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Biomedical subjects

J M Shefner

Publications and source records attributed to J M Shefner.

At least 55 records · Page 3Linked to original sources
PubMed1995

Urodynamic findings in children with spinal cord ischemia.

PURPOSE: We identified urodynamic patterns of lower urinary tract dysfunction in children after anterior spinal artery injury. MATERIALS AND METHODS: Between 1981 and 1993, 6 boys and 1 girl in whom ischemic spinal cord injuries developed after umbilical artery catheterization (4), cardiovascular surgery (2) and spontaneous bleeding from an arteriovenous fistula (1) were evaluated radiologically, neurologically and urodynamically. RESULTS: Neurological assessment revealed a motor level from T8 to S1 in all 7 children, whereas only 3 had sensory denervation ranging from T8 to L4. Urodynamic studies demonstrated a mixed upper and lower motor neuron pattern in 3 patients, a lower motor neuron lesion only in 3 and a pure upper motor neuron deficit in 1. Treatment consisted of oxybutynin in 5 cases, Credé voiding in 1 and close observation in 1. All children are dry and kidney function has remained stable. CONCLUSIONS: Spinal cord ischemia in children is a rare condition that can arise from a variety of causes, the most common of which are embolism from umbilical artery catheterization and surgical repair of a patent ductus arteriosus. The urodynamic patterns are variable but characteristically they can be upper or lower motor neuron in nature. Precise testing will lead to appropriate therapy to prevent upper urinary tract disease and minimize incontinence.

Child↗
PubMed1994

Long lasting excitability changes in human peripheral nerve.

When pairs of equal but submaximal electrical stimuli are delivered to a peripheral nerve, the second stimulus does not always excite the same number of fibers as the first. The number of fibers responding to the second stimulus depends on the interstimulus interval; the refractory period, a well-defined period of hypoexcitability, is followed by longer lasting and less well-characterized periods of hyper- and hypoexcitability. These cycles last at least 200 ms after the initial stimulus. We have carefully studied these cycles of excitability in human peripheral nerve in 12 normal subjects. The magnitude of excitability changes were found to be much greater in motor fibers than in mixed nerve; under some conditions, the motor response was reduced by more than 80% at interstimulus intervals of 40 ms, while the mixed nerve response never varied by more than 20%. In addition, the amplitude of the excitability changes varied as a function of the stimulus strength, so that stimuli that were near threshold or evoked near maximal responses were associated with smaller excitability changes than stimuli evoking midrange responses. Given that the excitability fluctuations are of large magnitude and occur at interresponse intervals easily achieved during physiological firing, it is suggested that they may be important modifiers of firing rate under experimental or physiological conditions.

Adult↗
PubMed1994

Submaximal stimuli activate different nerve fiber populations at different sites.

Theoretically, the largest and fastest nerve fibers are preferentially stimulated with submaximal stimuli. However, it is also well known that intraneural fascicular topography changes substantially along a proximal to distal axis. Because of this change in fascicular topography, we hypothesized the percutaneous submaximal stimuli applied to a nerve at different locations would stimulate different subpopulations of large fibers. We performed a series of collision studies by stimulating the ulnar nerve submaximally at proximal and distal sites at varying levels of stimulation intensity from motor threshold to supramaximal stimulation. The results suggest that variation in intraneural topography at different sites allows different large diameter nerve fiber subpopulations to be activated at submaximal stimuli, and emphasizes the importance of supramaximal stimulation to determine a valid conduction velocity.

Action Potentials↗
PubMed1994

Conduction velocity in motor, cutaneous afferent, and muscle afferent fibers within the same mixed nerve.

Mammalian axons subserving different functions have different conduction velocities (CV); motor fibers conduct more slowly than cutaneous fibers, which conduct slower than muscle afferents. However, human studies have yielded conflicting results. We studied isolated fiber populations in human sciatic nerve to examine further this question. Motor studies were performed in standard fashion, stimulating at gluteal fold (GF) and popliteal fossa (PF) and recording soleus. In addition, conduction velocity of a pure motor nerve volley was calculated for 3 subjects. Stimulating and recording electrodes were needles placed close to the nerve. Cutaneous afferents were studied by stimulating the sural nerve at the ankle and recording at PF and GF. Muscle afferent velocity was assessed by comparing soleus H reflex latency with stimulation at PF and GF. Results in 10 subjects showed muscle afferent CV of 57.6, cutaneous afferent CV of 55.1, motor CV of 52.4, and mixed nerve CV of 56.3 m/s. Although statistically significant, these differences are much smaller than in animal studies. These results have implications for understanding what fibers contribute to spinal reflexes.

Action Potentials↗
PubMed1994

Sensory and mixed nerve conduction studies in the evaluation of ulnar neuropathy at the elbow.

The relative sensitivities of sensory, mixed nerve, and motor conduction studies in assessing ulnar neuropathy at the elbow have not yet been established. Using surface electrodes, we performed conduction studies across the elbow segment in 43 patients with symptoms referable to the ulnar nerve and 40 control subjects. Segmental slowing of motor conduction localized the lesion to the elbow in 14 of 21 patients (67%) with clear evidence of ulnar neuropathy on physical examination but only in 2 of 22 (9%) with subtle or no physical examination abnormalities. The diagnostic yield was increased by the finding of segmental slowing of sensory or mixed nerve conduction across the elbow to 86% and 68%, respectively, for each of the groups. We conclude that surface-recorded sensory and mixed nerve conduction studies appear to be more sensitive than motor studies in the electrodiagnosis of ulnar neuropathy at the elbow and are especially valuable in patients with subtle clinical involvement.

Action Potentials↗
PubMed1994

Recurrent inhibition is decreased in patients with amyotrophic lateral sclerosis.

Recurrent inhibitory pathways are powerful modulators of motor neuron excitability. Renshaw cell activation can both inhibit homologous motor neurons and disinhibit antagonists. In spastic spinal-cord-injured patients, recurrent inhibition is consistently increased, and clinical reductions in spasticity are associated with reduced recurrent inhibition. In this study, we evaluated 12 spastic patients with amyotrophic lateral sclerosis (ALS) to see whether a similar mechanism was operating. In contrast to spinal-cord-injured patients, spastic patients with ALS showed strikingly reduced recurrent inhibition, as assessed by a conditioned H-reflex technique, which produces a response (H') whose amplitude is inversely correlated with activity in recurrent inhibitory pathways. The mean ratio of the maximum H' response to the maximum H-reflex response (H'/H ratio) was 0.55, significantly greater than the ratio seen in normal subjects. Amplitude of the H' correlated with amplitude of the Achilles tendon reflex. Thus, in patients with classical ALS, recurrent inhibition appears to be abnormally reduced compared with control subjects, suggesting a different physiology for spasticity in this setting than in spinal cord transection.

Adult↗
PubMed1993

Relationship between stimulus strength and the cutaneous silent period.

During sustained muscle contraction, an interval of reduced activity follows an electrical cutaneous stimulus, called the cutaneous silent period (CSP). To evoke a CSP, a single stimulus must be painful. We used single sural nerve stimuli to evoke a CSP in ipsilateral soleus muscle, and studied the relationships between stimulus strength, sensory action potential (SAP) morphology, and subjective experience. Near nerve electrodes were employed to record the sural SAP in order to record activity in slower conducting fibers in addition to A alpha fibers. In 6 normal subjects, the stimulus strength required to evoke a CSP ranged from 8 to 10 times threshold intensity. Pain threshold was slightly below that necessary to evoke the CSP. SAP shape changed with stimulus strength; main component amplitude occasionally increased as strength increased beyond 10 times threshold, and slowly conducting late components became more prominent. At stimulus intensities or at less than CSP threshold, components were seen conducting from 15-20 m/s that were not observed at lower intensities. We suggest that activation of sensory axons with conduction velocities in the range of A delta fibers are necessary to evoke the CSP, and that their potentials can be discerned in the SAP.

Action Potentials↗
PubMed1993

The effect of nicotine on recurrent inhibition in the spinal cord.

Recurrent inhibition via Renshaw cells provides a mechanism by which spinal and supraspinal centers exert control over movement. The conditioned H-reflex technique of Pierrot-Deseilligny and Bussel permits noninvasive assessment of recurrent inhibitory pathways. We employed this technique to investigate changes in Renshaw cell activity due to nicotine (a potent CNS cholinergic agonist that excites Renshaw cells in animals) contained in inhaled tobacco smoke. In 10 normal subjects, cigarette smoking caused a large, rapid drop in the conditioned H-response amplitude, implying increased activation of Renshaw cells. The time course of the change in conditioned H-response amplitude closely approximated the known pharmacokinetics of inhaled nicotine. Nicotine administered via chewing gum had a much slower and less dramatic effect, probably due to the slower rise in blood levels with this mode of administration. Increased activity in Renshaw cells may contribute to spasticity in spinal cord-injured patients, raising the possibility that cigarette smoking could cause further increases in tone in such patients.

Administration, Cutaneous↗
PubMed1993

Nonvasculitic, steroid-responsive mononeuritis multiplex.

We report two patients with mononeuritis multiplex, both of whom had focal inflammation of the perineurium and endoneurium on sural nerve biopsy without necrosis of blood vessel walls, histologic evidence of lymphoid malignancy, or mycobacterial infection. The predominant early sensory symptoms were asymmetric pain and paresthesias; subsequently, muscle weakness developed. Electrophysiologic studies showed an asymmetric sensorimotor axon loss radiculoneuropathy with denervation of limb and paraspinal muscles. Spinal fluid protein was elevated in one patient. There was no cause or underlying systemic disease. Marked improvement occurred with steroid therapy.

Action Potentials↗
PubMed1992

Peripheral sensory abnormalities in patients with multiple sclerosis.

Although multiple sclerosis primarily affects myelin within the central nervous system, both pathologic and physiological studies suggest that mild deficits in peripheral nervous system myelin may be common. To evaluate this question further, we performed near nerve studies on sural nerves of 14 patients with multiple sclerosis. Peak-to-peak amplitude and maximum conduction velocity were normal in 9 of 14 patients, while minimum conduction velocity, or the velocity of the slowest-conducting component of the sensory action potential, was abnormally reduced in 9 patients. In addition, the supernormal period was evaluated for patients and compared with a control sample; multiple sclerosis patients showed a significant reduction in the amplitude of supernormality. Both the reduction in minimum conduction velocity and the alteration in the supernormal period are consistent with a mild defect in peripheral myelin.

Action Potentials↗
PubMed1992

Lower motor neuron dysfunction in patients with multiple sclerosis.

A patient in whom multiple sclerosis (MS) was ultimately diagnosed presented with a lower motor neuron syndrome involving 1 hand, with EMG evidence of denervation. Twelve other patients were subsequently identified with definite MS and asymmetric hand atrophy. These patients were studied clinically and electrophysiologically. Evidence of chronic and ongoing denervation was noted in the hands of 12 of the 13 patients; in only 3 patients could the EMG abnormalities be accounted for by peripheral nerve lesions. Thus, lesions resulting in lower motor neuron damage may occur in the central nervous system in MS patients. We suggest that demyelination in the region of the ventral root exit zone may account for these findings.

Action Potentials↗
PubMed1992

Recurrent potentials in human peripheral sensory nerve: possible evidence of primary afferent depolarization of the spinal cord.

To study slowly conducted components of the orthodromic compound sensory action potential (CSAP), the response evoked at the lateral malleolus in the sural nerve was recorded through near-nerve needles at two to four sites along the nerve at midcalf. When 500 to 2000 responses were averaged at high gain, components with latencies of 30 to 80 ms were often recorded. In contrast to the main component and late components with latencies of less than 15 to 20 ms, the latencies of these extremely late components diminished the closer to the spinal cord that they were recorded. This suggested that the components were conducted antidromically from proximal to distal. This assumption was supported by abolishing the components by local anesthesia of the nerve proximal to the recording electrodes. These antidromic potentials therefore appear to be due to recurrent discharges in the sural nerve. Recurrent discharges were recorded from 65% of 60 subjects (18 normal subjects and 42 patients with peripheral or central nervous system disorders). The latencies of the recurrent discharges allowed conduction to and back from the spinal cord. Although the origin of these potentials remains unknown, we suggest that they are due to dorsal root reflexes within the spinal cord. In this case, the responses may be a direct expression of primary afferent depolarization (PAD) seen in presynaptic inhibition, and may be of value in further studies on the physiology and pathophysiology of presynaptic inhibition of cutaneous fibers in man.

Action Potentials↗
PubMed1992

Urodynamic dysfunction in walking myelodysplastic children.

We evaluated urodynamically and radiologically 54 children with myelodysplasia and neurological deficits at or below the S1 level. Baseline urodynamic testing was normal in 13 patients (24%), while 12 (22%) had an upper motor neuron and 13 (24%) had a lower motor neuron type of dysfunction. A total of 7 patients (13%) had a mixed upper and lower motor neuron type, and 9 (17%) had only lower motor neuron dysfunction of the urethral sphincter with a normally contractile bladder. Followup studies varying in time from 1 month to 10 years showed a changing neurourological lesion in 29 patients (54%): 25 deteriorated while 4 improved. Of the 54 children 20 had hydronephrosis and/or vesicoureteral reflux. Incontinence was the major problem in 41 patients (75%), and was managed initially with pharmacological agents and/or clean intermittent catheterization, with 9 of the 41 eventually requiring surgery. Urodynamic assessment reveals a variable picture that does not correlate well with the apparent neurological examination. Despite the low level of the neurological deficit, many children may be at risk for urinary tract deterioration. These findings emphasize the importance of continuous surveillance and appropriate management in this group of myelodysplastic children who have the greatest potential for a normal life.

Child↗
PubMed1992

Recurrent inhibition is increased in patients with spinal cord injury.

Mechanisms underlying the development of spasticity after spinal cord injury are not understood. One spinal interneuron likely to be affected is the Renshaw cell, which acts to produce recurrent inhibition in motor neurons as well as inhibiting Ia interneurons. Descending pathways exert both excitatory and inhibitory control over Renshaw cell activity. We studied Renshaw cell activity in normal subjects and in patients with varying levels of spasticity after spinal cord injury using the conditioned H-reflex technique of Pierrot-Deseilligny and Bussel. A submaximal stimulus to the tibial nerve is presented prior to a supramaximal stimulus so that action potential collision permits an H reflex (H') to be elicited in response to the supramaximal stimulus. The amplitude of this H' reflex is affected by activity in recurrent inhibitory pathways. Patients with both complete and partial spinal cord lesions were studied; date of injury ranged from 1 month to 216 months prior to evaluation. In the 18 patients in whom H reflexes could be recorded, H' reflexes were absent in 13, in contrast to their uniform presence in normal subjects. We conclude that recurrent inhibition via Renshaw cell activity is increased in spinal cord injury, and that measures of recurrent inhibition may correlate well with some clinical measures of spasticity.

Adult↗
PubMed1991

Slowly conducting myelinated fibers in peripheral neuropathy.

The main component of the compound sensory action potential reflects the activity of large myelinated sensory fibers with diameters of greater than 9 micron(s). By recording the averaged potential using a needle electrode placed close to the nerve, small late components can be measured. The latency of these late components can be used to calculate minimum conduction velocity (CV); in normal subjects, average minimum CV is 15 m/s, corresponding to conduction in fibers of about 4 micron(s) in diameter. Minimum CV was measured in median, ulnar, and sural nerves of 187 patients with mild to severe neuropathic symptoms. A reduction in minimum CV was a sensitive measure of peripheral nerve dysfunction, often showing abnormalities when measures derived from the main component were normal. Patients with isolated abnormalities in minimum CV tended to have neuropathic symptoms but no signs of neuropathy. In addition, reduced minimum conduction velocity has implications for the pathology of different types of neuropathy. Slowing conducting potentials may originate from regenerating fibers, which may be of particular relevance in patients with neuropathic pain.

Action Potentials↗
PubMed1991

Abnormalities in the sensory action potential in patients with amyotrophic lateral sclerosis.

Sensory function in patients with amyotrophic lateral sclerosis (ALS) is thought to be normal; however, there is convincing morphologic evidence that sensory systems are affected in addition to motor systems. In this study, compound sensory action potentials were recorded with near nerve electrodes from 18 patients with ALS. Up to 1024 responses were averaged at high gain to determine minimum conduction velocity; that is, the conduction velocity of the slowest conducting component of the sensory action potential. Nine of 18 patients had abnormally reduced minimum conduction velocity, even when peak-to-peak amplitude and maximum conduction velocity (calculated from the latency to the initial positive peak) were normal. Only 3 of 18 patients showed abnormalities in peak-to-peak amplitude. Thus, subtle abnormalities in the sensory action potential can be detected in many patients with ALS.

Action Potentials↗
PubMed1991

Clinical neurophysiology of focal spinal cord injury.

A focal spinal cord injury produces a host of electrophysiologic abnormalities, most of which reflect either local spinal cord destruction or the functional disconnection of rostral and caudal portions. Routine electromyographic and nerve conduction studies are most useful to investigate the deficits that result from local cord damage and have demonstrated that the injury zone most often extends at least 3 to 4 myotomes. Electromyographic studies of muscles innervated by more caudal myotomes have also shown abnormalities, but the extent to which these reflect primary changes of spinal cord injury is uncertain. Electrophysiologic studies of spinal cord function caudal to the site of injury have shown alterations in autonomic output, as well as changes in the level of excitability of reflex pathways. These changes may provide insight into the mechanisms that underlie the development of spasticity.

Humans↗