Brain Advance Access originally published online on May 3, 2006
Brain 2006 129(6):1481-1492; doi:10.1093/brain/awl095
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MRI and clinical studies of facial and bulbar muscle involvement in MuSK antibody-associated myasthenia gravis
1 Department of Clinical Neurology, University of Oxford Oxford, UK 2 Neurosciences Group, Weatherall Institute of Molecular Medicine, University of Oxford Oxford, UK 3 Centre for Functional Magnetic Resonance Imaging of the Brain, University of Oxford Oxford, UK 4 Oxford University Centre for Clinical Magnetic Resonance Research, University of Oxford Oxford, UK 5 Department of Neuroradiology, Radcliffe Infirmary Oxford, UK
Correspondence to: Prof. Angela Vincent, Neurosciences Group, Weatherall Institute of Molecular Medicine, John Radcliffe Hospital, Oxford OX3 9DS, UK E-mail: angela.vincent{at}imm.ox.ac.uk
A proportion of patients with myasthenia gravis (MG) without acetylcholine receptor (AChR) antibodies have antibodies to muscle-specific kinase (MuSK). MG with MuSK antibodies (MuSK-MG) is often associated with persistent bulbar involvement, including marked facial weakness and tongue muscle wasting. The extent of muscle wasting in MuSK-MG, and whether it is also found in the few acetylcholine receptor (AChR-MG) patients who have persistent bulbar involvement, is not clear. We studied 12 MuSK-MG patients and recruited 14 AChR-MG patients matched broadly for age, sex ratio, duration of disease and degree of ocular, bulbar and facial weakness. We used coronal and sagittal T1-weighted (T1W) and T2-weighted (T2W) magnetic resonance imaging (MRI) to assess muscle wasting in facial and tongue muscles. Hyperintense signal on T1W MRI and comparison of axial T1W sequences with cUTE sequences were used to assess fibrous/fatty tissue in the tongue. We compared the results with those of four patients with myotonic dystrophy and 12 healthy individuals. We correlated the changes with clinical and treatment histories, and established a new ocular-bulbar-facial-respiratory (OBFR) score. At the time of study, none of the clinical measures, including the OBFR score, differed between the two MG groups. MRI demonstrated thinning of the buccinator, orbicularis oris (O.oris) and orbicularis oculi (O.oculi) muscles in MuSK-MG patients compared with healthy controls, whereas thinning of these muscles was not significant in AChR-MG. Tongue areas with T1W high signal were increased in MuSK-MG patients and the intensity of the signal on axial T1W sequences was greater in MuSK-MG than in controls. To look for possible correlations between imaging and clinical findings, we pooled results from all MG patients. The duration of treatment with prednisolone at >40 mg on alternate days (AD) correlated positively with the percentage of tongue area with high signal (P = 0.006) and negatively with MRI measurements of individual muscles and with the mean muscle dimensions (P = 0.001). The new OBFR score correlated positively with current Myasthenia Gravis Foundation of America grades and with the percentage of high signal (P = 0.004) and negatively with the mean muscle dimensions (P < 0.001). The results show that bulbar and facial muscle weakness and wasting are associated with significant muscle atrophy and fatty replacement in MuSK-MG, which was not found in the AChR-MG patients. MuSK antibodies per se may predispose to muscle thinning, but the difficulties in obtaining clinical remission under steroid therapy in some patients, resulting in long duration of treatment with higher doses (>40 mg AD), may be an additional factor.
Key Words: myasthenia gravis; seronegative myasthenia gravis; muscle-specific kinase; magnetic resonance imaging; ultra-short echo time
Abbreviations: AChR, acetylcholine receptor; AD, alternate day; cUTE, conventional ultra-short echo time; MD, myotonic dystrophy; MG, myasthenia gravis; MGFA, Myasthenia Gravis Foundation of America; MRI, magnetic resonance imaging; MUAP, motor unit action potential; MuSK, muscle-specific tyrosine kinase; O.oculi and O.oris, orbicularis oculi and orbicularis oris; OBFR, oculobulbar facial respiratory score; AChR-MG, seropositive (acetylcholine receptor antibody positive) MG; T1W and T2W, T1 and T2 weighted
Received December 6, 2005. Revised March 16, 2006. Accepted March 22, 2006.
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