People who are old and those who have been confined in bed for a long time get weak. Their muscles seem to lose the power to act. In a disease called myasthenia gravis the muscles get weak but seem to recover strength after rest, becoming weak again almost immediately after the least exertion. During their action the muscles use up certain materials. Through actions of the nervous system the use of this material, called cholinesterase, is inhibited. The giving of drugs like neostigmine prevents the destruction of cholinesterase and thus helps these patients.
As in the case of lassitude a number of conditions can also produce asthenia. A long-continued infection, an excessive action of the thyroid with too-rapid beating of the heart, severe anemia, nutritional deficiencies, or habitual taking of drugs or poisons may be a prime factor. The doctor has to make a thorough study using many laboratory tests to determine the cause with certainty.
Dr. Tinsley Harrison has said that nearly all patients with true asthenia have lassitude but the majority of patients with lassitude do not have asthenia.
When people are troubled by faintness, lightheadedness or dizziness a great variety of conditions must be investigated. Usually this symptom results from disturbance of the supply of blood to the brain. The difficulty may be in the blood vessels of the brain, in the power of action of the heart or in the quality of the blood. The symptom can occur also in epilepsy and in hysterical conditions. Sudden drop in the sugar in the blood also causes this symptom. This symptom makes people anxious, although the conditions causing it are seldom fatal.