An Unusual and Rare Presentation of Dermatomyositis Sine Dermatitis Complicated by Neuromyositis

Dermatomyositis sine dermatitis (DMSD) is a rare autoimmune disease. It's distinguished from classical dermatomyositis (DM) by a lack of skin involvement. DM is known to have a variety of extramuscular manifestations, including interstitial lung disease, myocarditis, and dysphagia. However, peripheral nervous system involvement in DM termed "Neuromyositis" is less often encountered. Neuromyositis diagnosis is controversial due to its rarity, unknown mechanism, and heterogeneity of nerve pathology findings. A 53-year-old woman presented to our hospital following a fall. Six months prior to presentation, she had a sensory disturbance in her right foot. On admission, she had a right foot drop that progressed to right then left lower extremity weakness. A biopsy of the superficial peroneal nerve and long peroneal muscle showed large fiber nerve axonal loss, CD20 B-cell and CD4 T-cell predominant inflammatory infiltrate involving the perimysial connective tissue of the muscle, as well as myocyte hypertrophy and fibrosis with type I fiber predominance. These findings were compatible with dermatomyositis with neuropathic features. Electrophysiological studies of lower extremities revealed severe widespread axonal dysfunction, as evidenced by decreased tibial compound muscle action potential (CMAP), no peroneal motor responses, absent sural sensory nerve action potential (SNAP), and extensive active denervation throughout the left lower extremity. Three months later, she developed bilateral upper extremity weakness. A biopsy of the deltoid muscle that was done eight months after admission showed CD20 B-cell and CD4 T-cell predominant inflammatory infiltrates involving the perimysial connective tissue. These findings were pathologically similar to the first biopsy. Subsequently, a repeat electromyography (EMG) of upper extremities revealed myopathic changes with normal nerve conductions. She ultimately became quadriplegic and ventilator-dependent nine months after admission. She never exhibited any skin findings throughout her course of illness. An extensive imaging and laboratory workup did not reveal any occult malignancy, inflammation, or nutritional deficiency. Our patient did not respond to steroids or intravenous immunoglobulin (IVIg) and ultimately passed away. The clinical, pathological, and electrophysiological features suggested the presence of neuromyositis. This case illustrates the importance of recognizing peripheral nervous system involvement as a significant and yet underreported extramuscular manifestation of DM. There are currently no formal management guidelines for neuromyositis.