To describe a case of biopsy proven familial Inclusion body myositis with negative anti-cytosolic 5'-nucleotidase 1A  antibody test.

We describe a 53-year-old African-American man, who presented with one year of bilateral lower extremity weakness. Family history was significant for inclusion body myositis diagnosed in father at 60 years of age, who passed away at age of 74 years

His neurological exam showed decreased muscle bulk throughout, mild bilateral hip flexor weakness and left knee extensor weakness. Laboratory work-up revealed elevated creatine kinase (peak value – 2100 U/L), aspartate aminotransferase 37 U/L, alanine aminotransferase 50 U/L and alkaline phosphatase 79 U/L. Myositis panel was negative. Patient tested negative for cytoplasmic 5’ Nucleotide 1A Antibody. Electromyography  studies showed irritable myopathy. Magnetic resonance imaging  of the pelvis showed no active inflammation or myositis. Muscle biopsy identified neuromyopathic changes with vacuolization and cytoplasmic aggregates. Subsarcolemmal and sarcoplasmic collection of myeloid whorls, membrane bound vesicles, autophagosomes and circular bodies were seen on electron microscopy. The diagnosis of familial IBM was made based on the specific clinical and histological findings and positive family history.  The patient was started on strength training exercises and reported improvement in the strength in his lower extremities.

We here describe a case of familial IBM presenting with lower extremity weakness and negative anti-cytosolic 5'-nucleotidase 1A  antibody. Although not fully understood, familial IBM susceptibility has been linked to HLA DR3 and DR15 alleles and warrants further research.