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Clinical features of Mycoplasma pneumoniae pneumonia in older adults

underlying mechanism. In this case, urinalysis indicated an impairment in urine dilution. Furthermore, increased ADH levels were observed, and insufficient solute intake was predicted because the patient had skipped breakfast and lunch on the day of hospitalization. Therefore, the diagnosis of water intoxication is considered appropriate. Increased ADH levels indicate syndrome of inappropriate antidiuresis. Typical causes of syndrome of inappropriate antidiuresis include the use of certain drugs or the presence of malignancies, which were inconsistent in the present patient. She had nausea and vomiting, which might have affected the ADH release; however, the cause–effect relationship is still unclear. It is also uncertain if these symptoms were sufficient to trigger such severe hyponatremia. Therefore, mineralocorticoid-responsive hyponatremia of the elderly (MRHE), an important differential diagnosis of syndrome of inappropriate antidiuresis, might have been implicated. In MRHE, excessive sodium loss occurs owing to an impaired response of the distal renal tubule to aldosterone, causing mild dehydration, compensative ADH hypersecretion and dilutional hyponatremia. Mild dehydration, recovery with salt supplementation, elimination of other possible causes or effectiveness of fludrocortisone support the diagnosis although remission without fludrocortisone is possible in MRHE-like dysfunctions. In the present case, besides the examination data, recovery with enough sodium and fluid supplementation, but without water restriction and no detection of apparent other possible causes, were compatible with MRHE. Mild dehydration might have been present, but was possibly masked by water loading, as more water retention occurs in MRHE Acute water intoxication was implied to relate to aging. Taken together, these findings suggest that MRHE was a possible underlying cause. In summary, although the relationship of MRHE with water intoxication has rarely been suggested, it is reasonable to attribute acute water intoxication to impaired urine dilution and coexisting sodium loss as a result of renal tubule dysfunction similar to MRHE. There might be a high prevalence of MRHE in elderly people, which can easily progress to severe hyponatremia. Physicians should note that older people are susceptible to water and electrolyte imbalance, and attention should be paid to free-water loading.