Motor end-plate (MEP) analysis of muscle biopsies is much more effective in diagnosing immune-mediated myasthenia gravis (MG) for seronegative patients with myasthenia-like symptoms, a new study published in Elsevier’s Journal of the Neurological Sciences found.

The definitive diagnosis of MG is difficult as patients complaining about MG-like symptoms are seronegative for the routine acetylcholine receptor (AChR) and muscle-specific kinase (MuSK) antibody tests and display no significant waning phenomenon with the repetitive nerve stimulation and evasive responses to the intravenous edrophonium (Tensilon) test, the researchers said.

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Prior studies have reported that nearly 50% of patients are negative for routine antibodies, and around 70% of patients display no waning phenomenon on the repetitive nerve stimulation, particularly in ocular MG, the authors noted.

The research team formulated that complement deposition and reduced AChR densities at the MEP would be found if a patient is experiencing MG. They also looked for antibodies related to AChR only, clustered on the cell surface as found at the neuromuscular junction, and those which can be found in about 50% of patients with negative routine tests. Furthermore, they examined the clinical course data of all the previously seronegative patients to certify their final diagnoses.

The study included 20 seronegative patients with myasthenia-like symptoms, including 13 females and seven males with an average age of 49 years at the symptom onset, and a disease duration of 20 months. They defined myasthenia-like symptoms as at least ocular muscle weakness in line with muscular fatigue. Moreover, muscle biopsies were discussed with patients meeting the inclusion criteria of having myasthenia-like symptoms, seronegative for routine AChR and MuSK antibody tests, and <10% of the waning phenomenon with repetitive nerve stimulation.

Study results indicated that 5 out of 20 seronegative patients remained positive for complement C3 deposition in MEPs, 4 of which had lowered AChR densities, and 2 (of the four) experienced clustered-AChR Abs. The remaining 15 patients reported no complement C3 deposition, no reduction in AChR density, and no clustered-AChR antibodies (Abs). In particular, 14 seronegative patients reported having other diseases in follow-up over the ten years. 

The above results suggested the advantage of MEP and Cell-based assays for clustered AChR Abs in diagnosing immune-mediated MG in patients with myasthenia-like symptoms but negative routine AChR Abs. The researchers also noticed that complement deposition differentiated the patients with MG with ocular or generalized MG from those patients who received other diagnoses. 

To summarize, this study found that MEP analysis of muscle biopsies helps diagnose immune-mediated MG for seronegative patients exhibiting myasthenia-like symptoms.

“Ideally, clustered-AChR Ab assays, if available, should be used first, and muscle biopsies only performed when negative Ab results are found and the diagnosis remains in doubt,” the authors commented.

Reference

Nagaoka A, Tsujino A, Shiraishi H, et al. Motor end-plate analysis to diagnose immune-mediated myasthenia gravis in seronegative patients. J Neurol Sci. Published online December 15, 2022. doi: 10.1016/j.jns.2022.120494