Electrodiagnostic Update 2004: Carpal Tunnel Syndrome

Source: Witt JC, et al. Carpal tunnel syndrome with normal, nerve conduction studies. Muscle Nerve. 2004;29:515-522.

How valuable are nerve conduction studies (NCS) in the diagnosis and management of carpal tunnel syndrome (CTS)? Over a 7-month period, patients who both met CTS diagnostic criteria and were referred to the Mayo Clinic Electromyography (EMG) laboratory for suspected CTS, were voluntarily enrolled in a study designed to address this question. Criteria included 1) hand paresthesiae in a median and/or ulnar nerve distribution which were 2) aggravated by activity; 3) relieved by shaking; and 4) awoke the patient from sleep; 5) subjective hand weakness; 6) clumsiness; or (7) Tinel or Phalen sign. For enrollment, patients had to have criteria #1 and at least 1 (possible CTS) or 2 (definite CTS) other criteria. Patients with confounding neurological conditions were excluded, including prior CTS surgery or other potential cause for paresthesiae (eg, radiculopathy, polyneuropathy, ulnar neuropathy, myelopathy, stroke). All patients underwent electrodiagnostic studies encompassing NCS of the median and ulnar motor and sensory nerves, performed using standard technique, as well as needle EMG study of relevant muscles. Statistical analysis included the Pearson chi-square test, Fisher exact test, two-sample t-test, one-way analysis of variance, and multiple logistic regression analysis.

Among 99 eligible CTS patients, NCS were abnormal in 74 and normal in 25 (25%). Only 84 agreed to participate, including 60 of 74 normals and 24 of 25 abnormals. Patients categorized as definite CTS were more likely to have abnormal NCS compared to the possible CTS group (78% vs 47%) but, in both groups, the frequency of mild, moderate, or severe NCS abnormalities was similar. Normal NCS were more likely in patients with bilateral symptoms (83% vs 57%; P = 0.02), while patients with abnormal NCS were more likely to be older (P < 0.001), heavier (P = 0.015), and have higher body mass index (P = 0.002). Treatment outcome, whether conservative or surgical, did not differ between the definite and possible groups and, similarly, nonsurgical outcome did not differ between the normal or abnormal NCS groups. NCS are of diagnostic value, particularly when clinical clues are scarce, but they are not predictive of nonsurgical treatment outcome.

Ulnar Neuropathy

Source: Shakir A, et al. Which motor nerve conduction study is best in ulnar neuropathy at the elbow. Muscle Nerve. 2004;29:585-590.

Ulnar neuropathy at the elbow (UNE) is often difficult to localize electrodiagnostically. Slowing of ulnar motor nerve conduction velocity (MNCV) across the elbow provides supportive evidence, but controversy remains on how best to determine its presence. Two methods include comparing the across-elbow ulnar MNCV to that of an adjacent segment of the ulnar motor nerve (either the forearm or above-elbow segment) or comparing the across-elbow MNCV to an absolute normal standard. Which yields optimal results?

Receiver operator characteristic (ROC) curves, a statistical tool which graphically depicts the relationship between sensitivity and specificity of a measurement as a function of normal cut-off values, were used in this study to compare the 2 methods for determining across-elbow slowing. In a retrospective 8-year review of ulnar MNCS performed for suspected neuropathy in an arm (n = 1703), UNE was the referring diagnosis in 283 and the final diagnosis in 165. Of these, 99 had ulnar MNCS with recording from both the first doral interosseous and the abductor digiti minimi muscles, of which 85 yielded a recorded response from both muscles and were not repeat studies. These 85 served as the true-positive UNE group and were compared to 77 carpal tunnel syndrome patients without UNE who had similar electrodiagnostic studies. NCS were performed in the standard fashion, ROC curves were plotted as sensitivity against 1, minus specificity for all cut-off points, and 95% confidence intervals were calculated.

Across-elbow MNCV, as an absolute measure, was significantly more sensitive in determining the presence of UNE than comparing the across-elbow segment to an adjacent segment. This was true both when recording from the first doral interosseous (sensitivity, 80% vs 51%) and the abductor digiti minimi (sensitivity, 77% vs 38%), and was even more pronounced, though not significantly so, when ulnar evoked compound muscle potential amplitudes were very low. Across-elbow ulnar MNCV, compared to a normal standard, is preferred to adjacent segment comparison for diagnosing UNE.

Demyelinating Polyneuropathy

Source: Van Den Bergh PYK, et al. Electrodiagnostic criteria for acute and chronic inflammatory demyelinating polyradiculoneuropathy. Muscle Nerve. 2004;29:565-574.

Acquired demyelinating neuropathies are eminently treatable. Timely diagnosis has significant ramifications on patient management and outcome. Both intravenous immune globulin and plasma exchange are effective in the appropriate setting but costly, occasionally requiring in-patient hospitalization if only to satisfy third-party payers. Accurate diagnosis of demyelination is crucial to the patient, doctor, and insurance carrier.

Ten sets of published criteria for demyelinating neuropathy were compared, retrospectively, in 53 Guillain-Barre syndrome (GBS) and 28 chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) patients to establish an 11th set of criteria with greatest specificity, without significant loss of sensitivity. Patients with amyotrophic lateral sclerosis (ALS) and diabetic polyneuropathy (DPN) served as disease control groups. Diagnosis of GBS and CIDP was based on published clinical criteria, including cerebrospinal fluid analysis and response to immune therapy in most. Exclusionary criteria included drug or toxin exposure, family history of neuropathy, HIV infection, lupus, gammopathy, or central nervous system demyelinating disease. ALS patients all satisfied Airlie House criteria (revised El Escorial criteria) and DPN patients had long-standing diabetes mellitus with chronic symmetric sensorimotor polyneuropathy. Nerve conduction studies were performed using standard technique. Statistical analysis included chi-square 4-fold tables and calculation of Pearson’s goodness-of-fit chi-square with P-values for all sets of criteria.

For the 10 published sets of criteria, sensitivity ranged from 24% to 83% for GBS and 39% to 89% for CIDP. Specificity for 9 of the sets completely excluded ALS, but up to 9% of DPN patients satisfied demyelinating criteria in half the sets. As little as 20% motor conduction block (drop of amplitude on proximal compared to distal stimulation) was specific for GBS and CIDP, being present in at least 1 nerve in 70% and 93%, respectively. Increased temporal dispersion was specific but of low sensitivity.

Based on these findings a new set of criteria is proposed:

1. Motor distal latency prolonged by > 150% above upper limit of normal;

2. Motor velocity slowed to < 70% below lower limit of normal;

3. F wave latency prolonged by > 125% above upper limit of normal if motor amplitude is > 80% of normal, or > 150% if motor amplitude is < 80% of normal;

4. Abnormal temporal dispersion by > 30% in at least 2 nerves.

Compared to the original 10 sets of criteria, GBS and CIDP were diagnosed with 72% and 75% sensitivity, respectively, with 100% specificity in differentiating them from DPN or ALS. Validation of these results in other databases is warranted and awaited.

Myasthenia Gravis

Source: Rubin DI, et al. Trigeminal nerve repetitive stimulation in myasthenia gravis. Muscle Nerve. 2004;29:591-596.

A 10% decrement or more, of the compound muscle action potential amplitude (CMAP) on repetitive nerve stimulation (RNS), is the electrodiagnostic hallmark of myasthenia gravis (MG). Positivity of response varies from 41% to 95%, with higher positivity correlating not surprisingly with more muscles being tested. Distal muscles are less sensitive but technically more reliable than proximal muscles. Bulbar muscles, being most proximal, should provide the highest sensitivity and, in bulbar myasthenia, may be the only muscles to yield a decremental response. Facial nerve testing in generalized MG is positive in 62%. What about trigeminal nerve RNS?

Twenty-one MG patients, 2 with ocular involvement and 19 with weakness affecting other than ocular muscles, were compared to 26 normal controls. All underwent 2 Hz RNS of the trigeminal nerve, using percutaneous monopolar needle stimulation of the masseteric branch, with surface recording of the CMAP over the masseter muscle. MG patients additionally underwent transcutaneous ulnar, spinal accessory, and facial nerve RNS, recording from the abductor digiti minimi, trapezius, and nasalis muscles, respectively. 2 Hz stimulation was used, rather than 3-4 Hz, due to its relative comfort level, preferable artifact profile, and technical ease. RNS was thrice repeated at rest to ensure reproducibility, and repeated immediately after exercise and at 1, 2, and 3 minutes.

Trigeminal RNS was positive in 43% of MG patients compared to 57% of facial, 20% of ulnar, and 48% of spinal accessory RNS. Trigeminal RNS was better tolerated than facial RNS in all patients. Though less sensitive, trigeminal RNS may be added to the electrodiagnostic armamentarium in the evaluation of patients suspected of MG.

Quantitative Sensory Testing

Source: Chong PS, et al. AAEM Practice Topic in Electrodiagnostic Medicine. Technology literature review: quantitative sensory testing. Muscle Nerve. 2004;29:734-747.

Quantitative sensory testing (QST) non-invasively measures function of both large diameter nerve fibers (Aa and Ab) mediating light touch and vibration, and small diameter nerve fibers (Ad and C) mediating temperature and pain sensation. QST is often used in peripheral neuropathy clinical drug trials as a method of documenting progression or improvement, and its use in clinical practice is increasing as well. Yet, as a psychophysical test, it requires an honest, alert, and cooperative patient for results to be meaningful. No method exists to reliably differentiate subjects who wish to manufacture or magnify their impairment, although variability on repeat testing is likely to be greater in this group. How the stimulus is presented to the patient, how the subject’s response is obtained (yes-no or forced-choice method), and how the response is used to determine sensory threshold vary between sites, investigators, and computers used. Reproducibility varies among studies reported and is better among normal persons than in neuropathy patients. Given these issues, just how useful is QST?

Based on a Medline search and review of the literature from 1966 to 2001 dealing with this topic, the following conclusions are drawn: QST is a reliable psychophysical test of peripheral large and small nerve fibers, testing the entire sensory neuraxis from periphery to brain without localization value, and requiring complete patient cooperation and integrity. Reproducibility appears to be satisfactory during short-term studies, cold threshold testing moreso than warm threshold, and vibration perception threshold testing moreso than thermal. However, strict calibration and testing protocols must be followed. No one instrument may be recommended over another based on the literature, and the degree of allowable experimental error remains unknown for the individual patient. — Michael Rubin

Dr. Rubin, Professor of Clinical Neurology, New York Presbyterian Hospital-Cornell Campus, is Assistant Editor of Neurology Alert.