Thymus – what is it and what are its functions? What is myasthenia gravis?

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The thymus (myasthenia gravis) is an organ that few people are even aware of. This is because in most adults it is an organ in the state of extreme decline, weighing only a few or a dozen grams. However, if it does not go away or starts to grow larger, it could be associated with the development of myasthenia gravis, a chronic, potentially fatal disease.

The thymus (Latin thymus) is a lymphatic organ located in the chest, just behind the breastbone. Its name comes from the Greek word θυμός (thumos), which means “soul”, and for centuries this is where the soul has been located.

Only the works of Professor Jacques Miller from the 60s allowed us to understand the primary function of the thymus in the development and maturation of the human immune system. This is where they mature T lymphocytes, i.e. white blood cells responsible for our immunity.

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Already in the 2nd century AD, Galen noticed that the thymus disappears as a person develops. And in fact, the thymus grows from birth to 3-30 years of age, when it reaches its greatest mass (40-XNUMX grams), and then, under the influence of sex hormones appearing in the bloodstream during puberty, it gradually disappears.

Thymus – important for immunity

The proper functioning of the thymus in childhood determines the proper development of the immune system, which translates into the development of mechanisms necessary to fight infection.

In children, the large thymus is visible on chest X-rays, while in adults its remains are difficult to distinguish from the surrounding fat, even after opening the chest.

Sometimes, however, the thymus does not atrophy, and even begins to grow pathologically, and then it may be associated with the onset of myasthenia gravis. Thymus hypertrophy is seen in the majority of people suffering from this disease, although the role of the thymus gland in the development of myasthenia gravis is not entirely clear.

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Myasthenia gravis – what is it?

Myasthenia gravis, sometimes referred to as muscle fatigue, is an autoimmune disease that causes muscle weakness, which varies in severity over the course of the disease. It is estimated that the incidence of myasthenia gravis is 10-15 cases per 100 people. Approximately 000 people are most likely ill in Poland. People of all ages are ill, but epidemiological observations showed two peaks in the incidence of myasthenia gravis, i.e. in young people in the second and third decades of life, when women are more ill, and a later peak in people aged 5000-60 years, with a predominance of men illnesses.

Myasthenia gravis is an autoimmune disease, which means that the body itself, through a malfunctioning immune system, produces antibodies against its own tissues. Indeed, antibodies against the acetylcholine receptor are detected in the blood of sick people (anti-AChR) or against muscle-specific tyrosine kinase (anti-MuSK). By attaching to the molecules they are directed at, these antibodies interfere with the proper transmission of signals between the nervous system and muscles. Under normal conditions, the nerve signal that stimulates the muscles to contract is transmitted by means of acetylocholiny, a substance released in nerve endings that attaches to its receptor on the surface of muscle cells, causing them to contract. The antibodies produced in myasthenia gravis block the access of acetylcholine to its receptors, preventing signal transmission.

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This causes muscle weakness and fatigue, known as myasthenia gravis, which can vary in severity. In about 50 percent of patients, the first symptoms result from the involvement of the muscles that move the eyeball.

Less commonly, in about 15 percent of cases, problems with the muscles of the face and neck appear initially. Sometimes (about 5% of cases) the weakness of the limbs is revealed first. The disease may be confined to the oculomotor muscles or take a generalized form. Depending on which muscles are affected, patients complain of ptosis (it may occur unilaterally, bilaterally or alternately), double vision.

A characteristic feature is the change in facial expressions and the appearance of the so-called a transverse smile or a drooping mandible making it difficult to keep the mouth closed. Patients report problems with biting, chewing and swallowing food, which worsen with eating. Speech problems are a typical symptom, manifested as a weakening and lowering of the voice that increases with the length of speech. Involvement of the neck muscles causes the head to droop forward and the neck muscles to sore. A typical symptom of myasthenia gravis on the part of the upper limbs is problems with keeping the hands up while, for example, brushing the teeth or brushing. Muscle weakness in the lower extremities causes problems with climbing stairs and falls. The most dangerous is the involvement of the respiratory muscles, which can lead to respiratory disorders and, in extreme cases, even death.

Development of myasthenia gravis

The dynamics of myasthenia gravis is different, at the beginning of its duration the ocular symptoms appear most often, however clinical observations show that within two years in about 50% of patients the disease becomes more generalized. The course of relapses and remissions of the disease, which may last up to several weeks, is very characteristic of myasthenia gravis. A common symptom is also apokamnoza, i.e. progressive muscle weakness, a decrease in the strength of their contraction with activity. Symptoms also worsen with the passage of the day and in the evening they are much more severe than in the morning after a night of rest.

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The most dangerous is the so-called myasthenic crisis, i.e. a situation in which, due to the involvement of the respiratory muscles, leading to respiratory disorders, respiratory failure occurs, which requires immediate intubation of the patient and connection to a ventilator. The cause of a breakthrough may be a banal infection, surgery, and administration of certain medications. This serious complication may also occur as a consequence of pregnancy or childbirth. Even 40 years ago, the mortality rate in myasthenic crisis was 75%, now, thanks to the progress in treatment, it is about 5%.

Diagnostics of myasthenia gravis

The diagnosis of myasthenia gravis is made on the basis of the typical symptoms of muscle fatigue, which is confirmed in various studies, mainly using electrophysiological tests (electrostimulation test of fatigue and electromyographic examination of a single muscle fiber with the so-called jitter). The Tensilon test is also helpful, as intravenous administration reduces the severity or disappears symptoms of the disease. Immunological tests are aimed at detecting the presence of (anti-AChR) or (anti-MuSK) antibodies, although it should be remembered that in about 6-12% of patients they are absent.

Chest imaging tests (computed tomography or magnetic resonance imaging), which are used to assess the size of the thymus, are also a very important element of diagnostics. It is known that 60-70% of patients have thymic hyperplasia (hyperplasia), and about 10-15% of patients have the presence of a benign neoplasm of the thymus – thymoma.

The role the thymus plays in the development of the disease is not fully understood. The studies conducted so far suggest that in the uncompressed thymus lymphocytes are “sensitized” to the elements of the muscle cell membrane. The thymus has cells that look like striated muscle cells. As a result of abnormal interactions, the maturing lymphocytes in the thymus “learn” to recognize the anti-acetylcholine receptors on the surface of these cells as “foreign” and stimulate the production of antibodies against them.

Treatment of the disease depends on its severity. In myasthenic crisis, apart from intubation and mechanical ventilation, we use plasmapheresis (procedures to “clear” the blood of antibodies circulating in it), intravenous immunoglobulins (IVIG) and glucocorticosteroids.

In some patients, however, the only drug used is Mestinon, which slows the breakdown of acetylcholine. Sometimes it is necessary to administer immunosuppressants to suppress the abnormal activity of the immune system, such as glucocorticoids and other drugs that modulate the immune system. Surgical removal of the thymus is often a prerequisite for successful pharmacological treatment. It is also important to avoid exacerbation medications – the list is long and includes substances that are used to treat a variety of conditions, including antibiotics, anesthetics, ophthalmology and cardiology medications.

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