Fatal familial insomnia (FFI) affects the thalamus, the part of the brain that controls the sleep-wake cycle. The most common symptoms are sleep disturbance, psychiatric problems, weight loss, and balance problems. Other symptoms include high blood pressure, excess sweating, and difficulty controlling body temperature. These symptoms tend to get worse over time. Almost all cases of FFI occur due to a specific variant in the PRNP gene and are inherited in an autosomal dominant pattern. Diagnosis is based on the symptoms, clinical exam, sleep study, and imaging studies. The results of genetic testing can help confirm the diagnosis.