Adult Langerhans cell histiocytosis: An unusual cause of referred knee pain.

Langerhans cell histiocytosis (LCH) in adults is rare and regarded as an 'orphan disease.' The systemic symptoms of LCH can mimic many other undifferentiated diseases seen at the primary care level. Failure to diagnose and delays in referral are common pitfalls in the management of this disease. We present a case of a 34-year-old woman with referred knee pain who was eventually diagnosed with multi-system LCH 4 years after the initial presentation. The mean age of presentation of LCH symptoms in adults is 33. Bone lesions are the frequent presentation of LCH in this age group. Endocrine involvement in LCH is seen in the form of diabetes insipidus (DI), which remains the most common extraskeletal presentation of LCH in adults. In the case discussed here, a definitive diagnosis of LCH was established through tissue biopsy. The spectrum of undifferentiated symptoms underscores the difficulty and delay in making a diagnosis associated with the condition. Most GPs not only face the predicament of initial recognition but also fail to merge presenting symptoms to form a purposeful referral of this elusive disease to a tertiary care unit.