Background
Syncopes and transient loss of consciousness affect a large number of patients. Determining the underlying mechanism of a syncope is key to effectively treating and preventing future events. However, given the broad differential diagnosis of transient loss of consciousness, it can be challenging to determine the exact etiology.

Case presentation
This case presents a 42-year-old Caucasian female patient with recurrent transient loss of consciousness due to a hitherto undiagnosed impaired glucose tolerance and hyperinsulinism. The patient had been thoroughly tested for all typical causes of syncope without finding any causal explanation. An oral glucose tolerance test confirmed rapidly dropping blood glucose levels associated with rapidly fading consciousness as the underlying cause of transient loss of consciousness. Further diagnostic workup revealed that the patient suffered from impaired glucose tolerance and subsequent hyperinsulinism without overt diabetes mellitus. Nutritional counseling including reduction of glucose intake and frequently eating smaller meal portions led to a significant reduction in the frequency of transient loss of consciousness and overall improvement in quality of life.

Conclusions
The current European Society of Cardiology (ESC) guideline on syncope does not list hypoglycemia as a cause of transient loss of consciousness. However, this case report stresses that metabolic dysregulation can indeed lead to self-limited transient loss of consciousness. Thus, in the case of recurrent syncope with an unclear underlying mechanism, physicians should consider transient hypoglycemia and metabolic workup as a possible differential diagnosis.