Distal upper limb involvement in myasthenia-myositis association

Myasthenia gravis-inflammatory myopathy (MG-IM) association has been rarely reported as specific clinical entity characterized by variable myositis manifestations, ranging from subclinical to diffuse muscle involvement with characteristic distal upper limb weakness. Although, in view of this, it has been hypothesized that distal muscle weakness in MG-IM could be due to the muscle inflammation instead of a pure neuromuscular transmission impairment, a biopsy-proven myositis process of distal muscles of upper limbs has not yet been provided.

We report on clinical, immunological, and myopathological characterization of a novel case affected by MG-IM association showing the typical distal upper limb weakness, including muscle biopsy of a weak forearm muscle.

Histological and immunohistochemical studies showed a marked inflammatory process on muscle biopsy of extensor digitorum communis. The patient, a 47-year-old man with 10-year history of anti-acetylcholine receptor (AChR) and anti-titin antibody-positive MG with thymoma, developed a progressive, diffuse, and non-fatigable weakness predominant in distal upper limb muscles, unresponsive to acetylcholinesterase inhibitors associated to myalgia and creatine kinase (CK) elevation.

We provide the histopathological evidence of a prominent inflammatory process responsible of distal upper limb weakness in MG-IM association. Muscle biopsy does not reveal any typical histopathological feature of other nosologically definite inflammatory myopathy, leading MG-IM association to come close to the group of overlap-myositis (OM) with the myopathological features of non-specific myositis (NSM).