KLS can be diagnosed when there is confusion, apathy, or derealization in addition to frequent bouts of extreme tiredness and prolonged sleep. The earliest it can be diagnosed is the second episode, this is not common. The condition is generally treated as a diagnosis of exclusion. Because KLS is rare, other conditions with similar symptoms are usually considered first. MRIs can determine if the symptoms are caused by certain brain disorders, stroke, and multiple sclerosis. Lumbar puncture can determine if encephalitis is the cause. KLS must be differentiated from substance abuse by toxicology tests. The use of electroencephalography (EEG) can exclude temporal status epilepticus from consideration. EEGs are normal in about 70% of KLS patients, but background slowing may sometimes be detected. In addition, low-frequency high-amplitude waves can be observed during waking hours. Initially, KLS appears similar to bipolar depression. Patients with frontal-lobe syndromes and Klüver-Bucy syndrome also display similar symptoms, but these conditions can be differentiated by the presence of brain lesions. KLS should also be distinguished from very rare cases of menstruation-caused hypersomnia.
Kleine–Levin syndrome
Bulimia nervosa
Temple–Baraitser syndrome
Adams–Nance syndrome
Frey's syndrome
Urban–Rogers–Meyer syndrome
Wallis–Zieff–Goldblatt syndrome
Nance–Horan syndrome