Predictive Value of Electrophysiology for Presence of Thymic Pathology in Myasthenia Gravis

Single fibre electromyography (SFEMG) and repetitive nerve stimulation (RNS) are routinely performed investigations in evaluation of patients with myasthenia gravis (MG). Significant number of MG patients have a thymic pathology. We aimed to explore the relationship between the SFEMG and RNS findings with the presence of thymic pathology.

We studied 159 consecutive patients with MG over a 10 year period. The SFEMG parameters - mean jitter (MJ) and percentage of abnormal fibres (POAF) and the RNS result were correlated with the thymic findings.

As compared to patients with normal thymus (MJ:58.3 μsec; POAF:63.5%), patients with thymic pathology had a significantly higher MJ (80.9 μsec; p < 0.0005) and POAF (83.5%; p < 0.0005). MG patients with thymic hyperplasia had the highest MJ (87.6 μsec) and POAF (84.4%) followed by patients with thymoma (MJ:78.6 μsec; POAF:83.2%). The MJ and POAF did not correlate with the stage of thymoma. Mean jitter values above 34.9 μsec and POAF above 31% had 100% sensitivity for the presence of thymic pathology. A positive RNS increased the risk of thymic pathology (OR = 3.9, CI = 1.8-8.5) and thymoma. (OR = 3.5; CI = 1.5-8.1).

Electrophysiology could be valuable complimentary tool to identify MG patients at high risk for thymic pathology. However, it does not aid us to identify the exact thymic pathology and does not correlate with the stage of thymoma. All OMG patients with higher MJ and PAOF values should also be screened for thymoma. The results reinforce the immunological role of thymic pathology in neuromuscular transmission interference.