, surgical procedures and even emotional stress.

Diagnosis of the disease.

Diagnosis of MG is based on the symptoms, physical examination and paraclinical explorations, and the electromyogram (EMG and SF-EMG) and the Tensilon test (Tensilón®) are the most extensively used. It is possible to identify the presence of antibodies (AChR and MuSK) in serum in most patients. A small percentage of patients are seronegative for these antibodies. The study must be completed with a computed tomography scan (CT) to rule out the presence of a thymoma.

Treatment

Acetylcholinesterase inhibitors associated with immunosuppressant drugs are usually used to treat myasthenia gravis. Most of these drugs have secondary effects (generally dose-dependent) which must be taken into account. The choice of treatment must be agreed with the patient, taking into account the recommendations of the guidelines for the disease, the clinician’s experience and the clinical form of the MG. This treatment of a patient with myasthenia means that the patient will have a lifelong association with the neurologist.

The surgical removal of the thymus (thymectomy) can improve the symptoms, prevent a number of complications throughout the patient’s life, and reduce the dosage of immunosuppressant drugs. Thymectomy is recommended in generalized forms of MG in patients less than 55 old and in all patients with thymoma, regardless of their age.

Plasmapheresis and intravenous immunoglobulin can be used when myasthenia deteriorates. Mechanical ventilation may be required in myasthenic crises.

With treatment, most patients lead relatively normal lives and have a normal life expectancy.

Special Situations

Pregnancy. The physiological changes that take place during pregnancy (nausea, vomiting, weight gain, increased blood volume, changes in renal function and alterations in gastrointestinal absorption) frequently alter the course of myasthenia gravis and an adjustment of the dosage may be required during the entire pregnancy. Pregnant women with MG are consequently at greater risk of complications and adverse effects. There is a clinical worsening of myasthenia in a third of pregnancies (mainly in the first and third trimesters and during the postpartum period). Many drugs administered to myasthenic women may have teratogenic effects, while others are associated with higher miscarriage rates and low birth weights.

Newborns with mothers with myasthenia may present symptoms of MG in the initial hours of life (transient neonatal myasthenia) due to the transfer of antibodies from the mother.

Given the unpredictable nature of complications during pregnancy and childbirth, these patients should be monitored in Specialist Units with a multidisciplinary team, at least including a neurologist, obstetrician, anesthetist and a neonatologist.

Surgical procedures. Surgical procedures, and thymectomy in particular, may precipitate a myasthenic crisis. High scores on the functional scale for the disease, low spirometric values and serious concurrent diseases are the main risk factors. A joint risk assessment with pneumologists, surgeons and anesthetists must therefore be carried out.

Procedures with local, regional or epidural anesthesia do not appear to involve a risk of myasthenic crisis.

Contraindicated drugs. Some drugs may lead to a deterioration in myasthenic symptoms, and patients should inform doctors when they are prescribed new drugs. Some vaccines, drinks and foods may also be contraindicated. Our website contains information about those that are permitted: https://www.neuromuscularbarcelona.org/miastenia/

Associated Autoimmune Diseases

Many patients with MG also have other associated autoimmune diseases, of which hypothyroidism is the most common.

Physical measures. Mild daily exercise is advisable, with rest periods given the fluctuating nature of the symptoms. The exercises can be completed at home with respiratory exercises. Overweight must be avoided.

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