Reversible Conduction Block Research Articles

Delayed spontaneous return of hearing after acoustic tumor surgery: Evidence for cochlear nerve conduction block

Unlike the eventual resolution of facial paralysis in most patients with intact facial nerves, delayed hearing return after acoustic tumor resection is rare. This discrepancy in recovery has been ascribed to the inherent resilience of the facial nerve (a special visceral efferent nerve) to injury versus the cochlear nerve (a special somatic afferent nerve). In the presence of an intact cochlear nerve, hearing loss has been attributed to transection or spasm of the internal auditory artery or to direct mechanical trauma to the cochlear nerve during manipulation of the tumor. The possibility of a reversible conduction block in the cochlear nerve has not been considered. A review of three instances of delayed spontaneous hearing recovery several months after acoustic tumor resection suggests that a conduction block phenomenon can exist. This report describes the pathophysiology and the possible intraoperative predisposing features of this condition.

Cochlear Nerve Conduction Block: An Explanation for Spontaneous Hearing Return after Acoustic Tumor Surgery

In the presence of an intact cochlear nerve, hearing loss has been attributed to either transection or spasm of the internal auditory artery or direct mechanical trauma to the cochlear nerve during tumor manipulation. Such events have been correlated with changes in intraoperative auditory evoked potentials. The possibility of a reversible conduction block in the cochlear nerve, however, has not been investigated. Review of four cases of delayed spontaneous recovery of hearing several months after acoustic tumor resection suggests that a conduction block phenomenon may exist. By comparing recent pertinent animal data with clinical intraoperative electrophysiologic data obtained during posterior fossa surgery in human subjects, we attempt to elucidate further the pathophysiology and intraoperative predisposing factors to cochlear nerve injury during hearing preservation procedures.

Descending influences on the cutaneous receptive fields of postsynaptic dorsal column neurones in the cat.

1. The influence of activity in descending systems on the cutaneous receptive field properties of postsynaptic dorsal column (PSDC) neurones has been investigated in chloralose-anaesthetized cats. The main aim of the study was to determine whether the receptive field boundaries of PSDC neurones are under the control of systems descending from the brain. 2. Single-unit recordings were made from the ascending axons of PSDC units in the dorsal columns. Receptive fields were analysed using light tactile and noxious mechanical and thermal stimuli, both before and during a reversible block of spinal conduction produced by cooling the cord rostral of the recording site. 3. The light tactile excitatory fields of PSDC neurones were largely unaffected by the cold-block procedure. 4. In contrast, both the sensitivity of PSDC neurones to noxious stimuli and the area of skin from which they could be effectively excited by such stimuli were found to be profoundly modified by interruption of descending activity. Two-thirds of the units excited by noxious pinch responded more vigorously in the cold-blocked state and one-half from an expanded area of skin. Responses to noxious radiant heat were similarly modified. 5. Inhibition evoked in PSDC neurones, whether by light tactile or noxious stimuli, involved predominantly segmental mechanisms since it remained effective in the cold-blocked state. 6. It is concluded that neurones of the PSDC system are amongst those dorsal horn neurones with receptive field geometries which may be modified by activity in descending systems.

Cochlear Nerve Conduction Block: An Explanation for Spontaneous Hearing Return After Acoustic Tumor Surgery

At the spring meeting of the American Neurotology Society, Palm Beach, Fla, John F. Kveton, MD, Lahey Clinic, Burlington, Mass, reviewed two cases of delayed spontaneous recovery of hearing to preoperative levels several months after acoustic tumor resection. He theorized the possibility of a reversible conduction block of the cochlear nerve. While previous theories of transient hearing loss after surgery were attributed to spasm of the internal auditory artery or to direct mechanical trauma of the cochlear nerve during tumor manipulation, Kveton speculated that a conduction block phenomenon might exist. He went on to compare animal data with clinical intraoperative electrophysiologic data to further elucidate this pathophysiology.

Reversible proximal conduction block underlies rapid recovery in Guillain-Barré syndrome.

Three patients with Guillain-Barré syndrome underwent electrophysiological examination prior to and during a period of rapid clinical recovery. In each case, improved strength of the abductor digiti minimi coincided with electrophysiologic evidence of a marked reversal of proximal conduction block. In contrast, the degree of distal conduction block remained relatively unchanged after stimulation at the wrist, elbow, axilla, and Erb's point. These findings indicate that rapid motor recovery, early in the course of Guillain-Barré syndrome, can result from reversal of proximal conduction block and explains the often noted dissociation between clinical improvement and conventional distal nerve conduction studies.

Transient conduction block following acute peripheral nerve ischemia

We studied the effects of transient focal ischemia on the electrophysiologic function of rat sciatic nerves. Focal and generalized impairment of impulse conduction, measured by falling evoked muscle and nerve compound action potential amplitudes, occurred within 10 minutes of femoral artery occlusion. Conduction failure reached a nadir at 45-60 minutes and then improved to normal within 24 hours. Fastest motor and mixed nerve conduction velocities were reduced less than 15% of baseline values during the period of acute conduction block. There were no detectable morphological abnormalities at the site of conduction failure. Transient ischemia produces reversible conduction block without evident structural changes. The fall in amplitude without significant conduction slowing implies that slower conducting myelinated fibers are relatively more sensitive to the effect of acute ischemia.

Neurological Deficits Following the Hot Bath Test in Multiple Sclerosis

To the Editor.— In their article entitled Persistent Neurological Deficit Precipitated by Hot Bath Test in Multiple Sclerosis, Berger and Sheremata 1 conclude that hyperthermia can provoke an exacerbation in multiple sclerosis (MS). This is in contradistinction to the well-known reversible worsening of signs and symptoms in MS induced by hyperthermia, which is the result of a reversible block of impulse conduction caused by a critical decrease in action current available for conduction. 2,3 We have never observed persistent neurological deficits in more than 20 years' experience with the hot bath test; however, unlike Berger and Sheremata, we have rarely studied patients already in an exacerbation, and then only in a single experimental protocol. 4 I don't believe the authors have demonstrated what they conclude. They have, I believe, shown that in patients already in acute exacerbation, hyperthermia effects on signs and symptoms may not always reverse. The mechanism, however,

Functional and Structural Changes in the Rabbit Vagus Nerve in Vivo following Exposure to Various Hypoosmotic Solutions

Hypoosmolar solutions were recently shown to produce a reversible conduction block of rabbit vagus nerve and to potentiate local anesthetic agents. The object of the present study was to determine the ability of neural tissue to recover structurally and functionally following exposure to hypoosmotic solutions. Cervical vagus nerves of rabbits were bathed in situ for 2 hours in a control solution or in 0.4, 0.5, and 0.6 aqueous dilutions of physiologic salt solution. Nerves excised immediately after exposure, or 1 to 4 weeks later, were subjected to light and electron microscopic examination. Following exposure to control and 0.6 dilution, nerves were normal in all respects at 8 days. However, nerves exposed to 0.4 and 0.5 dilutions, although apparently functionally intact as tested by conduction of C fiber action potentials, showed evidence of axonal damage characterized by accumulation of macrophages and proliferation of Schwann cell processes. It may be inferred that the osmotic fragility of axons is similar to that of erythrocytes and that immersion in 0.6 N osmotic solution is probably harmless to the nerve.

Nature of the reversible white matter lesion in multiple sclerosis. Effects of acute inflammation on myelinated tissue studied in the rabbit eye.

Arthus reactions of different intensities were induced in the rabbit eye using measles vaccine as the immunogen. In the presence of a mild Arthus reaction in the vitreous, myelinated nerve fibre bundles on the retinal surface revealed extracellular oedema together with marked swelling and rupture of astrocyte processes, including those bearing a special relationship to nodes of Ranvier. These changes improved or disappeared over the course of six days. No demyelination occurred. The findings implicate an astrocytic lesion as a possible cause of reversible neurological deficit and conduction block associated with acute inflammation in central white matter.

Experimental studies of the effects of extrinsic factors on conduction in normal and demyelinated nerve. 1. Temperature.

Previous studies in experimentally demyelinated mammalian nerves have demonstrated that a reversible conduction block occurs with small increases of temperature within the animal's normal body temperature range. This phenomenon is believed to be the mechanism for clinical temperature effects in multiple sclerosis. This study examines some quantitative thermal relationships in demyelinated nerves of guinea pigs with experimental allergic neuritis. The observed results in normal and experimental animals are in good agreement with previous theoretical calculations based on the effects of temperature on the voltage and time-dependent behavior of the ionic permeabilities of the nodes of Ranvier. Guinea pigs with increasing motor dysfunction generally exhibited corresponding increases in the overall latency of the conducted action potential, as well as decreases in amplitude. In addition, the lower the initial velocity increment per degree of temperature elevation, the lower was the temperature at which conduction block began to occur. Except for a few cases in which the recorded action potential was bimodal, with response at both normal and prolonged latency, the results tended to indicate a remarkedly uniform involvement of the sciatic nerve within the region of temperature control.

The effects of temperature on conduction in demyelinated single nerve fibers.

Increases of temperature of 0.5 C within the physiological range of temperature cause reversible conduction block at identifiable internodes of experimentally demyelinated rat ventral root fibers; such internodes invariably have greatly increased internodal conduction times. At less severely affected internodes, increased temperature causes a reduction in internodal conduction time, as at normal internodes. Such reversible conduction block with increased temperature in demyelinated fibers is suggested as the explanation for the temperature sensitivity of signs and symptoms of patients with multiple sclerosis. It is also suggested that the chronic slowing of conduction that is known to occur in demyelinated nerve fibers is probably of minor clinical significance in human demyelinating disease.

The contribution of membrane hyperpolarization to adaptation and conduction block in sensory neurones of the leech.

The factors underlying sensory adaptation and conduction block have been studied in cutaneous mechanoreceptor neurones of the leech. A touch-sensitive cell was activated by applying mechanical or electrical stimuli to its receptive field on the skin. Impulses were recorded extracellularly from its axons and intracellularly from its cell body, which is situated within the C.N.S.1. Activation of the touch cell by mechanical stimuli revealed two distinct types of adaptation with characteristically different time courses. Sustained pressure on the skin caused a brief burst of impulses at the onset of the stimulus. This rapid adaptation to pressure was restricted to the part of the receptive field that had been stimulated mechanically. A second type of adaptation developed more slowly during the course of repetitive mechanical stimulation. It persisted for many seconds after the end of a train of impulses and appeared as an increase in the threshold to mechanical stimuli not only in the region of skin that had been rubbed but throughout the receptive field of the cell.2. Impulses initiated in the cell body propagated antidromically towards the skin and also raised the threshold to touch, indicating that after-effects of impulse activity were responsible for the long-lasting threshold increase.3. Repetitive mechanical stimulation could also produce a reversible conduction block in branches of the touch cell. The block occurred in discrete regions of low safety factor such as axonal branch points both within the ganglion and in the periphery. In some experiments impulses intermittently failed to reach one axonal branch yet continued to invade a separate branch of the same cell.4. Several lines of evidence indicate that both conduction block and the slow component of adaptation are linked to a prolonged hyperpolarization that follows repetitive stimulation of the touch cell. Strophanthidin, which blocks the after-hyperpolarization in touch cells, reduced the adaptation following trains of impulses and also relieved a conduction block previously established by repetitive stimulation. Furthermore, a comparison of the effects of hyperpolarizations produced by current injection and by repetitive firing showed that most of the threshold increase in the cell body after a train of impulses could be attributed directly to the membrane hyperpolarization.5. These experiments suggest several ways in which repetitive activity can have pronounced and long-lasting effects on the performance of a highly branched sensory cell. Thus a relatively small number of impulses in a touch cell can markedly decrease its sensitivity to touch. The functional role of the conduction block observed during vigorous stimulation is not as clear because activity for many seconds or minutes is usually needed to establish a block in the larger branches of the cell.

Orbicularis oculi reflex in coma: clinical, electrophysiological, and pathological correlations.

The orbicularis oculi reflex was studied in 48 comatose patients and the results were correlated with clinical and pathological findings. The late component of the reflex was absent or of minimal amplitude in all cases regardless of the site of lesions, reflecting a diffuse suppression of the reticular system rather than a specific or local block in the brain-stem. Alteration of the early component of the reflex, on the other hand, generally indicated primary or secondary structural changes in the pons, although reversible functional or pharmacological block of pontine conduction was also documented.

Effects of descending impulses on transmission through the spinocervical tract.

1. Micro-electrode recordings were made from ascending axons of the spinocervical tract in unanaesthetized decerebrate cats before, during and after reversible cold block of impulse conduction in the spinal cord rostral to the recording site.2. Most units (forty-one of forty-four) fell into one of four categories as defined by their evoked responses to mechanical stimulation of identified cutaneous receptors. These categories were; Type I excited by movement of tylotrichs (hairs) in the decerebrate preparation but by movement of all types of hairs after block of descending impulses; Type II excited by movement of guard hairs and usually weakly by pressure in the decerebrate state but by movement of all types of hairs and by pressure in the spinal state; Type III excited by movement of all types of hairs and often by pressure in the decerebrate animal but by movement of all types of hairs and always by pressure in the spinal animal; Type IV weakly excited by heavy pressure or with no receptive field in the decerebrate state but excited by pressure in the spinal state.3. The descending influences depressed the spontaneous activity and the evoked responses to harmful stimuli.4. The descending influences depressed inhibitory inputs from segmental levels.5. The functional significance of the descending control is discussed.

Relationship of acetylcholinesterase activity to axonal conduction

We overcame previous difficulties in correlating electrical and acetylcholinesterase (AChE) activity, after exposure to organophosphate inhibitors, by using the intact squid giant axon, acetyl-β-methylcholine as substrate and a sensitive radiometric assay to measure AChE activity. Reversible conduction block by a 6-min exposure to 10 −2M diisopropylfluorophosphate (DFP) was associated with 80 per cent inhibition of enzyme activity, while a 30-min exposure caused irreversible block and 99 per cent inhibition. Exposure to 5 × 10 −2 M DFP for 1–5 min caused irreversible conduction block and 70 per cent enzyme inhibition. Pyridine aldoxime methiodide (PAM) restored electrical function in about one-half of the axons “irreversibly” blocked by DFP. Even in those axons in which PAM did not restore electrical activity, 30–50 per cent of the original AChE activity was present. The tertiary analogue of phospholine (TP) had no effect on electrical activity, although it inhibited 98–99 per cent of AChE. After cottonmouth venom pretreatment, TP blocked conduction in one-half of the axons and caused 80 and 93 per cent inhibition of AChE in the conducting and nonconducting axons respectively. There does not appear to be a simple and direct relationship between electrical and AChE activity.

The action of acetylcholine and cholinesterase inhibitors on single axons of the lobster

Acetylcholine (ACh), physostigmine and related substances have been tested for their effects on the electrical activity of single giant axons of the circumesophageal connectives of the lobster. Acetylcholine and physostigmine in concentrations of 5 × 10 −3 to 1 × 10 −2M depolarize the membrane. The simultaneously recorded action potential is prolonged, and with progressive depolarization reversible block of conduction occurs. Both compounds cause initially spontaneous or repetitive firing. Neostigmine, 3-hydroxyphenyltrimethylammonium and edrophonium prevent the effect of ACh without having an effect of their own on membrane or action potential. Choline, cholinethiol and phenyltrimethylammonium do not antagonize the action of ACh, nor do they change the electrical parameters of the axon. Curare depolarizes the membrane by 15 mV, but does not cause block of conduction nor does it inhibit the ACh effect. These observations are discussed and possible mechanisms of action are suggested.

NERVE EXCITABILITY TESTS IN FACIAL PARALYSIS.

Introduction IT is generally believed that the decision as to the therapeutic approach to be adopted in patients with peripheral facial paralysis will be greatly facilitated if the severity of the conductive disorder can be established in an early stage after the onset of paralysis. In any individual case of paralysis, especially complete paralysis, therefore, two concrete questions are to be answered as early as possible: (1) Is this paralysis still in a reversible stage (reversible conduction block), or is the nerve already degenerated? (2) In case the paralysis is still in a reversible stage, which signs (if any) are to be feared as heralding an imminent or incipient unfavorable development? From publications by Cawthorne, 3 Jongkees, 11,12 Kettel, 13 Taverner, 19 and many others, as well as from our own study (Laumans 16,17 ), we know that on the basis of clinical examination alone the above-mentioned questions cannot be answered.

Effects of acetylcholine on axonal conduction of lobster nerve.

Acetylcholine and related compounds have been tested for their effects on the electrical activity of lobster walking-leg nerve bundles. Changes in action and membrane potential were recorded with the “sucrose-gap” technique. ACh in concentrations of 5·10−3–10−2 M decreases the membrane potential; the simultaneously recorded action potentials initially display an increased amplitude and a marked prolongation and elevation of the descending phase which appears oscillatory. As the depolarization progresses, the spike height decreases and the effects on the descending phase decline. Finally reversible block of conduction occurs. d,l-Acetyl-β-methylcholine (10−2 M) produces similar effects on the action potential but the membrane potential is only slightly depolarized. Choline, acetate and bromide in concentrations equal to those of ACh have no effect; in 10-fold higher concentrations only choline produces minor changes in the action potential. Physostigmine (5 mM) produces effects similar to those seen with ACh while lower concentrations inhibit the action of ACh. Atropine and tetracaine reversibly block conduction and like physostigmine, also inhibit the effect of ACh. Although curare was nerve demonstrated to block conduction, it does inhibit the action of ACh. Irreversible cholinesterase inhibitors (phospholine, Paraoxon) block conduction. Paraoxon much faster than phospholine. The membrane potential is decreased by 20–30 mV, and while the effects of these inhibitors on the membrane potential are reversible, conduction remains blocked. The difficulties of interpreting some of these actions on a nerve bundle are discussed.

Some Effects of the Ionic Composition of the Extracellular Fluid On the Electrical Activity of the Cockroach Abdominal Nerve Cord

ABSTRACT Irrigation of abdominal ganglia of Periplaneta Americana with salines containing excess potassium ions resulted in the development of a reversible conduction block. Removal of portions of the cellular and fibrous nerve sheath produced an accelerated potassium depolarization as described by Twarog & Roeder (1956). Elevation of the cation concentrations to correspond to the extracellular levels resulted in delayed conduction block in desheathed preparations irrigated with highpotassium saline. At 70 mM./l. K the rate of depolarization was delayed to approximately half that of intact ganglia. At extremely high potassium concentrations this effect became relatively small. It is suggested that the enhanced rates of potassium depolarization obtained in desheathed preparations partly result from the changed ionic composition of the extracellular fluid resulting from the desheathing procedure. The possible additional factors involved in the desheathing procedure are discussed. 50 mM./l. acetylcholine had little effect on the synaptic transmission in intact terminal abdominal ganglia but caused rapid loss of conduction in desheathed preparations. Injection of equivalent amounts of acetylcholine beneath the nerve sheath did not cause rapid conduction block, and it is suggested that the effects of desheathing are not necessarily caused by the removal of a relatively impermeable superficial diffusion barrier.

Reversible block of axonal conduction by curare after treatment with cobra venom

Reversible block of axonal conduction by curare after treatment with cobra venom