Objective:

N/A

Background:

There are three subtypes of immune-mediated necrotizing myopathies (IMNM) including anti-3-hydroxy-3-methylglutaryl-coenzyme-A reductase (HMGCR), anti-signal recognition particle (SRP), and seronegative. We present a case of a 19-year-old female with acute on chronic progressive severe proximal extremity weakness with rapidly progressive bulbar and respiratory dysfunction, found to have seronegative IMNM.

Design/Methods:

N/A

Results:

A 19-year-old female with history of hypothyroidism presented with acute on chronic progressive bilateral proximal upper and lower extremity weakness and rapidly progressive bulbar dysfunction with dyspnea. She was hospitalized two months earlier for one year of slowly progressive proximal weakness in a limb-girdle pattern with creatine kinase (CK) of 400 concerning for genetic etiology. Limb-girdle-muscular dystrophy genetic testing was normal. During follow-up, examination was remarkable for use of respiratory accessory muscles, proximal>distal upper and lower extremity weakness, MRC grade 0/5 strength neck flexion/extension, bilateral lower facial weakness, reduced symmetrical reflexes, and intact sensation. Labs were significant for elevated CK 2000 and aspartate transaminase/alanine aminotransferase 803/202. Further diagnostics including myasthenia gravis antibody panel, voltage-gated calcium channel antibody, myositis and necrotizing myopathy panel including anti-SRP and anti-HMGCR were negative. EMG showed an irritable myopathy. Empiric IVIG 2g/kg and high dose corticosteroids were initiated. A CT chest/abdomen revealed pulmonary embolus and hepatomegaly. No malignancy was found. The patient was determined to have probable autoimmune hepatitis as a cause of hepatomegaly. Muscle biopsy results were consistent with necrotizing myopathy. The patient had robust response to treatment with full recovery of strength over the course of one month and remains on low-dose prednisone, maintenance IVIG, and mycophenolate mofetil.

Conclusions:

Seronegative IMNM is the least understood out of the IMNM subtypes. While fulminant bulbar symptoms, axial weakness, and respiratory distress are more likely to be present in anti-SRP necrotizing myopathies, our case highlights that seronegative INMN can mimic this pattern.