Quantitative EMG of facial muscles in myasthenia patients with MuSK antibodies

• Published in: Clinical neurophysiology Vol. 118; no. 2; pp. 269 - 277
• Main Authors: Farrugia, Maria E, Kennett, Robin P, Hilton-Jones, David, Newsom-Davis, John, Vincent, Angela
• Format: Journal Article
• Language: English
• Published: Shannon Elsevier Ireland Ltd 01.02.2007; Elsevier Science
• Subjects: Acetylcholine receptor; Action Potentials; Adult; Aged; Autoantibodies - blood; Biological and medical sciences; Botulinum Toxins - pharmacology; Diseases of striated muscles. Neuromuscular diseases; Electrodiagnosis. Electric activity recording; Electromyography - methods; Facial muscles; Facial Muscles - innervation; Facial Muscles - physiopathology; Female; Humans; Interference pattern; Investigative techniques, diagnostic techniques (general aspects); Male; Medical sciences; Middle Aged; Motor Neurons; Muscle atrophy; Muscle Fibers, Skeletal - immunology; Muscle Fibers, Skeletal - pathology; Muscular Atrophy - diagnosis; Muscular Atrophy - immunology; Muscular Atrophy - physiopathology; Muscular Dystrophies - diagnosis; Muscular Dystrophies - physiopathology; MuSK; Myasthenia gravis; Myasthenia Gravis - diagnosis; Myasthenia Gravis - immunology; Myasthenia Gravis - physiopathology; Nervous system; Neurology; Neuromuscular Junction - physiopathology; Predictive Value of Tests; Receptor Protein-Tyrosine Kinases - immunology; Receptors, Cholinergic - immunology; Receptors, Nicotinic - immunology; Turns amplitude analysis; Type II muscle fibre; MG, myasthenia gravis; CI, confidence interval; AChR-MG, myasthenia gravis associated with antibodies to AChR; MuSK; Turns amplitude analysis; MUAP, motor unit action potential; MD, muscular dystrophy; MGFA, myasthenia gravis Foundation of America score of disease; Myasthenia gravis; NMJ, neuromuscular junction; SFEMG, single fibre EMG; MuSK, muscle specific tyrosine kinase; AChR, acetylcholine receptor; Botox, botulinum toxin; SD, standard deviation; FSH MD, facioscapulohumeral muscular dystrophy; MuSK-MG, myasthenia gravis associated with antibodies to MuSK; MyoD, myotonic dystrophy; Muscle atrophy; MA, mean change in amplitude per turn; Interference pattern; EDC, extensor digitorum communis; O oculi, orbicularis oculi; IP, interference pattern; T/s, turns per second; MRI, magnetic resonance imaging; O oris, orbicularis oris; Log, logarithm; TAA, turns amplitude analysis; MuRF-1, muscle ring finger 1; CNE, concentric needle electrode; Hz, Hertz; Facial muscles; Acetylcholine receptor; EMG, electromyography; Type II muscle fibre; Orbicularis oris muscle; Electrophysiology; Metalloendopeptidases; Action potential; Motor unit; Atrophy; Bontoxilysin; Cholinergic receptor; Electromyography; Pattern analysis; Human; Neuromuscular diseases; Nervous system diseases; Enzyme; Interference; Orbicularis oculi muscle; Peptidases; Treatment; Needle electrode; Hydrolases; Models; Dystrophy
• ISSN: 1388-2457; 1872-8952
• DOI: 10.1016/j.clinph.2006.10.004

Abstract Objective Our aim was to study the pathophysiological process leading to facial muscle atrophy in 13 patients with MuSK antibody positive myasthenia gravis (MuSK-MG), and to compare with findings from 12 acetylcholine receptor antibody positive myasthenia patients (AChR-MG), selected because they suffered from the same degree of disease severity and required similar treatment. Methods Motor unit action potential (MUAP) and interference pattern analysis from orbicularis oculi (O oculi) and orbicularis oris (O oris) muscles were studied using a concentric needle electrode, and compared with findings in 20 normal subjects, 6 patients receiving botulinum toxin injections (representing a neurogenic model) and 6 patients with a muscle dystrophy (representing a myopathic model). The techniques and control data have been reported previously. Results The mean MUAP durations for O oculi and O oris were significantly reduced ( p < 0.001) in both MG cohorts when compared with healthy subjects, and were similar to those in the myopathic control group. They were significantly different from those obtained from the neurogenic control group ( p < 0.001 for both O oculi and O oris). The MUAP findings in O oculi occurred independently from neuromuscular blocking on single fibre EMG (SFEMG) in the same muscle. On turns amplitude analysis (TAA), 50% of MuSK-MG patients and 42% of AChR-MG patients had a pattern in O oculi which was similar to that in the myopathic control group, and 62% of MuSK-MG patients and 50% of AChR-MG patients had a pattern in O oris that was also similar to that in the myopathic control group. The TAA findings for O oculi and O oris in both MG cohorts were different from those obtained from the neurogenic control group. Conclusions Facial muscle atrophy in MuSK-MG patients is not neurogenic and the pathophysiological changes are akin to a myopathic process. The selected AChR-MG patients also show evidence of a similar pathophysiological process in the facial muscles albeit to a lesser degree. Significance We propose that muscle atrophy in MuSK-MG is a myopathic process consisting of either muscle fibre shrinkage or loss of muscle fibres from motor units. The duration of disease and long-term steroid treatment may be further contributory factors.