Impaired sweating responses to a passive whole body heat stress in individuals with multiple sclerosis

This study is the first to assess the reflex control of the thermoregulatory system in individuals living with multiple sclerosis (MS). The novel findings are twofold. First, attenuated increases in sweat rate in subjects with MS compared with healthy controls were observed in response to a moderate increase (0.8°C) in core temperature via passive whole body heat stress. Second, it appears the reflex control of the cutaneous vasculature is preserved in MS.

Abstract

Multiple sclerosis (MS) is an autoimmune disease that affects the central nervous system (CNS), disrupting autonomic function. The aim of this study was to test the hypothesis that individuals with MS have blunted control of thermoregulatory reflex increases in sweat rate (SR) and cutaneous vasodilation compared with controls during a passive whole body heat stress (WBH). Eighteen individuals with relapsing-remitting MS and 18 healthy controls (Con) participated in the study. Core temperature (T_core), skin temperature, heart rate, arterial blood pressure (10-min intervals), skin blood flow (laser-Doppler flux, LDF), and SR were continuously measured during normothermic baseline (34°C water perfusing a tube-lined suit) and WBH (increased T_core 0.8°C via 48°C water perfusing the suit). Following WBH, local heaters were warmed to 42°C, inducing peak cutaneous vasodilation at the site of LDF collection. Cutaneous vascular conductance (CVC) was calculated as the ratio of LDF to mean arterial pressure and expressed as a percentage of peak achieved during local heating. Individuals with MS had attenuated SR responses to WBH (ΔSR from baseline: Con, 0.65 ± 0.27; MS, 0.42 ± 0.17 mg·cm⁻²·min⁻¹, P = 0.003), whereas Δ%CVC_42C from baseline was similar between groups (Con, 42 ± 16%; MS, 38 ± 12%, P = 0.39). SR responses were blunted as a function of T_core in MS (interaction: group × T_core, P = 0.03), of which differences were evident at ΔT_core 0.7°C and 0.8°C (P < 0.05). No interaction was observed in Δ%CVC_42C. Taken together, the findings show MS blunts sweating responses, whereas control of the cutaneous vasculature is preserved, in response to WBH.

NEW & NOTEWORTHY This study is the first to assess the reflex control of the thermoregulatory system in individuals living with multiple sclerosis (MS). The novel findings are twofold. First, attenuated increases in sweat rate in subjects with MS compared with healthy controls were observed in response to a moderate increase (0.8°C) in core temperature via passive whole body heat stress. Second, it appears the reflex control of the cutaneous vasculature is preserved in MS.

multiple sclerosis (MS) is an autoimmune disease affecting the central nervous system (CNS). As the most prominent neurological disorder in young adults, the National Multiple Sclerosis Society (NMSS) estimates that 400,000 individuals are currently living with MS in the United States, and 2.3 million worldwide (NMSS 2016). The pathology of MS is marked by chronic autoimmune attacks via activated T cells, leading to the penetration of the blood-brain barrier that ultimately results in demyelination of axons within the CNS, replacing the healthy myelin with lesions (i.e., damaged myelin). Through demyelination of axons within the CNS, there is a loss of conductive properties that increase the risk for reduced conduction velocity (Eisen and Odusote 1980; Schauf and Davis 1974), conduction strength, and, potentially, conduction block (Davis 1970; Smith 1994). Loss of adequate neural conduction has been shown to cause a wide range of MS-related symptomatology, including but not limited to visual dysfunction (Frohman et al. 2011), gait ataxia (Givon et al. 2009; Kalron et al. 2010; Martin et al. 2006), bladder dysfunction (Betts et al. 1993; Miller et al. 1965), and fatigue (Bol et al. 2012; Krupp and Christodoulou 2001; Mollaŏglu and Üstün 2009), as well as autonomic dysfunction (Andersen and Nordenbo 1997; Huang et al. 2015, 2016; Keller et al. 2014; Noronha et al. 1968; Saari et al. 2009).

Although the effects of MS can widely vary among individuals, the majority (60–90%) of MS patients experience a profound heat intolerance known as Uhthoff’s phenomenon (Davis et al. 2010; Flensner et al. 2011; Frohman et al. 2011, 2013), which elicits a temporary worsening of disease-related symptomatology as a result of increased internal temperatures (e.g., warm environments and/or exercise). In some cases, an increase as little as 0.5°C in core temperature (T_core) is sufficient to elicit Uhthoff’s phenomenon (Rasminsky 1973). Although dangers of heat intolerance (e.g., loss of vision, loss of motor control, extreme fatigue, etc.) in MS are well established (Frohman et al. 2013; Guthrie and Nelson 1995; Romberg et al. 2012; White et al. 2013), little progress has been made in regard to understanding the effect of MS on the control of thermoregulatory reflex responses (i.e., skin blood flow and sweating) that are employed to prevent excessive increases in T_core.

Autonomic thermoregulatory responses to increases in T_core require appropriate neural integration within the CNS. Heretofore, little research has been focused on understanding the effect of MS on the reflex control of cutaneous vascular and sudomotor function. Although primarily qualitative, sudomotor dysfunction in MS patients has been reported (Cartlidge 1972; Noronha et al. 1968; Saari et al. 2009; Vas 1969). Using a gross qualitative investigative approach, during a thermoregulatory sweat test (quinzarin powder) investigators have observed alterations or impairments in sweating as a result of MS (Cartlidge 1972; Noronha et al. 1968; Vas 1969). Using a more direct approach, Davis et al. (2005) demonstrated that individuals with MS had a significantly reduced sweat rate and sweat output per gland despite similar total sweat gland recruitment relative to healthy controls when maximal sweat responses from a small area of the dorsal forearm were evoked using iontophoresis of an acetylcholine agonist (pilocarpine). These data are noteworthy because they identify evidence of end-organ impairments, despite MS being primarily a CNS disease, but only provide inferences regarding the maximal capacity rather than a sweat rate (SR) obtained during a patient-relevant heat stress. Additionally, the other primary thermoregulatory mechanism of heat dissipation, skin blood flow, has not been assessed in individuals with MS.

The use of a water perfusion suit to induce a whole body heat stress (WBH) provides a reliable technique to investigate thermoregulatory reflex control of the cutaneous vasculature and sweat glands. By covering ~85% of the body’s surface area with a suit perfusing warm water, investigators can use the suit as a tool to heat the body, ultimately leading to a significant increase in T_core. Subsequent measurements recorded from an exposed limb (e.g., an uncovered forearm) are thought to be representative of the thermoregulatory reflex (i.e., adequate sensing of increased T_core, leading to proportional activation of efferent thermoregulatory end organs), because direct local heating from the suit does not influence the unexposed measurement site. Therefore, the use of this technique in assessing the neural control of efferent thermoregulatory responses could provide valuable insight into MS involvement in the neural control and regulation of the cutaneous vasculature and sweat glands.

To date, the effect of MS on the thermoregulatory reflex has yet to be investigated. Therefore, the primary aim of this study was to test the hypothesis that individuals with MS will have impaired reflex control of sweating and skin blood flow to increases in T_core via passive WBH, leading to diminished sweating and skin blood flow responses compared with matched healthy controls (Con). Collected data, when combined with previous data, also may be able to provide insight into the origin of impairments (central vs. end-organ) in sweat glands and cutaneous blood vessels in individuals with MS.

METHODS

Subjects.

Individuals with definite relapsing-remitting MS (MS; n = 18; expanded disability status scale, EDSS: range 0–6) and age-, sex-, height-, weight-, and body surface area (BSA)-matched healthy controls (Con; n = 18) voluntarily participated in this study. Based on pilot data, an anticipated difference of ~45 mg·cm⁻²·min⁻¹ in sweating between individuals with MS (n = 4) and healthy controls (n = 4) was expected (effect size = 1.47). A sample size of six subjects per group was derived from a power analysis (α = 0.05, power = 0.80) to find an anticipated difference in SR between groups during WBH. Because of the heterogeneity of symptomatology and disease course in individuals with MS, additional subjects were recruited.

All individuals with MS were otherwise healthy, but clinically diagnosed and currently being treated by neurologists specializing in MS at the MS Clinic at the University of Texas Southwestern Medical Center. This study focused on relapsing-remitting MS because it is the most common disease course: ~85% of individuals with MS are initially diagnosed with this form of the disease (NMSS 2016). This form of MS is characterized by acute attacks (exacerbations) followed by periods of partial or complete recovery (remissions) (NMSS 2016). All individuals with MS were on disease-modifying therapies. According to the 2015 consensus paper by the Multiple Sclerosis Coalition on the use of disease modifying therapies, FDA-approved disease-modifying treatment (DMT) is recommended and should be continued indefinitely (Costello et al. 2015). Individuals with MS on symptom-modifying therapies that are known to affect the CNS and/or thermoregulatory responses (i.e., antidepressants, psychostimulants, anticonvulsants, antispasmatics, and anticholinergics) were excluded from the study. Individuals with MS remained on their DMT as prescribed by their neurologists but were asked to refrain from taking any additional supplements and/or over-the-counter medications before testing. All individuals with MS were tested during a stable phase of their disease and were at least 6 mo removed from their most recent relapse. In an effort to avoid any differences associated with acclimation status, healthy controls were tested during the same time of year and during the same time of day (9:00 AM–12:00 PM) as their MS counterparts. All procedures were approved by the Institutional Review Boards at Southern Methodist University (Dallas, TX) and the University of Texas Southwestern Medical Center (Dallas, TX), and all conformed to the Declaration of Helsinki. All subjects were instructed on the protocol and provided their written informed consent before participation in the study.

Instrumentation.

Internal temperature (T_core) was assessed using an ingestible telemetry thermometry pill (HQ, Palmetto, FL) taken ~1–2 h before the collection of baseline measurements. Mean skin temperature (T_sk) was measured via the weighted average of six thermocouples attached to the skin (Taylor et al. 1989). Arterial blood pressure was obtained via auscultation of the brachial artery (SunTech Medical Instruments, Raleigh, NC). Heart rate was identified using an electrocardiogram (ECG; Solar 8000i; General Electric, Schenectady, NY) interfaced with a cardiotachometer (CWE, Ardmore, PA). Sweat rate (SR) was measured using capacitance hygrometry (Vaisala, Woburn, MA) via the ventilated capsule method (Bullard 1962; Lang et al. 1993) on a proximal and a distal site on the skin of the exposed dorsal forearm as used previously (Davis et al. 2007). Laser-Doppler flux (LDF; an index of skin blood flow) was measured at a proximal and a distal site on the skin of the exposed dorsal forearm positioned at heart level via integrated laser-Doppler probes (PF413; Perimed, Ardmore, PA) connected to a laser-Doppler flowmeter (PF5010; Perimed). Probes were fitted inside thermostatic probe holders (PF450; Perimed) connected to a local heating device (PF5020; Perimed) capable of controlling local skin temperature at that site. For both SR and LDF, the mean values from the proximal and distal sites were averaged and recorded at each of the corresponding changes in T_core of interest. To modify T_sk and T_core, subjects were fitted in a tube-lined perfusion suit (Allen-Vanguard Technologies, Ottawa, ON, Canada), allowing the manipulation of water temperature to provide a thermal stress while leaving the instrumented forearm (as well as the head and feet) exposed for collection of LDF and SR data. All measurements were taken from participants lying in the supine position on a patient bed.

Protocol.

This study consisted of 1 experimental day lasting ~3.5 h. Baseline measurements were taken from the final minute of a 10-min normothermic (NT) baseline period, in which 34°C water perfused the tube-lined suit. After confirmation of adequate baseline measurements, WBH was performed by circulating 48°C water in the suit until an increase in T_core of 0.8°C was observed. This moderate level of heat stress has been used in similar studies and has been shown to be of sufficient magnitude to induce changes in skin blood flow and sweating (Davis et al. 2007; Low et al. 2011). The following variables were continuously measured throughout the heating protocol: T_core, T_sk, heart rate, LDF, and SR. Upper arm cuff blood pressures were taken at 10-min intervals. Because the suit was not in contact with the regions of skin where LDF and SR data were assessed, responses from these areas are presumed to be due entirely to thermoregulatory reflex-mediated neural modulation of efferent activity in response to changes in T_core induced by the suit. Once the desired increase in internal temperature (0.8°C) was achieved, T_core was stabilized by lowering the temperature of the water perfusing the suit to 42°C. Once a stable elevated T_core was established, WBH measurements were recorded before subjects were returned toward NT T_core levels by decreasing the temperature of the water perfusing the suit to 16°C. During this time period, local heating was performed at the sites of LDF collection on the forearm by setting thermostatic probe holders to 42°C. Local heating was terminated after a plateau in LDF values was observed (~30 min). LDF was then normalized relative to peak vasodilation observed at 42°C for each site.

Data analysis.

All data were continuously acquired at a sampling rate of 100 Hz on a 16-channel data acquisition system (Biopac, Santa Barbara, CA). Mean values from all measured variables, with the exception of cuff blood pressure, were obtained from the final minute of NT and WBH. Because of the dramatic increase in heart rate of 40–45% in both groups as a result of WBH, mean arterial pressure (MAP) during both NT and WBH was calculated according to two different equations: 1) the traditional equation MAP_trad = [(systolic blood pressure − diastolic blood pressure)/3 + diastolic blood pressure]; and 2) an adjusted equation (MAP_adj), which applies mathematical weighting of systolic blood pressure based on the proportionally less time spent in systole (Moran et al. 1995). Skin blood flow is reported as cutaneous vascular conductance (CVC) and was calculated by dividing mean LDF values (arbitrary units, au) by MAP (both MAP_trad and MAP_adj) calculated from brachial artery auscultation. CVC data were also normalized to peak vasodilation obtained during the final minute of local heating at 42°C for 30 min and are expressed as a percentage of peak CVC (%CVC_42C). Changes in these variables relative to NT baseline conditions are plotted as means ± SD.

Statistical analysis.

All values are means ± SD. A two-way repeated-measures ANOVA was utilized to compare SR of groups (MS vs. Con) as a function of T_core (0.1°C increments). Because of the necessity of using estimations of MAP to calculate CVC, it was not possible to appropriately calculate CVC as a function of T_core. A two-way repeated-measures ANOVA was utilized to compare LDF responses (au) of groups (MS vs. Con) as a function of T_core (0.1°C increments). Post hoc multiple comparisons were performed utilizing paired t-tests with a Bonferroni adjustment. Paired t-tests were performed on ΔCVC from baseline as a result of WBH (0.8°C increase in T_core) and local heating responses. For any significant differences between groups, an effect size (Cohen’s d) was calculated to provide practical significance. All analysis was conducted using SPSS version 23 (IBM, Armonk, NY) and plotted via GraphPad Prism 6 (GraphPad Software, La Jolla, CA). Statistical significance was accepted at P < 0.05.

RESULTS

Subject characteristics.

Subject characteristics for both groups are presented in Table 1. The ranges for MS and Con groups were as follows: age (MS: 28–54; Con: 28–53 yr), height (MS: 152–193; Con: 160–196 cm), weight (MS: 48.2–102.9; Con: 48.2–93.1 kg), body surface area (MS: 1.46–2.34; Con: 1.53–2.26 m²), and body mass index (MS: 17.6–35.2; Con: 17.1–30.0 kg/m²). All individuals with MS were diagnosed with relapsing-remitting MS 14 ± 4 yr before participation in the study. Disease-modifying treatments used by individuals with MS included the following: Avonex (interferon β-1a), n = 1; Gilenya (fingolimod), n = 3; Copaxone (glatiramer acetate), n = 3; Tecfidera (dimethyl fumarate), n = 4; Tysabri (natalizumab), n = 5; Plegridy (peginterferon beta-1a), n = 1; and no DMT, n = 1.

Table 1.

Subject characteristics

	MS	Con	P Value
Age, yr	41 ± 7	39 ± 8	0.32
Weight, kg	72 ± 17	70 ± 14	0.68
Height, cm	169 ± 11	172 ± 10	0.50
BMI, kg/m2	24.8 ± 4.3	23.6 ± 3.2	0.51
BSA, m2	1.8 ± 0.3	1.8 ± 0.2	0.86

Values are means ± SD in individuals with multiple sclerosis (MS; n = 18) and healthy control subjects (Con; n = 18). BMI, body mass index; BSA, body surface area.

Normothermic baseline.

Baseline heart rate was significantly greater in the Con group (Con: 72 ± 16; MS: 65 ± 7 beats/min, P = 0.19), whereas baseline systolic (Con: 115 ± 12; MS: 108 ± 13 mmHg, P = 0.13), diastolic (Con: 74 ± 8; MS: 70 ± 10 mmHg, P = 0.26), MAP_trad (Con: 88 ± 9; MS: 83 ± 11 mmHg, P = 0.17), and MAP_adj (Con: 88 ± 9; MS: 83 ± 11 mmHg, P = 0.15) were similar between groups. Additionally, there were no group differences in T_core (Con: 37.1 ± 0.3; MS: 37.2 ± 0.3°C, P = 0.22), T_sk (Con: 35.1 ± 0.4; MS: 34.9 ± 0.5°C, P = 0.12), and SR (Con: 0.11 ± 0.02; MS: 0.10 ± 0.03 mg·cm⁻²·min⁻¹, P = 0.16). All indexes of skin blood flow were similar during NT baseline: LDF (Con: 18 ± 8; MS: 17 ± 7 au, P = 0.60), mean CVC_trad (Con: 0.20 ± 0.09; MS: 0.20 ± 0.08 au/mmHg, P = 0.98), mean CVC_adj (Con: 0.20 ± 0.09; MS: 0.20 ± 0.08 au/mmHg, P = 0.96), %CVC_42C:trad (Con: 9.1 ± 5.1; MS: 7.7 ± 3.2, P = 0.32), or %CVC_42C:adj (Con: 9.1 ± 5.1; MS: 7.8 ± 3.2, P = 0.38).

Heat stress.

Hemodynamic responses, represented as change from baseline, to WBH are presented in Table 2. During the last minute of WBH, heart rate increased to a similar level (Con: 100 ± 16; MS: 95 ± 14 beats/min, P = 0.44), whereas diastolic blood pressure was similarly maintained (Con: 69 ± 9; MS: 66 ± 5 mmHg, P = 0.14). Interestingly, although systolic blood pressure following heat stress was not statistically different (Con: 125 ± 15; MS: 115 ± 17 mmHg, P = 0.09), MAP_trad (Con: 88 ± 8; MS: 83 ± 8 mmHg, P = 0.08) and MAP_adj (Con: 92 ± 8; MS: 86 ± 10 mmHg, P = 0.06) trended higher in the Con group.

Table 2.

Thermal and hemodynamic responses to heat stress

	MS	Con	P Value
ΔTcore, °C	0.80 ± 0.01	0.80 ± 0.01	0.13
ΔHeart rate, beats/min	30 ± 11	28 ± 9	0.67
ΔSystolic blood pressure, mmHg	8 ± 12	10 ± 10	0.51
ΔDiastolic blood pressure, mmHg	−4 ± 8	−5 ± 9	0.94
ΔMAPtrad, mmHg	0 ± 8	0 ± 17	0.78
ΔMAPadj, mmHg	4 ± 8	4 ± 7	0.74

Values are means ± SD, expressed as changes from baseline following whole body heating, in individuals with multiple sclerosis (MS) and healthy control subjects (Con). T_core, core temperature; MAP_trad, mean arterial pressure calculated traditionally [(systolic blood pressure − diastolic blood pressure)/3 + diastolic blood pressure]; MAP_adj, mean arterial pressure as a function of heart rate.

Thermoregulatory responses, represented as change from baseline, to WBH are presented in Figs. 1, 2, and 3. Of importance, a large effect size (d = 0.81 and d = 1.01) for SR differences between groups was observed at ΔT_core of 0.7°C and 0.8°C, respectively. During the final minute of WBH, the maximal SR achieved by individuals with MS was significantly lower than that of Con (Con: 0.76 ± 0.27; MS: 0.52 ± 0.18 mg·cm⁻²·min⁻¹, P = 0.003). T_core (Con: 37.9 ± 0.3; MS 38.0 ± 0.2°C, P = 0.20), T_skin (Con: 38.6 ± 0.5; MS: 38.7 ± 0.5°C, P = 0.50), LDF (Con: 109 ± 38; MS: 106 ± 28, P = 0.92), CVC_trad (Con: 1.3 ± 0.4; MS: 1.3 ± 0.5 au/mmHg, P = 0.55), CVC_adj (Con: 1.2 ± 0.4; MS: 1.2 ± 0.5 au/mmHg, P = 0.79), %CVC_42C:trad (Con: 53 ± 17; MS: 47 ± 14, P = 0.41), and %CVC_42C:adj (Con: 51 ± 16; MS: 45 ± 14, P = 0.30) were similar during the final minute of heating.

Fig. 1.

Mean change in sweat rate (ΔSR) from baseline (±SD) as a function of changes in core temperature (ΔT_core) during passive whole body heating (A) and mean group (bars) and individual summary data (squares) showing ΔSR from baseline following a 0.8°C increase in T_core (B) in persons with multiple sclerosis (MS) and healthy control subjects (Con). *Differences observed at 0.7°C and 0.8°C (Bonferroni-adjusted P < 0.05).

Fig. 2.

Mean change in laser-Doppler flux (ΔLDF; arbitrary units, au) from baseline (±SD) as a function of changes in core temperature (ΔT_core) during a whole body heat stress in individuals with multiple sclerosis (MS) and healthy control subjects (Con).

Fig. 3.

Mean group (bars) and individual (squares) changes in peak cutaneous vascular conductance (ΔCVC_42C:adj) from baseline during passive whole body heating in individuals with multiple sclerosis (MS) and healthy control subjects (Con).

Local heating.

Peak LDF responses to 42°C local heating are presented in Fig. 4. In short, there were no significant differences in any peak values for LDF (P = 0.41), CVC_42C:trad (P = 0.32), or CVC_42C:adj (P = 0.32).

Fig. 4.

Mean group (bars) and individual (squares) responses in peak heart rate-adjusted cutaneous vascular conductance (CVC_42C:adj; A), peak traditionally calculated cutaneous vascular conductance (CVC_42C:adj:trad; B), and peak absolute laser-Doppler flux (LDF; C) values during the final minute of local heating to 42°C in individuals with multiple sclerosis (MS) and healthy control subjects (Con).

DISCUSSION

The present study is the first to provide evidence of impairments of thermoregulatory reflex sweating in individuals with MS during passive WBH. Confirming our hypothesis, sweating responses were blunted in individuals with MS during a moderate increase in core temperature compared with healthy controls. Individuals with MS had significantly reduced sweat rates as a function of core temperature and this difference increased as temperature increased. Counter to our hypothesis, the reflex control of the cutaneous vasculature (represented as %CVC_42C) appears to be preserved in individuals with MS, because increases in skin blood flow were similar between individuals with MS and healthy controls. Taken together, these findings indicate that MS impairs the reflex control of thermoregulatory responses to passive heat stress.

The thermoregulatory reflex requires complex integration of afferent and efferent signal processing. On exposure to a rising core temperature, thermoreceptors at the skin and the core provide afferent information that is relayed to the preoptic area of the hypothalamus (thermoregulatory center of the brain) (Gordon and Heath 1986; Shibasaki et al. 2006). Once integrated, it is thought this brain center modulates heat dissipation via increasing autonomic signaling to thermoregulatory effector organs (i.e., cutaneous vasculature and sweat glands). The observed reduction in the neural control of sweating in the MS group provides evidence of autonomic impairments as a result of the disease. The mechanisms responsible for this are not readily apparent. With MS being an autoimmune disease affecting the CNS, it is probable that neural conduction within the brain and spinal cord plays a pivotal role in the blunted SR responses observed in this study. As previously mentioned, reduction in myelin (the underlying pathophysiology of MS) has detrimental effects on neural conduction. Reduced conduction velocity, impaired saltatory conduction, and increased incidence of conduction block stems from inadequate insulation properties that are typically provided by an intact myelin sheath (Frohman et al. 2011). Reduced myelin cross sectional area, and thus increased axonal exposure, leads to decreases in the current available relative to the current required for signal propagation (i.e., axonal “safety factor”). Therefore, it remains possible that a dampening of the signal for sweating, impaired integration of the signal for sweating, and/or the efferent signal being relayed from the CNS occurs in MS. Additionally, the increased axonal exposure as a result of decreased myelin cross-sectional area increases the relative reliance on voltage-gated sodium and potassium channels responsible for signal transmission down demyelinated areas of the axon. Over time, this increased voltage-gated channel reliance ultimately leads to increased expression of sodium channels at exposed demyelinated sites (Smith 1994, 2007; Smith et al. 1982), in an effort to ensure action potential propagation. Although this adaptation along the axon may be beneficial for maintaining conduction during normothermic resting conditions, it is thought that these sodium channels are temperature sensitive (Frohman et al. 2011), potentially altering neural signaling/processing within the CNS when internal temperature is increased. On relatively modest increases in T_core (~0.2–0.5°C), these temperature-sensitive sodium channels close, thus terminating the depolarization phase of subsequent action potentials (Frohman et al. 2011, 2013). Although not directly tested in this study, it is likely this adaptation contributed to the reduction in reflex-mediated sweating responses observed herein.

In the present investigation, the magnitude of increase in SR was blunted during passive WBH. Altered control of sweating in individuals with MS has been identified previously (Cartlidge 1972; Noronha et al. 1968; Saari et al. 2009). Initially, investigators used the technique of staining the skin of participants with quinizarin powder before increasing their body temperature via passive heating (Noronha et al. 1968; Vas 1969). Quinizarin powder, initially yellow in color, stains to a dark shade of purple when in contact with moisture (i.e., perspiration). The use of this technique allows for visual inspection of sweating abnormalities but lacks the capability to quantify sweating responses (qualitative only measurement). More recently, with the use of more quantifiable methods, peripheral sweat gland dysfunction has been identified (Davis et al. 2005). In this study, Davis et al. (2005) demonstrated that postganglionic sympathetic nerves and sweat glands were responsive to maximal peripheral cholinergic stimulation in individuals with MS. However, individuals with MS demonstrated a diminished ability to sweat (decreased output per sweat gland) despite similar sweat gland recruitment compared with healthy counterparts. As a follow-up study, Davis et al. (2005) addressed the potential for decreases in sweat function being attributed to detraining or de-adaptation in this clinical population by exercise training a cohort of MS patients. Expected improvements in sweating responses (e.g., increases in maximal sweating) following aerobic exercise training previously demonstrated in healthy individuals (Crandall et al. 2003; Wilson et al. 2010) were not observed in individuals with MS, suggesting impairments in thermoregulatory control. Given these findings, the current study utilized a passive WBH stress model to investigate sweating responses on an exposed forearm, to target thermoregulatory reflex-mediated modulation of efferent activity in response to changes in T_core. In agreement with aforementioned studies, the findings of the current study provide further evidence of sweating impairments and suggest these impairments could be attributed to altered neural control of sudomotor function. Taken together, the above findings indicate that MS impairments in sudomotor control within the CNS are complex and could be attributed to interactions of one or more of the following: 1) altered sensory processing, 2) neural integration abnormalities within the CNS, and/or 3) inappropriate end-organ effector processing. Ultimately, these alterations of neural function could lead to end-organ maladaptations in the sweat glands themselves, including a downregulation of cholinergic receptors (number and/or sensitivity) on the eccrine sweat gland, which would ultimately result in lower sweating responses per quanta of acetylcholine released and/or atrophy of sweat glands themselves.

The finding of preserved cutaneous vascular function in individuals with MS in response to an increase in T_core is an interesting one. The %CVC_42C:adj values from the groups in this study, which align well with values reported in a similar study (Davis et al. 2007), showed no apparent differences at any T_core value. This suggests that not only is the control of the cutaneous vasculature in MS similar to controls in response to heat stress, but sweating and skin blood flow may potentially be under separate neural control. However, it also remains possible that the change in T_core utilized in this study was not severe enough to reveal significant cutaneous vascular impairment. For healthy participants, in response to light exercise (~30% V̇o_2max), T_core increases nearly 0.8°C, whereas increases in T_core of ~1.5°C and 2.0°C are seen with moderate (~51% V̇o_2max) and moderate-high (~70% V̇o_2max) exercise intensities, respectively (Saltin and Hermansen 1966). Additionally, in a study by Low et al. (2011) investigating more severe levels of heat stress in healthy participants (~Δ1.5°C T_core), the authors demonstrated that CVC, skin sympathetic nerve activity (SSNA), and muscle sympathetic nerve activity (MSNA) increase as a function of T_core. This increase in SSNA as a result of WBH was likely attributed to activation of sweat glands and increasing dilation of the cutaneous vasculature (i.e., increasing CVC) to facilitate heat dissipation. Although in the current study, differences in %CVC_42C:adj responses to a passive heat stress that elicits an 0.8°C change in T_core were not apparent between groups, potential differences could possibly be revealed at higher levels of heat stress in which more autonomic activity is required. However, a modest change (0.8°C) in T_core was chosen in the current study in effort to elicit measureable SR and LDF responses while avoiding the onset and/or minimizing symptomatology associated with Uhthoff’s phenomenon. Further research is warranted to investigate the neural control of the cutaneous vasculature at higher T_core levels in individuals with MS. Given that thermoregulatory reflexes utilize postganglionic nerves to induce both vasodilation and sweating, it is perplexing that SR responses are blunted while the control of skin blood flow is seemingly left intact. Clearly, more research is needed to elucidate these observations and overlying mechanisms.

In this investigation, blunted reflex control of sweating in MS was apparent at relatively moderate increases in T_core (~0.8°C), whereas control of skin blood flow remained intact. These levels of elevated T_core are likely encountered on a daily basis among most individuals and lie within the minimum core temperatures necessary to elicit Uhthoff’s phenomenon. Because the evaporation of sweat is the most effective form of heat dissipation among humans, suppressed control of this mechanism is particularly detrimental to individuals with MS. The disease likely renders these individuals unable to adequately prevent heat gain (increased T_core) via blunting of sweating responses. The combination of heat sensitivity (Uhthoff’s phenomenon) and apparent thermoregulatory dysfunction creates a vicious cycle for individuals vulnerable to excess rises in T_core, with known heat intolerance.

Limitations and future considerations.

Although our findings are consistent with CNS impairments (Andersen and Nordenbo 1997; Frohman et al. 2011; Noronha et al. 1968; Vas 1969), it remains possible that our results were influenced by DMT prescribed to individuals with MS. Patients were carefully screened for additional medications (i.e., antidepressants, psychostimulants, anticonvulsants, antispasmatics, and anticholinergics), and precautions were taken to remove any clear outliers that might influence group data. With this in mind, immediate prescription of DMT upon diagnosis is the current standard of care (Costello et al. 2015), and thus a critical understanding of individuals with MS currently on these medications is warranted.

CVC data from the current study are referenced to nonnoxious local skin heating to 42°C (Johnson et al. 2014). It may be possible that further vasodilation could have occurred at higher, potentially noxious, local heating temperatures (e.g., 43–46°C). Cutaneous nociceptors robustly express transient receptor potential vanilloid type 1 (TRPV1) channels (Ständer et al. 2004), which are necessary in the heat-pain pathway (Caterina et al. 2000; Davis et al. 2000). These receptors are stimulated at temperatures <42°C (Bevan et al. 2014) even if not consciously processed as pain in normal healthy participants. Given that individuals with MS present with somatosensory abnormalities (e.g., reduction in skin temperature detection thresholds) and central pain (with incidence rates of 50–55%), the use of higher temperatures could become more noxious (Leocani et al. 2003), thus affecting between-group comparisons. Thus, to avoid a potential noxious stimulus and collateral physiological responses, a 42°C local heating protocol was utilized (Johnson et al. 2014). Additionally, as part of the thermoregulatory reflex, elevated local skin temperatures can increase sweat output to aid in heat loss. Because local skin temperature at the SR measurement site was not recorded in the current study, it remains possible that there could be local influences aiding in the responses seen here. That being said, the autonomic control of local skin temperature is part of the thermoregulatory reflex and would only serve to strengthen our findings based on these results.

The use of perfusion suits to induce a tightly controlled passive heat stress limits the ability to assess certain aspects of thermoregulatory function. By covering ~85% of BSA, this heat stress technique effectively reverses skin heat transfer covered by the suit to that of an “outside-in” direction rather than “inside-out.” Although there are circumstances in which humans can face this form of heat stress, the majority of physiological heat stress is encountered from increases in metabolism (i.e., physical activity). To fully understand the impact of impaired sweating in MS on heat dissipation, further research is needed with the use of exercise as an experimental paradigm to increase core temperature. Finally, the experimental design required subjects be positioned in the supine position. Many heat stresses experienced by individuals with MS involve exposures or activity in the upright position. This may be doubly problematic to these patients, because current data suggest that dysautonomia may also manifest itself with the control of arterial blood pressure (Huang et al. 2016; Keller et al. 2014).

Conclusions.

This study utilized a novel approach to examine the reflex control of thermoregulatory responses to a passive heat stress in individuals with MS. These results demonstrate individuals with MS attenuated increases in SR, leading to suppressed SR in response to moderate increases in T_core. Furthermore, it appears that MS does not affect the reflex control of the cutaneous vasculature in response to moderate elevations in T_core. These findings are particularly troubling for individuals with MS, because this altered reflex control of thermoregulatory responses could contribute to decreased heat loss capacity. This apparent thermoregulatory dysfunction can compound heat intolerance (Uhthoff’s phenomenon), which may result in increased frequency and susceptibility to this acutely debilitating symptomatology. Further research is warranted to determine whether this attenuated reflex control translates to thermoregulatory dysfunction, especially during physical activity.

GRANTS

This study was supported by National Heart, Lung, and Blood Institute Grant R15-HL-117224 (to S. L. Davis) and National Multiple Sclerosis Society Grants RG4043A1/1 and RG4696A3/2 (to S. L. Davis).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

D.R.A., E.M.F., and S.L.D. conceived and designed research; D.R.A., M.H., I.M.P., A.R.D., and S.L.D. performed experiments; D.R.A., M.H., I.M.P., and S.L.D. analyzed data; D.R.A., M.H., E.M.F., and S.L.D. interpreted results of experiments; D.R.A., M.H., and S.L.D. prepared figures; D.R.A. and S.L.D. drafted manuscript; D.R.A., M.H., I.M.P., A.R.D., E.M.F., and S.L.D. edited and revised manuscript; D.R.A., M.H., I.M.P., A.R.D., E.M.F., and S.L.D. approved final version of manuscript.