Microscopic polyangiitis and myasthenia gravis: the battle of Occam and Hickam.

Microscopic polyangiitis (MPA) is a pauci-immune, necrotising, small-vessel vasculitis with an incidence of 3.6 per million population that typically presents in adulthood. Myasthenia gravis (MG), the most common disorder of the neuromuscular junction is rare, with an incidence of four per million population. We present the case of an adolescent girl previously diagnosed with MPA at age 7 years who presented with breathlessness and respiratory failure aged 15 years. The respiratory symptoms were due to thymoma-MG, which was successfully treated with cholinesterase inhibitors and thymectomy. This case report illustrates that the well-established doctrines of Occam's razor and of 'common conditions occurring commonly' are not universally applicable, and that in the adolescent age group, one should still consider Hickam's dictum.