Diagnostic Techniques in Neuromuscular Disease
By Jonathan Katz, M.D., director, Neuromuscular Program, Forbes Norris MDA/ALS Research and Treatment Center
Electromyography (EMG) and nerve conduction studies (NCS) are common diagnostic procedures in neuromuscular disease. Recent literature has provided useful information on diagnosis and prognosis in common entrapments, ultrasound of muscle, and the risk of EMG in anticoagulated patients.
One interesting study looked at the ability of general practitioners to diagnose carpal tunnel syndrome (CTS) at the bedside (1): 232 patients referred by general practitioners (GP) for nerve conduction studies with suspected CTS were followed through subsequent appointments with the neurologist and then with nerve conduction testing. In 187 patients (81%), the neurologist agreed with the diagnosis. The neurologist disagreed in the other 45 (19%), with the most common reason being no neurological problem. When the clinical impressions were put to the test by NCS, the neurologist’s clinical diagnosis actually turned out to be correct in 180/187 of the suspected cases and in all 45 cases where they suspected that there was no CTS. The authors concluded that clinical pathways should probably allow direct referrals from GPs for NCS, given the high accuracy of the clinical diagnosis. These findings differ somewhat from the American Academy of Neurology (AAN) website, which suggests direct referrals to neurologists would lead to a 25% reduction in diagnostic testing. The work provides useful information for centers that wish to develop clinical pathways.
Electrodiagnostic testing is often used to prognosticate in simple entrapments. One method has been to use algorithms that rely on the degree of axonal loss, slowing, or conduction block with focal demyelinating injury to predict outcomes. Prior work has found that the first dorsal interosseus muscle (FDI) and the abducator digiti minimi (ADM) can be differentially affected in ulnar neuropathy and may provide different information about outcomes. In one study, the prognostic value of electrodiagnostic studies for ulnar neuropathy at the elbow (UNE) was determined by comparing specific findings with the subsequent likelihood of resolution of symptoms after surgery(2); 59 patients with “definite” UNE were included in the analysis. The authors found that the combination of conduction block across the elbow to FDI with normal distal compound muscle action potential (CMAP) amplitude in ADM was strongly associated with full recovery (86%). This compared to only 7% if there was no conduction block (only slowing) and low amplitude CMAP. The correlation of outcome with conduction block (CB) makes intuitive sense, but it is unclear why ADM and FDI were not both associated with outcome. This may have something to do with the location of fibers within the nerve. Not surprisingly, there were no clear predictors of who went to surgery, suggesting a need for developing clear protocols to help with this decision.
Another study finally addressed the risk of doing EMG in patients using Coumadin or antiplatelet agents (3). The authors did ultrasound immediately following the needle study, looking for hematoma formation in “high risk” muscles (paraspinals, flexor pollicis longus, flexor digitorum longus, posterior tibialis, and iliopsoas). At least 100 muscles were examined in patients who had undergone the testing on Coumadin, antiplatelet agents, or placebo. Only two small hematomas were detected and there were no clinical complaints, suggesting the risk is very low.
Multifocal Motor Neuropathy
From the more technical side, “activity-dependent CB” is a concept mentioned to explain weakness in patients with multifocal motor neuropathy (MMN) who have no evidence of focal conduction block on testing. There may be inducible focal abnormalities in nerve membrane channels that appear only with tetanic stimulation of nerve or exercise. Similar to certain subatomic particles, the idea is theoretical, and a clear example has never been observed. Here, a Dutch group performed detailed NCS in 22 nerve segments of 19 MMN patients, before and immediately after 60 seconds of maximal voluntary contraction (MVC) of the relevant muscle (4). Sadly, MVC induced no block or changes in CMAP. The work suggests that we should consider other causes of weakness in these patients, such as autoimmune lesions that focally induce axonal loss without affecting conduction or very proximal focal nerve lesions that are difficult to find on routine NCS.
Finally, developing noninvasive techniques for studying muscle may eventually offer advantages over EMG. In one report, ultrasonography of muscle was used to look at muscle thickness, echo intensity, and fasciculations in 31 ALS patients. Diagnostic information could be gleaned from studying echo intensity, thickness, and fasciculations (5). The study of electrical impedance myography (EIM) is also gaining ground. This is a noninvasive procedure where the basic approach is applying an electrical current and measuring the resulting voltages across a region of muscle tissue via surface electrodes (6). EIM is based on the premise that changes in the microscopic structure of tissue such as hypertrophy or atrophy, muscle fibrosis and fatty infiltration, and increased free water will alter impedance data. These techniques are being evaluated as a biomarker in a variety of neuromuscular diseases, ranging from amyotrophic lateral sclerosis to inflammatory myositis.
- Claes F, Bernsen H, Meulstee J, Verhagen WI. Carpal tunnel syndrome diagnosed by general practitioners: an observational study. Neurol Sci. 2011 Dec 24.
- Friedrich JM, Robinson LR. Prognostic indicators from electrodiagnostic studies for ulnar neuropathy at the elbow. Muscle Nerve. 2011 Apr; 43 (4):596-600.
- Boon AJ, Gertken JT, Watson JC, Laughlin RS, Strommen JA, Mauermann ML, Sorenson EJ. Hematoma risk after needle electromyography. Muscle Nerve. 2012 Jan; 45 (1):9-12.
- Straver DC, van den Berg LH, van den Berg-Vos RM, Franssen H. Activity-dependent conduction block in multifocal motor neuropathy. Muscle Nerve. 2011 Jan; 43 (1):31-6.
- Arts IM, Overeem S, Pillen S, Schelhaas HJ, Zwarts MJ. Muscle changes in amyotrophic lateral sclerosis: a longitudinal ultrasonography study. Clin Neurophysiol. 2011 Mar;122 (3):623-8.
- Rutkove SB. Electrical Impedance Myography: Background, Current State, and Future Directions. Muscle Nerve 2009, 40, 936–946
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Common Conditions for EMG Testing
- Brachial and lumbar plexopathies
- Carpal tunnel and ulnar neuropathies or cubital tunnel syndrome
- Cranial nerve disorders
- Motor neuron disease
- Muscle cramps or hyperexcitable nerve syndrome
- Myopathies (congenital and acquired)
- Neuromuscular junction disorders (e.g., congenital and acquired myasthenia gravis, Lambert-Eaton myasthenic syndrome)
- Nerve terminal disorders
- Peripheral neuropathy
- Pediatric muscular dystrophies, inherited neuropathies, metabolic myopathies, congenital myasthenia syndromes