[A case of chronic myopathy associated with an antibody to signal recognition particle (SRP) following long-term asymptomatic hypercreatinekinasemia].

A 65-year-old man first visited our hospital due to hypercreatinekinasemia (hyperCKemia) (669 IU/l) 12 years ago at age 53. At that time, he had normal muscle strength without other neurological deficits, electromyography (EMG) was normal, and a muscle biopsy obtained from the biceps brachii was intact in routine histochemical studies. These findings led to a diagnosis of idiopathic hyperCKemia that lasted for over a decade. At age 65, the patient became aware of muscle weakness and serum CK was elevated to 4,846 IU/l. Neurological examination revealed very mild atrophy in both thighs, proximal muscle weakness in the left upper and right lower limbs without myalgia, grasping pain, joint pain, and skin lesions. A typical myogenic pattern was detected on EMG exclusively in proximal limb muscles, and fat-suppressed MRI showed high intensity signal areas in adductor magnus muscles. The clinical diagnosis was limb-girdle muscular dystrophy, but MRI findings suggestive of an inflammatory process prompted us to perform muscle biopsy at the rectus femoris. The pathology had characteristic features of necrotizing myopathy containing necrotic and regenerating fibers without prominent inflammatory cell infiltration. Serum anti-signal recognition particle (SRP) antibodies were found to be positive and the final diagnosis was anti-SRP antibody myopathy. Muscle weakness progressed slowly despite therapy with oral corticosteroids. Addition of intravenous high-dose immunoglobulin therapy led to an apparent improvement of muscle weakness in parallel with lowering of the serum CK level. In those who were thought to be idiopathic hyperCKemia or hereditary muscle disorders, potential immunotherapy-effective group does exist. We suggest considering such cases including anti-SRP antibody myopathy during diagnosis, and non-invasive MRI study may be useful to differentiate immunotherapy-effective group from hereditary muscle disorders.