As a rule, in the myxedematous patient, cognitive activity is slowed; in exceptional cases, there is a significant confusional state or stupor. When such changes have been observed, we have noted drowsiness, inattentiveness, and apathy as early features. In 2 cases observed by our colleagues, the somnolence was so extreme that the patients could not stay awake long enough to be fed or examined. They were in a state of hypothermic stupor but exhibited no other neurologic abnormality. In extreme form, the state progresses to “myxedema coma.” This state is often precipitated by stresses, particularly surgery and sepsis, mainly in the elderly. Hypothermia, hyponatremia and elevation of serum creatine kinase (CK) concentration, hypoventilation, and elevation of the CSF protein can be expected. The clinical state and laboratory abnormalities are reversed within a few days by thyroid medication. The treatment of myxedema coma has several refined aspects, including the need to administer thyroid hormone cautiously; details can be found in Harrison’s Principles of Internal Medicine.
Hypothyroidism is associated with a number of distinctive myopathic disturbances, which are discussed in Chap. 45. The ataxia and peripheral neuropathy that are sometimes observed in patients with myxedema were described earlier and in Chap. 43.
This form of severe intrauterine hypothyroidism (in mother and fetus) or postnatally as a hereditary or acquired thyroid disease, is probably the most frequent and potentially preventable and correctable metabolic cause of encephalopathy in the world. Although the condition is most common in goitrous regions where there is a lack of iodine, it may also be the result of any of several genetically determined defects in thyroxin synthesis that have come to light (Vassart et al). In areas of endemic cretinism, additional factors may be operative, such as the widespread ingestion of cassava, which contains a toxic goitrogen that inhibits the uptake of iodine by the thyroid.
The symptoms and signs of congenital thyroid deficiency are not usually recognizable at birth but become apparent only after a few weeks; more often the diagnosis is first made between the 6th and 12th months of life. Physiologic jaundice tends to have been severe and prolonged (up to 3 months), and this, along with widening of the posterior fontanelle and mottling of the skin, should raise suspicion of the disease.
Two types of early life hypothyroidism are recognized—sporadic and endemic. The sporadic type occurs occasionally in developed countries (less than once in 4,000 live births) and is a consequence of a congenital metabolic or anatomic disorder of the thyroid gland. At birth, the gland is either absent or represented by cysts, indicating a failure of development or a destructive lesion. In the sporadic form, in the latter part of the first year, stunting of growth and delay in psychomotor development become evident. Untreated, the child is severely developmentally delayed but placid and good natured; such children sleep contentedly for longer periods than normal children. Sitting, standing, and walking are delayed. Movements are slow, and if tendon reflexes can be obtained, their relaxation time is clearly delayed. The body temperature is low, and the extremities are cold and cyanotic. Although the head is small, the fontanelles may not close until the sixth or seventh year, and there is delayed ossification. This type of hypothyroidism is preventable by treatment with thyroid hormone.
Endemic cretinism is most common in developing countries, with an estimated incidence in some areas of 5 to 15 percent. DeLong and colleagues, on the basis of epidemiologic surveys mainly in western China, have distinguished 2 forms of endemic cretinism: neurologic and myxedematous. The occurrence of the 2 different types is governed by the timing, duration, and severity of the iodine deficiency (Thilly et al).