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Brain Advance Access published online on May 3, 2006
Brain, doi:10.1093/brain/awl095
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© The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org. The online version of this article has been published under an open access model. Users are entitled to use, reproduce, disseminate, or display the open access version of this article for non-commercial purposes provided that: the original authorship is properly and fully attributed; the Journal and Oxford University Press are attributed as the original place of publication with the correct citation details given; if an article is subsequently reproduced or disseminated not in its entirety but only in part or as a derivative work this must be clearly indicated. For commercial re-use, please contact journals.permissions@oxfordjournals.org
Received December 6, 2005
Revised March 16, 2006
Accepted March 22, 2006

Article

MRI and clinical studies of facial and bulbar muscle involvement in MuSK antibody-associated myasthenia gravis

Maria Elena Farrugia 1, Matthew D. Robson 2, Linda Clover 3, Phil Anslow 4, John Newsom-Davis 5, Robin Kennett 5, David Hilton-Jones 5, Paul M. Matthews 6, and Angela Vincent 1 *

1 Department of Clinical Neurology, Weatherall Institute of Molecular Medicine, Oxford, UK; Neurosciences Group, Weatherall Institute of Molecular Medicine, Oxford, UK
2 Oxford University Centre for Clinical Magnetic Resonance Research, University of Oxford, Oxford, UK
3 Neurosciences Group, Weatherall Institute of Molecular Medicine, Oxford, UK
4 Department of Neuroradiology, Radcliffe Infirmary, Oxford, UK
5 Department of Clinical Neurology, Weatherall Institute of Molecular Medicine, Oxford, UK
6 Department of Clinical Neurology, Weatherall Institute of Molecular Medicine, Oxford, UK; Centre for Functional Magnetic Resonance Imaging of the Brain, University of Oxford, Oxford, UK

* To whom correspondence should be addressed.
Angela Vincent, E-mail: angela.vincent{at}imm.ox.ac.uk


  Abstract

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.

Keywords: myasthenia gravis; seronegative myasthenia gravis; muscle-specific kinase; magnetic resonance imaging; ultra-short echo time.
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