Cervical nerve root stimulation. Part II: findings in primary demyelinating neuropathies and motor neuron disease.

OBJECTIVE: Cervical nerve root stimulation (CRS) allows the assessment of conduction in the proximal segments of motor fibers destined to the upper extremities, which are not evaluated by routine nerve conduction studies (NCS). Since many primary demyelinating polyneuropathies (PDP) are multifocal lesions may be confined to the proximal nerve segments. CRS may therefore increase the yield of neurophysiologic studies in diagnosing PDP. METHODS: We reviewed clinical and neurophysiologic data from 38 PDP patients and compared them to 35 patients with motor neuron disease (MND), and 21 healthy controls (HC). RESULTS: Mean onset-latency was significantly prolonged in PDP patients. The optimal onset-latency cutoff necessary to distinguish PDP from MND and controls was 17.5 ms for the abductor pollicis brevis (APB) and abductor digiti minimi (ADM), and 7 ms for Biceps and Triceps. Mean reduction in proximal to distal CMAP amplitude to APB and ADM was significantly greater in PDP patients, with an optimal cutoff in proximal to distal CMAP amplitude reduction necessary to distinguish PDP from MND and HC being 45%. CONCLUSIONS: CRS is effective in distinguishing PDP from MND and HC based on prolonged onset latency and conduction block criteria. SIGNIFICANCE: CRS may increase the diagnostic yield in cases where demyelinating lesions are confined to the proximal peripheral neuraxis.     

Phrenic nerve conduction studies: technical aspects and normative data

In our clinical work we have occasionally encountered difficulties (e.g., no response, concomitant brachial plexus stimulation) in performing phrenic nerve conduction studies. The aim of this study was to overcome these difficulties and obtain our own set of normative data. In 29 healthy volunteers (15 men), aged 21-65 years, phrenic nerve conduction studies were performed using bipolar surface stimulation electrodes and a standard recording montage. Stimulation just above the clavicle, between the sternal and clavicular heads of the sternocleidomastoid muscles, elicited responses at the lowest stimulation strength, without concomitant brachial plexus stimulation. M-wave amplitude and duration changed with respiration, whereas latency and area did not. The normative limit for M-wave latency was 8.0 ms (upper), for amplitude it was 0.46/0.33 mV (lower: inspiration/expiration), and for area it was 4.4 mVms (lower). We suggest a slight modification of the generally used position for phrenic nerve stimulation, and the use of M-wave latency and area (unaffected by the respiratory cycle) in future phrenic nerve conduction studies.     

Cervical root stimulation in the diagnosis of radiculopathy

Cervical root stimulation (CRS) was compared with conventional EMG, nerve conduction, and late response studies in 34 patients with possible cervical radiculopathy. Cervical roots were stimulated by monopolar needles inserted into paraspinal muscles, recording compound muscle action potentials in biceps, triceps, and abductor digiti minimi muscles. In 18 patients with clinical evidence of radiculopathy, EMG was abnormal in 11 (61%), but CRS was abnormal in all 18. Of 16 patients with symptoms but no signs of radiculopathy, EMG was abnormal in 5 (31%) and CRS was abnormal in 9 (56%).     

A guide to benzodiazepine selection. Part I: Pharmacological aspects

Since absorption rates, volumes of distribution and elimination rates differ greatly among the benzodiazepine derivatives, each benzodiazepine has a unique plasma concentration curve. Although the time to peak plasma levels provides a rough guide, it is not equivalent to the time to clinical onset of effect. Two half-lives can be described: the alpha half-life, the rate of decline in plasma concentrations due to the process of drug redistribution from the central to the peripheral compartment, and the beta half-life, the rate of decline due to the process of drug elimination due to metabolism. The frequent classification of benzodiazepines into long, intermediate, and short-"acting" categories based on their terminal beta half-lives is unfounded; the duration of action is much more dependent on the alpha half-life. Benzodiazepines with short beta half-lives are commonly thought to be preferable because they accumulate less. However, with repeated use, sedation and cognitive neuromotor impairment usually diminish progressively despite stable or even rising benzodiazepine plasma concentrations, whereas anxiolytic effects generally persist over time.     

Improved methodology for lumbosacral nerve root stimulation

The site of S1–S2 root activation following percutaneous high-voltage electrical (ES) and magnetic stimulation were located by analyzing the variations of the time interval from M to H soleus responses elicited by moving the stimulus point from lumbar to low thoracic levels. ES was effective in activating S1–S2 roots at their origin. However, supramaximal motor root stimulation required a dorsoventral montage, the anode being a large, circular surface electrode placed ventrally, midline between the apex of the xiphoid process and the umbilicus. Responses to magnetic stimuli always resulted from the activation of a fraction of the fiber pool, sometimes limited to the low-thresholds afferent component, near its exit from the intervertebral foramina, or even more distally. Normal values for conduction velocity in motor and 1a afferent fibers in the proximal nerve tract are provided. © 1996 John Wiley & Sons, Inc.     

Thoracic spinal nerve and root conduction: A magnetic stimulation study

We describe a technique of magnetic coil (MC) stimulation of the thoracic spinal nerves and roots in 12 normal subjects and a patient with diabetes mellitus. We kept the MC flat against the vertebral column in the midline over T-7, T-8, and T-9 spinous processes and obtained compound muscle action potentials from the upper rectus abdominis, external oblique, and intercostal muscles. We obtained mean latencies to these muscles after stimulation in the posterior axillary line. We noted that the onset latencies remained fixed despite increasing the intensity of stimulation from 30% to 100% and on moving the coil up to 3 cm lateral to the spinous processes suggesting that the stimulation of the fastest conducting fibers was occurring at a fixed site, most likely at the intervertebral foramina. Prolonged latencies in the diabetic patient confirmed the diagnosis of radiculoneuropathy. © 1995 John Wiley & Sons, Inc.     

Technical and practical aspects of magnetic nerve stimulation

Magnetic stimulators consist of a high-voltage (400 V to more than 3 kV), high-current (4 kA to more than 20 kA) discharge system producing a brief pulse of magnetic field (1-10 T lasting up to a few milliseconds). In order to ensure safety and reliability, care has to be taken in the construction of magnetic stimulators and especially their stimulating coils, which may come into contact with the patient, by the use of adequate mechanical and electrical insulation and the incorporation of sensors to monitor temperature, voltage, etc. Where possible, equipment should be manufactured to comply with safety standards such as UL504 and IEC601. The effectiveness of a magnetic stimulator depends on many parameters such as the maximization of the peak coil energy, fast magnetic field rise times, and good coil design. It is also important to choose a coil based on its intended clinical application and not simply on account of its high magnetic field strength. A small coil, producing a high surface magnetic field, is suitable in the stimulation of superficial nerves, whereas a larger coil, with a low magnetic field intensity, may well be more suitable for the stimulation of deep nerves. Double, figure-eight, or butterfly coils produce more localised induced currents allowing for more selective stimulation. The exposure of the brain to high magnetic field strengths can also be reduced by using larger coils.     

Cutaneous nerve stimulation and motoneuronal excitability: I, soleus and tibialis anterior excitability after ipsilateral and contralateral sural nerve stimulation.

Modification of soleus and anterior tibial anterior horn cell excitability following ipsilateral and contralateral stimulations of the sural nerve was studied by either the H reflex (for the soleus and anterior tibial muscles) or the F response (for the anterior tibial muscles). Several intensities of stimulation were employed. In every instance the recovery curves showed two distinct peaks of facilitation, which appeared with the same delay in muscles with antagonist functions. Also, reciprocal facilitation and inhibition phenomena which occurred after a 25 ms delay and which lasted more than 1000 ms were observed. The intervention of suprasegmentary neuronal mechanisms is proposed to explain the facilitation peaks, while the longer lasting phenomena are probably dependent on spinal processes.     

Cervical root stimulation in a case of classic neurogenic thoracic outlet syndrome.

We performed C8 nerve root stimulation in addition to other electromyographic (EMG) studies in a surgically proven case of classic thoracic outlet syndrome (TOS). The patient was a 19-year-old woman with a 2-year history of right hand cramps and progressive weakness and atrophy of hand muscles, especially the thenar eminence. Routine EMG studies showed evidence for an axon-loss lower trunk brachial plexopathy. Stimulation studies of the C8 nerve roots demonstrated proximal conduction block on the affected side only. The diagnosis was further supported by cervical spine radiographs, which demonstrated a cervical rib, and surgical exploration of the brachial plexus, which demonstrated upward compression and stretching of the lower trunk by a fascial band extending from the anomalous cervical rib to the first thoracic rib. The patient noted a modest improvement in hand function postoperatively. Root stimulation studies can help in the diagnosis of classic TOS by providing more precise localization and information regarding the degree, if any, of proximal motor conduction block.     

Soleus H-reflex to S1 nerve root stimulation

H-reflexes in normals were elicited by percutaneous electrical and magnetic stimulation of proximal nerve roots at the cauda equina. H-M interval to S1 nerve root stimulation at the level of the S1 foramen was 6.8 +/- 0.33 ms, with side to side difference of 0.16 +/- 0.13 ms. Compression/ischemia of the sciatic nerve in the mid-thigh abolished the H-reflex to stimulation of the tibial nerve at the popliteal fossa when the H-reflex to S1 nerve root stimulation was preserved. The length of the S1 nerve root in human cadavers was measured to be 17.5 +/- 03 cm, providing an estimated dorsal root conduction velocity of 67.3 m/s and a ventral root conduction velocity of 54 m/s. We conclude that the H-M interval to S1 root stimulation can provide reliable measures of conduction within the spinal canal including proximal afferents, anterior horn cells and ventral roots.     

Glial hyperpolarization upon nerve root stimulation in the leech Hirudo medicinalis.

Hyperpolarizing responses in neuropil glial cells evoked by nerve root stimulation were studied in the central nervous system of the leech Hirudo medicinalis using intracellular recording and extracellular stimulation techniques. From a mean resting potential of -60.5 +/- 1.0, the glial membrane was hyperpolarized by -8.6 +/- 0.8 mV, via stimulation of the dorsal posterior nerve root in an isolated ganglion. Nerve root stimulation evoked biphasic or depolarizing responses in glial cells with resting potentials around -70 mV (Rose CR, Deitmer JW. J. Neurophysiol. 73:125-131, 1995). The hyperpolarizing response was reduced by the ionotropic glutamate receptor antagonist CNQX (50 microM) to 58% of its initial amplitude. In 15 mM Ca2+/15 mM Mg(2+)-saline the hyperpolarization was reduced by 44%. The hyperpolarization that persisted in high-divalent cation saline was not affected by CNQX. Bath-applied glutamate (500 microM) and kainate (2 microM) elicited glial hyperpolarizations that were sensitive to CNQX and 10 mM Mg2+/1 mM Ca(2+)-saline. The 5-HT-antagonist methysergide did not affect the hyperpolarizations evoked by nerve root stimulation. The results show that in the leech glial membrane responses to neuronal activity include not only depolarizations, as shown previously, but also hyperpolarizations, which are mediated by direct and indirect neuron-glial communication pathways. In the indirect pathway, glutamate is a transmitter between neurons.     

Lumbosacral nerve root stimulation comparing electrical with surface magnetic coil techniques.

Stimulation of lumbosacral nerve roots using a monopolar needle electrode was compared with magnetic stimulation using a 7-cm diameter surface coil. Compound muscle action potentials were recorded from the tibialis anterior (TA) and flexor hallucis brevis (FHB) muscles. Although the mean latency of CMAPs did not differ using the two techniques, amplitudes were considerably larger using a needle. Mean amplitudes were 66% (TA) and 64% (FHB) of the direct M response obtained by distal, supramaximal stimulation compared with mean values using maximal magnetic coil stimulation of 36% (TA) and 25% (FHB). Minimum F-wave latencies from FHB were used to estimate the site of nerve root stimulation using both techniques. Although there was a large amount of variability in the data from individual subjects, the results suggested that, on the average, both forms of stimulation act proximal to the intervertebral foramen. We conclude that a needle electrode is a more suitable technique for stimulating lumbosacral nerve roots.     

Ambulatory polysomnography: technical aspects and normative values.

Ambulatory polysomnographic (APSG) assessment of sleep disorders is now possible, but the technique of APSG is sufficiently different from in-laboratory PSG that normative data from in-laboratory PSG may not apply to APSG. This paper reviews the technical aspects of APSG and presents normative APSG data from 20 older healthy males. Subjects underwent medical and psychiatric screening before completing APSG in their homes. Total sleep time and the rapid-eye-movement sleep latency (RL) were both shorter than those reported by others using traditional in-laboratory techniques. The shorter total sleep may be related to behaviors at home that impinge upon sleep. The shorter RL may be related to differences in calculation methods. Periodic limb movements were common in our subjects but did not contribute to sleep disturbance. We conclude that APSG is sufficiently different from traditional PSG as to warrant collection of a large normative data base.     

Polysomnography: technical aspects in adolescents and adults.

Polysomnography is a complex procedure that should be performed by a trained technologist. The routine clinical polysomnogram includes the monitoring of EEG, electro-oculography, electromyography, effort of breathing and airflow, oxygen saturation electrocardiography, and body position. Other more specialized studies may include parameters such as endoesophageal pressure, endoesophageal pH, or penile circumference change. Special considerations to keep in mind when studying adolescents include attention to psychological, social, and technical issues. Polysomnography is an essential tool in the formulation of diagnosis for sleep disorders patients and in the enhancement of our understanding of sleep and its disorders.     

Voluntary contraction and responses to submaximal cervical nerve root stimulation

Voluntary contraction of hand muscles increases compound muscle action potential (CMAP) amplitudes evoked by submaximal electrical percutaneous cervical stimulation (EPCS). This has been reported to be due to an intraspinal, presynaptic mechanism. We studied the effects of voluntary contraction on hypothenar CMAP amplitudes in 5 volunteers following electrical peripheral nerve stimulation at the wrist, EPCS, magnetic stimulation at the neck and the effects of a conditioning subthreshold cortical magnetic stimulus on CMAPs evoked by EPCS at rest. CMAP amplitudes increased with voluntary contraction of the target muscle, regardless of type or location of stimulus (P < 0.001). No increase in CMAP amplitude occurred when a conditioning transcranial stimulus was employed with EPCS (P = 0.35). Our findings indicate a peripheral rather than central mechanism underlies this effect of voluntary contraction. It is probably related to the recruitment order of motor axons, comparing voluntary activation with electrical or magnetic stimulation. © 1994 John Wiley & Sons, Inc.     

Repetitive nerve stimulation of the nasalis muscle: technique and normal values

A technique of repetitive nerve stimulation of the facial nerve with recording from the nasalis and orbicularis oculi muscles was developed and tested in 15 healthy subjects. Quality criteria were that the stimulus artifact should return to baseline before the onset of the compound muscle action potential (CMAP), and that the CMAP should begin with a negative phase, be biphasic in shape, and have an amplitude of over 1 mV. Repetitive nerve stimulation was only performed if all four criteria were met, which was the case in all 15 subjects for the nasalis muscle, but in only 5 subjects for the orbicularis oculi muscle. The largest observed decrement was 9% in area. Reptitive nerve stimulation of the nasalis muscle is thus feasible, but its diagnostic utility remains to be established in patients with disorders of neuromuscular transmission.