Wet beriberi, characterized by high cardiac output with predominantly 
right-sided heart failure and lactic acidosis, is a disease caused by thiamine 
deficiency, and is rarely seen in modern society. However, patients with social 
withdrawal syndrome, also known as hikikomori syndrome, may be a new population 
at risk of thiamine deficiency. Hikikomori syndrome, first recognized in Japan, 
is becoming a worldwide issue. A 39-year-old Japanese patient was brought to our 
hospital, with a 3-week history of progressive shortness of breath and 
generalized edema. The patient had right-sided high-output heart failure, lactic 
acidosis, and Wernicke-Korsakoff syndrome. Because of his history of social 
isolation, we diagnosed hikikomori syndrome according to the Japanese 
government's definition, which is as follows: lifestyle centered at home; no 
interest or willingness to attend school or work; persistence of symptoms beyond 
6 months; and exclusion of other psychiatric and developmental disorders. 
Considering his diagnosis of hikikomori syndrome and social isolation, we 
suspected malnutrition, particularly thiamine deficiency, and successfully 
treated him. Clinicians should be aware of the potential risk of thiamine 
deficiency associated with hikikomori syndrome and initiate thiamine replacement 
in cases of high-output heart failure associated with lactic acidosis.