2. Myxoedema coma is the extreme manifestation of (usually untreated)
hypothyroidism. It is one of a relatively small number of endocrine
emergencies, and it is a rare, but potentially fatal disorder. It does not
necessarily involve the presence of pretibial oedema or of coma.
»In a patient who has untreated or undertreated hypothyroidism several
physiological changes take place to compensate for the lack of thyroid
activity. This ability of the body to compensate for deficiency of T4 and T3
may, however, be overwhelmed - for example, by infection, drugs, other
diseases or hypothermia. The resulting state is referred to as 'myxoedema
coma' despite the fact that the patient may not be comatose or display the
skin changes of myxoedema (hence the alternative term 'myxoedema
crisis').
»Myxoedema coma can be regarded as a form of decompensated
hypothyroidism in which the adaptations of the body to untreated
hypothyroidism fail to maintain homoeostasis and become overwhelmed by
hypothermia, infection or other precipitating factors. These adaptations
include peripheral vasoconstriction to maintain core body temperature. This
process of adaptation and eventually failing function, affects all organs,
including the brain, heart, lungs, kidney and gastrointestinal tract.
3. » Central nervous system
» Deterioration in mental state is the cardinal symptom.-A largely unknown mechanism disrupts brain
function.
» Reduced cerebral blood flow, reduced oxygen and glucose consumption are all involved.
» Confusion.
» Apathy.
» Psychosis - all patients will display this to some degree.
» It may also produce lethargy, stupor or (very rarely) coma.-Reflexes will show a slow relaxation phase.
» Other metabolic effects may compound these effects (for example, hyponatraemia).
» Metabolic
» Reduced metabolic rate causes weight gain, growth reduction, lower energy production and many other
effects.
» Significantly, the metabolic effects impair drug metabolism (see the triggers listed under 'Factors which
may precipitate myxoedema coma', above).
» Hypothermia (core temperature <35.5°C is common, although temperature may be normal).
» Respiratory acidosis, hypoxia and hypercapnia are common.
» Skin:Dry, cool and doughy skin.-Non-pitting oedema of the hands and feet.
» Hair loss (alopecia).
» Cardiovascular: Bradycardia and reduced cardiac output resulting from profound effects on the heart
muscle.
» Pericardial effusions may occur and lead to cardiomegaly on X-ray.-There is increased peripheral vascular
resistance.
» Reduced heart sounds and reduced apical impulse are typically found.
» Blood pressure may be raised in the early stages. Raised diastolic and normal systolic pressures occur.
Hypotension can occur in the later stages.-Blood counts may show normocytic or macrocytic anaemia.
Pernicious anaemia may co-exist.
4. » The differential diagnosis of myxoedema coma will
includes other causes of a deterioration in mental
state:
» Hypothermia.
» Septic shock.
» Psychiatric disorders
» Dementia (including Alzheimer's disease),Depression
» Changes in mental state secondary to other medical
conditions and drugs,Hypoglycaemia (may co-exist)
» Encephalitis and meningitis, Hepatic encephalopathy,
Cerebrovascular disease.
