Aims:
A patchy inflammatory myositis is a recognised complication of systemic connective tissue disorders. EMG is useful in confirming myositis due to the large number of muscles that can be sampled (relative to muscle biopsy and MRI) and this influences treatment, however there is little evidence to guide the diagnostic approach in this patient group.  Our aim is to evaluate the EMG findings in patients with myositis and to see if we could determine which muscle groups offer the highest diagnostic yield.

Methods:
EMG findings and blood test results (including creatine kinase and autoantibody screen) were collected retrospectively for patients with known systemic connective tissue disorders referred for EMG investigation of suspected inflammatory myositis at the Royal Free (January 2008 to January 2018). All EMG was performed by a consultant neurophysiologist; repeat studies on the same patient were excluded. 

Results:
27 patients were identified with EMG confirmation of myositis (defined from the presence of fibrillations indicating active muscle fibre inflammation).  Creatine kinase was raised in 22 patients and 23 patients had positive autoantibodies. Systemic sclerosis was the most common connective tissue diagnosis (n=12).
When tested, the thoracic paraspinals showed EMG findings of fibrillations in 96% of myositis patients. Iliopsoas, trapezius, deltoid and rectus femoris showed fibrillations in 56%, 39%, 35% and 21% of patients, respectively.  Creatine kinase correlated positively with an estimate of the average amount of spontaneous activity per muscle tested (R=0.58, p=0.007).

Conclusion:
EMG of thoracic paraspinals offers the greatest sensitivity in detecting active inflammatory myositis in our patient group (at a regional centre for connective tissue disorders). Thoracic paraspinal sampling should be included in an EMG screen for the diagnosis and monitoring of active myositis in similar patient groups.