Effect of body temperature on visual evoked potential delay and visual perception in multiple sclerosis

Temperature effects on synaptic transmission and neuronal function in the visual thalamus

Numerous neuronal properties including the synaptic vesicle release process, neurotransmitter receptor complement, and postsynaptic ion channels are involved in transforming synaptic inputs into postsynaptic spiking. Temperature is a significant influencer of neuronal function and synaptic integration. Changing temperature can affect neuronal physiology in a diversity of ways depending on how it affects different members of the cell’s ion channel complement. Temperature’s effects on neuronal function are critical for pathological states such as fever, which can trigger seizure activity, but are also important in interpreting and comparing results of experiments conducted at room vs physiological temperature. The goal of this study was to examine the influence of temperature on synaptic properties and ion channel function in thalamocortical (TC) relay neurons in acute brain slices of the dorsal lateral geniculate nucleus, a key synaptic target of retinal ganglion cells in the thalamus. Warming the superfusate in patch clamp experiments with acutely-prepared brain slices led to an overall inhibition of synaptically-driven spiking behavior in TC neurons in response to a retinal ganglion cell spike train. Further study revealed that this was associated with an increase in presynaptic synaptic vesicle release probability and synaptic depression and altered passive and active membrane properties. Additionally, warming the superfusate triggered activation of an inwardly rectifying potassium current and altered the voltage-dependence of voltage-gated Na⁺ currents and T-type calcium currents. This study highlights the importance of careful temperature control in ex vivo physiological experiments and illustrates how numerous properties such as synaptic inputs, active conductances, and passive membrane properties converge to determine spike output.

Efferent Inputs Are Required for Normal Function of Vestibular Nerve Afferents

A group of vestibular afferent nerve fibers with irregular-firing resting discharges are thought to play a prominent role in responses to fast head movements and vestibular plasticity. We show that, in C57BL/6 mice (either sex, 4-5 weeks old), normal activity in the efferent vestibular pathway is required for function of these irregular afferents. Thermal inhibition of efferent fibers results in a profound inhibition of irregular afferents' resting discharges, rendering them inadequate for signaling head movements. In this way, efferent inputs adjust the contribution of the peripheral irregular afferent pathway that plays a critical role in peripheral vestibular signaling and plasticity.SIGNIFICANCE STATEMENT Vestibular end organs in the inner ear receive efferent inputs from the brainstem. Previously, electrical stimulation of efferents was linked to an increase in resting discharges of afferents and a decrease in their sensitivities. Here, we show that localized thermal inhibition of unmyelinated efferents results in a significant decrease in the activity of afferent nerve fibers, particularly those with irregular resting discharges implicated in responses to fast head movements and vestibular compensation. Thus, by upregulating and downregulating of afferent firing, particularly irregular afferents, efferents adjust neural activity sensitive to rapid head movements. These findings support the notion that peripheral vestibular end organs are not passive transducers of head movements and their sensory signal transmission is modulated by efferent inputs.

Quantification of Uhthoff's phenomenon in multiple sclerosis: a magnetic stimulation study

OBJECTIVE

To quantify temperature induced changes (=Uhthoff phenomenon) in central motor conduction and their relation to clinical motor deficits in 20 multiple sclerosis (MS) patients.

METHODS

Self-assessment of vulnerability to temperature and clinical examination were performed. We used motor evoked potentials to measure central motor conduction time (CMCT) and applied the triple stimulation technique (TST) to assess conduction failure. The TST allows an accurate quantification of the proportion of conducting central motor neurons, expressed by the TST amplitude ratio (TST-AR).

RESULTS

Temperature induced changes of TST-AR were significantly more marked in patients with prolonged CMCT (P=0.037). There was a significant linear correlation between changes of TST-AR and walking velocity (P=0.0002). Relationships were found between pronounced subjective vulnerability to temperature and (i) abnormal CMCT (P=0.02), (ii) temperature induced changes in TST-AR (P=0.04) and (iii) temperature induced changes in walking velocity (P=0.04). CMCT remained virtually unchanged by temperature modification.

CONCLUSIONS

Uhthoff phenomena in the motor system are due to varying degrees of conduction block and associated with prolonged CMCT. In contrast to conduction block, CMCT is not importantly affected by temperature.

SIGNIFICANCE

This is the first study quantifying the Uhthoff phenomenon in the pyramidal tract of MS patients. The results suggest that patients with central conduction slowing are particularly vulnerable to develop temperature-dependent central motor conduction blocks.

Cloth color preference under the influences of body heating due to hot bath immersion

The experiment is aimed at knowing the effect of "body heating" on color preference. Eleven subjects with normal color vision served as subjects. Two tests, one of the "No Bath" and the other of the "Bath" were conducted. Hot bath immersion with 38.5 degrees C was performed for 30 min from 07:30 h to 08:00 h. Then, they were instructed to choose a single colored cloth out of 41 cloth colors (24 x 52 cm), preferred by themselves, every five min from 08:00 h to 09:00 h under the ambient temperature (Ta) of 27 degrees C. Most subjects preferred cooler color after "body heating" than after "not body heating". This finding was discussed in terms of greater differences between core temperature and its set point after "body heating", because cooler color would be helpful psychologically in allowing raised core temperature approach its set point.

Influence of temperature changes on multiple sclerosis: critical review of mechanisms and research potential

In 1890, Uhthoff studied multiple sclerosis (MS) patients who developed amblyopia following exercise, a phenomenon later discovered to be secondary to elevated body temperature from muscular activity. Six decades later, the hot bath test and various other heating reactions (HR) began to be used diagnostically. They were essentially discontinued after 1983, being replaced by more specific and safer tests and procedures. Over 80% of MS patients develop a panoply of neurological signs during hyperthermia, 60% of which are "new" to that patient. The literature contains a number of unexplained paradoxical responses of MS patients during induced hyperthermia. These challenge the current hypothesis that, in MS, hyperthermia induces a heat-linked neuro-blockade of partially demyelinated axons. Some MS patients developed signs before temperature elevations occurred; others showed clearing of signs while temperatures were elevated or were ascending. Several MS patients improved for about 3 hours after being tested, a rebound phenomenon known as "overshoot." Conversely, other MS patients developed persistent neurological deficits after hyperthermia. The etiology of HR may be multifactorial. This includes heat itself, effects of serum calcium, blockade of ion channels, circulatory changes, heat shock proteins, and unidentified humoral substances. Research techniques are suggested to continue investigations into the enigma of HR, hopefully to widen knowledge of demyelination.

Conduction properties of central demyelinated and remyelinated axons, and their relation to symptom production in demyelinating disorders

The conduction properties of central demyelinated and remyelinated axons are discussed, and related to the expression of symptoms in central demyelinating disease. The mechanisms underlying the block and restoration of conduction in segmentally demyelinated axons are described, together with the range of deficits expressed by the conducting axons. These abnormalities are related to clinical relapses and remissions, and to the phenomena of weakness, fatigue, the temperature sensitivity of symptoms, and the generation of 'positive' symptoms (e.g. Uhthoff's and Lhermitte's symptoms). The potential role of circulating 'blocking factors' in the symptomatology of central demyelinating disease is examined, and some approaches are advanced for the symptomatic therapy of such diseases.

Uhthoff's symptom in optic neuritis: relationship to magnetic resonance imaging and development of multiple sclerosis

Eighty-one patients with a first attack of isolated optic neuritis, 40 with Uhthoff's symptom (Group 1) and 41 without (Group 2), were studied. All had a neurovisual examination, 74 of 81 patients had the pattern visual evoked potential recorded at rest, and 43 had magnetic resonance imaging brain scans. The pattern visual evoked potential P100 latency was prolonged, Group 1 with a mean of 136 +/- 19 msec. Group 2 with a mean of 131 +/- 19 msec (control subjects, 102 +/- 5 msec; n = 84), and the P100 amplitude was reduced, without significant difference between the groups. Abnormal magnetic resonance imaging scans were present in significantly more patients in Group 1 (p less than 0.025). Treatment of optic neuritis with corticosteroids had no effect on the evolution or duration of Uhthoff's symptom. Overall, 35 of 81 (43%) patients, followed for a mean of 3.5 years, developed multiple sclerosis. The incidence was significantly greater in Group 1 (p less than 0.01). Uhthoff's symptom also correlated with a higher incidence of recurrent optic neuritis. We conclude that Uhthoff's symptom is a prognostic indicator for the early development of multiple sclerosis.

Paradoxical effects of temperature in multiple sclerosis

Six cases of multiple sclerosis are reported in which cold produced a temporary exacerbation of symptoms and signs of the disease. Also, in one case investigated in detail by psychophysical methods, heating produced a paradoxical deterioration in vision and simultaneous improvement in sensory and motor function. The effect of temperature in multiple sclerosis is discussed and a physiological explanation for the paradoxical response to heating is suggested.

Fluctuations of visual evoked potential amplitudes and of contrast sensitivity in Uhthoff's symptom

Visual evoked potentials (VEPs) to pattern reversal and psychophysical contrast sensitivity (CS) were tested in five patients with Uhthoff's symptom before and after exercise. Four of the cases were related to a demyelinating disease and one patient had a severe brain injury. Uhthoff's symptom occurred also in neuropathies without previous clinical neuritis and in eyes with normal latency time of the VEPs. A depression of CS was observed by all five patients after exercise; a transient "overshoot" of CS following the depression was present in three of the patients. VEP amplitudes were reduced by exercise in three cases. Responses by stimulation with small checks were more affected. Paradoxical increased VEP amplitudes after exercise in the presence of a loss of psychophysical CS were found in one case.

Exposure times for colour discrimination in the parafoveal field: a new procedure to detect subtle visual dysfunction in multiple sclerosis patients

A new procedure to test colour discrimination in the parafoveal field is described. This method was used on 25 healthy controls and 25 subjects having a definite clinical or laboratory-supported diagnosis of multiple sclerosis, with or without a previous history of optic neuritis. The test proved to be sensitive in detecting visual dysfunction even in some patients in whom the standard technique of pattern-reversal evoked potential recordings yielded a normal result.

Critical flicker frequencies in MS patients with normal or abnormal pattern VEP

Critical flicker frequencies (CFF) were compared with the latencies of visual pattern reversal evoked potentials (VEP) in 2 groups of MS patients to evaluate the value of CFF in electrophysiological diagnostics of demyelination of the optic tract. A microprocessor-based device was constructed to obtain CFF thresholds with a 2-alternative forced-choice method. VEP latency was more frequently pathological than CFF, but CFF was abnormal in 2 optic neuritis patients with normal VEP. The frequency of pathological CFF and VEP was different in the patient group with a long history of MS compared with the new cases; in the freshly diagnosed group, CFF had a better diagnostic yield compared to the patients with a longer history.

Diagnostic techniques in multiple sclerosis

Although multiple sclerosis (MS) has been an area of great interest and effort in numerous scientific disciplines, its etiology, pathogenesis and therapy remain mysterious. Perhaps the most significant advances have resulted from the ability to be more precise in establishing the diagnosis of MS. Among the most important of the new diagnostic tools are electrophysiological techniques for examination of the visual and oculomotor systems. Tests such as the visually evoked potential (VEP) can confirm the existence of clinically suspected lesions and document the presence of asymptomatic ones. Various electrophysiological techniques for detection and evaluation of MS, as well as CSF analysis, psychological and neuroradiological procedures, and immunological observations are described. The relationship between optic neuritis and MS is reviewed, as are therapeutic regimens in use and under study.

Some recent advances in the clinical aspects of multiple sclerosis

Recent work on the clinical aspects of multiple sclerosis is reviewed with particular regard to symptomatology: New approaches to clinical symptoms and the identification of more subtle impairments are illustrated by recent studies of visual function in M.S. patients; pathophysiology: It is now widely appreciated that the dysfunction observed in patients is not determined solely by histologically demonstrable demyelination. The function of the demyelinated neuron is highly variable, being dependent upon factors which may change from day to day. Recent ideas about 'neuro-electric blocking factors' and other factors that may influence demyelinated neurons and hence symptoms are discussed; diagnosis: tests on C.S.F., electro-physiological and psychophysiological tests and computer tomography as aids to diagnosis and the controversy over 'specific' blood tests are reviewed; course and prognosis: Long term follow-up studies confirm that, in a significant proportion of cases, the course of M.S. may be benign and have identified some early prognostic indices;

TREATMENT

The results of trials of symptomatic (spinal cord stimulation) and would-be curative therapies (such as dietary supplementation with poly-unsaturated fatty acids and immunosuppression) are briefly discussed.

Effects of induced hyperthermia on visual evoked potentials and saccade parameters in normal subjects and multiple sclerosis patients

A convenient method for raising body temperature has been developed and used to evaluate temperature effects on visual evoked potentials and saccade reaction time and velocity in five normal subjects and five patients with multiple sclerosis.

Effect of raising body temperature on visual and somatosensory evoked potentials in patients with multiple sclerosis

The effects of raising body temperature on the visual (VEP) and somatosensory (SEP) evoked potentials were observed in normal subjects and in patients with multiple sclerosis. The amplitude of the VEP was significantly reduced to the same degree after heating in normal subjects and in patients with multiple sclerosis but there was no effect on the latency of the potential. Changes in amplitude could not be related to reduction in acuity. In contrast, the cervical SEP was greatly disorganised after heating in many patients with multiple sclerosis while the only effect in normal subjects was to reduce the latency by increasing peripheral conduction velocity. These results suggest that heat caused conduction block in demyelinated axons in the sensory pathways of the cervical spinal cord.