Is the decrement pattern in myasthenia gravis due to muscle-specific kinase antibodies different to that due to acetylcholine receptor antibodies?

A decrement on repetitive nerve stimulation (RNS) is essential for the diagnosis of myasthenia gravis (MG). The decrement pattern is typically "U-shaped" in MG caused by acetylcholine receptor antibodies (AChR-MG) but is less well described in MG caused by muscle-specific kinase antibodies (MuSK-MG). The aim of this study was to investigate RNS abnormalities in MuSK-MG, and to describe the differences in the decrement pattern as compared to AChR-MG. This retrospective case-control study included patients diagnosed with generalized MuSK-MG, compared to a control group of generalized AChR-MG. The five most frequently explored nerve-muscle pairs in RNS were analyzed: radial-anconeus, fibular nerve-tibialis anterior (TA), XI-trapezius, XII/V-submental complex (SMC), and VII-orbicularis oculi (OO). Decreased amplitude between the 1st and 4th responses (early decrement) and the late/early ratio were calculated (late/early ratio <100 %=U-shaped pattern, and ≥100 %=progressive pattern). For MuSK-MG, 25 patients were included and compared to 35 AChR-MG patients. An early decrement was present in 38/83 (54.2 %) muscles in MuSK-MG compared to 88/130 (67.7 %) muscles in AChR-MG; and in MuSK-MG was less frequently found in anconeus (4/22 [18.2 %] vs 27/31 [87.1 %], p < 0.001) and in TA (0/12 [0.0 %] vs 9/30 [30 %], p = 0.04). A progressive pattern was more frequent in MuSK-MG (19/38 [50.0 %] of muscles vs 15/88 [17.0 %], p < 0.001). The late/early ratio was greater in MuSK-MG (median value was 98.4 % [IQR, 86.8-106.8] vs 89.7 % [IQR, 79.5-96.5]). The first response with minimal amplitude during the RNS (Amin) was significantly different between the two groups (p < 0.001). Compared to AChR-MG, RNS in MuSK-MG showed fewer affected muscles, with less frequent involvement of anconeus and TA in particular; and a more progressive decrement pattern.