Magnetic resonance imaging changes of thigh muscles in myopathy with antibodies to signal recognition particle

Objective: The aim of this study was to evaluate muscle MRI changes and the role of MRI in monitoring therapy in patients with myopathy associated with antibodies to signal recognition particle (anti-SRP myopathy).

Methods: We identified 12 patients with anti-SRP myopathy [6 females and 6 males; mean age of onset 38.5 years (s.d. 12.4), mean duration 22.8 months (s.d. 20.6). The main symptoms were proximal limb muscle weakness. Mean serum creatine kinase levels were moderately increased. Muscle biopsies revealed necrotizing myopathy in all patients, with obvious connective tissue proliferation in five patients and a single focus of lymphocytic infiltration in the endomysium in one. The myositis disease activity assessment (MYOACT) visual analogue scales scores were assessed. Muscle MRI was performed through the thighs. All patients were treated with corticosteroids and other immunosuppressive drugs.

Results: MRI revealed fatty infiltration and oedema in the thigh muscles of all 12 patients. Prominent fatty infiltration was present in 4 of the 12 patients. The hamstrings and adductor magnus were the most severely infiltrated and the quadriceps femoris the least. Obvious oedema was observed in 10 of the 12 patients, the most severely affected muscles being the vastus lateralis, rectus femoris, biceps femoris and adductor magnus, with relative sparing of the vastus intermedius. The degree of oedema was not correlated with creatine kinase levels or MYOACT scores. The four patients with striking fatty infiltration were refractory to therapy.

Conclusion: MRI of the thigh muscles shows a distinct pattern of oedema and fatty infiltration and can be used to monitor the treatment of patients with anti-SRP myopathy.