In one study, the average age of the affected individual at the time of diagnosis was 4 years old. Slightly over 50% were aged 24 months or younger, and 75% were under six years old. The average duration from onset of symptoms to diagnosis was 22 months. By the time of diagnosis, six percent of the affected persons were dead, mostly from apnea (a common result of smothering) or starvation, and seven percent had long-term or permanent injury. About half of the affected had siblings; 25% of the known siblings were dead, and 61% of siblings had symptoms similar to the affected or that were otherwise suspicious. The mother was the perpetrator in 76.5% of the cases, the father in 6.7%.