By Michael Rubin, MD
Professor of Clinical Neurology, Weill Cornell Medical College
SYNOPSIS: To accomplish accurate diagnosis of nerve and muscle disorders, clinical collaboration with ultrasound and magnetic resonance imaging experts is invaluable and can provide critically important information beyond electrophysiology.
SOURCES: Wijntjes J, Gerritsen J, Doorduin J, van Alfen N. Comparison of muscle ultrasound and needle electromyography findings in neuromuscular disorders. Muscle Nerve 2023; Oct 25. doi: 10.1002/mus.27989. [Online ahead of print].
Venturelli N, Tordjman M, Ammar A, et al. Contribution of muscle MRI for diagnosis of myopathy. Rev Neurol (Paris) 2023;179:61-80.
In conjunction with nerve conduction studies, needle electromyography (EMG), and tissue biopsy, neuromuscular ultrasound provides real-time anatomic and physiologic information for the evaluation and diagnosis of nerve and muscle disease. Typical ultrasound findings of pathology include muscle atrophy, increased homogeneity, loss of bone shadow, and increased echogenicity, the last being the easiest to identify when there is replacement of normal muscle fibers with fibrosis, inflammation, and fat. In a prospective case-control study of children in 2007, the investigators reported a sensitivity of 67% to 81% among childhood neuromuscular diseases, with a specificity of 84% to 92%.1 How does EMG compare to muscle ultrasound as a diagnostic tool?
In this investigation, muscle ultrasound studies, performed within two weeks of needle EMG testing between May 2017 and August 2019, were retrospectively reviewed and compared to the EMG findings of individual muscles. EMG findings were designated as myopathic if motor units were found to be of short duration, polyphasic, and low amplitude with an early recruitment pattern, or neurogenic if they were large, of long duration, and decreased in number, with neurogenic findings further qualified as showing mild, moderate, or severe reinnervation depending on the degree of reduced recruitment pattern and the amplitude and duration of motor units.
Myotonic discharges, complex repetitive discharges, other forms of spontaneous activity, or absence of motor unit recruitment were separately noted. Muscle ultrasonographers used an Esaote MyLab Twice ultrasound system, with studies visually assessed using the four-point Heckmatt grading scale, and were blinded to all clinical and laboratory results, including prior EMG and ultrasound testing. Statistical analysis compared EMG and ultrasound results using crosstabs for paired measurements with dichotomous variables, and were classified as congruent or incongruent, with significance set at P < 0.05.
Among 218 patients who underwent muscle ultrasound and EMG within a two-week period, 796 muscles were analyzed. EMG demonstrated abnormalities in 71.8%, whereas quantitative and visual muscle ultrasound were abnormal in 19.3% and 35.4%, respectively. Among those with a neurogenic EMG, quantitative and visual muscle ultrasound were abnormal in 18.9% and 35.6%, respectively, with the most pronounced denervated muscles demonstrating abnormalities in 43.7% and 87.5%, respectively. Proximal muscles demonstrated more congruity between EMG and ultrasound than distal muscles. EMG is more sensitive than muscle ultrasound, particularly where the neurogenic abnormalities are mild.
COMMENTARY
Magnetic resonance imaging (MRI) also may be used for the diagnosis of nerve and muscle disease. Ultrasound can characterize neuropathy and myopathy, but imaging requires skilled operators and, although accurate for superficial polyneuropathy, it is less so in obese patients and for deep anatomic sites, particularly in the presence of tissue distortion from prior surgery.
MRI is a more objective imaging modality, offering sharp soft tissue contrast and excellent visualization of deep structures. With neuropathy, it can distinguish focal from diffuse processes, and characterize causes including tumors, cysts, or abnormal muscle. MR neurography is particularly sensitive to subtle nerve signal alterations, swelling, contrast enhancement, fascicular anatomy, and grades of muscle denervation. MRI may be highly useful in presurgical decision-making and therapy planning.
REFERENCE
- Pillen S, Verrips A, van Alfen N, et al. Quantitative skeletal ultrasound: Diagnostic value in childhood neuromuscular disease. Neuromuscul Disord 2007;17:509-516.
