Practice Points
- Increased core temperature during exercise can limit the dosage and intensity of exercise performed by people with multiple sclerosis (MS), which can limit the effectiveness of physical therapy.
- The use of cooling vests may limit the effects of thermosensitivity in people with MS, as evidenced by improved 6-minute walk test performance compared with an uncooled condition.
- Physical therapists working with people with MS should consider using cooling vests to increase exercise dosage and intensity.
Walking impairment is highly prevalent in people with multiple sclerosis (MS), and its remediation is a common rehabilitation goal. Due to the multifactorial nature of MS, walking impairments occur for heterogeneous reasons, making consistent rehabilitation strategies challenging. An intervention to improve walking across a spectrum of MS presentations would therefore be a useful tool.
Thermosensitivity—defined as a transient worsening of neurological function due to increases in core body temperature—is commonly associated with impaired walking in MS. Mobility tasks, such as walking, increase core temperature and limit the amount of exercise that can be tolerated. Lowering core temperature may improve walking performance in people with MS. Cooling has been studied in MS, but its specific impact on walking has not been well examined.
A systematic review identified 6 studies examining the impact of cooling on mobility in those with MS.3 Although the authors concluded that cooling generally resulted in positive outcomes, the studies reviewed had some issues with clinical generalizability. Buoite et al measured walking using distance to fatigue; however, this measurement lacks the validity and reliability of the 6-minute walk test (6MWT). Gonzales et al used an interventional pilot study combining walking and cooling; however, it is not known whether the improvement was due to the training, the cooling, or both. Grahn et al used cooling of 1 hand as an intervention as well as distance to fatigue as an outcome measure. Meyer-Heim and Nilsagård and their colleagues used extremely short walking tests as outcome measures, which may not be valid for assessing gait fatigue in people with MS.
In this study, we investigated the use of an inexpensive, commercially available cooling vest on walking endurance in participants with MS. We hypothesized that they would walk longer distances in the cooled vs the uncooled condition. If successful, this intervention would add a simple, inexpensive, and nonpharmacologic approach to improve walking endurance in people with MS.
Methods
Research Design
We used a randomized crossover design where each participant was randomly assigned (by pulling odd and even numbers out of an envelope during their first session) to either the cooled condition (CC) or the uncooled condition (UC). Participants were then reassigned to the opposite condition in their second session. There were 7 to 14 days between sessions. This study was approved by the Hunter College Institutional Review Board (Protocol #: 2019-0336).
Participants
We recruited a convenience sample through flyer distribution to local MS-related medical providers, patient organizations, and support groups. Inclusion criteria included a definitive diagnosis of MS, being at least 18 years of age, the ability to walk 6 minutes without the assistance of another person, with or without an assistive device, and the ability to read and provide informed consent. Exclusion criteria included steroid use within the past 30 days; MS exacerbation (neurologist or self-reported) in the past 30 days; or any orthopedic, cardiovascular, or pulmonary condition that would limit the participant’s ability to complete the intervention.
Procedure
Once consent was obtained, participant characteristics were collected, including age; sex; years since diagnosis; MS phenotype; and scores on the Expanded Disability Status Scale (EDSS), Multiple Sclerosis Impact Scale-29 (MSIS-29), Fatigue Severity Scale (FSS), and Multiple Sclerosis Walking Scale-12 (MSWS-12). Validity and reliability in MS for these tests have been established. Tests were administered by the first author (H.K.) except for the EDSS, which was given by the participant’s neurologist in the course of clinical care. These measures were included to better understand the population and allow for appropriate generalizability of study results to patients in the clinic.
Participants in the CC were seated and donned the cooling vest for 30 minutes. We used the Kool Max Adjustable Zipper Cooling Vest (Polar Products, USA). Each vest holds 12 ice packs. In the uncooled cohort, participants sat for
30 minutes without a cooling vest. All participants were instructed to wear a cotton T-shirt, comfortable pants or shorts, and the same shoes for each session. All data were collected in a temperature-controlled room, and no data were collected during the summer months to avoid the possibility of the impact of increased room temperature.
Outcome Measures
Participants in the CC completed a visual analog scale to measure perceptions of cold (VAS-C), which assessed their current subjective feeling of coldness before and after 30 minutes in the cooling vest. Following the 30-minute period of cooling (CC) or rest (UC), participants completed the 6MWT. During the test, participants were asked to walk in a 100-foot hallway at their fastest comfortable pace for 6 continuous minutes using the assistive device they would normally use. We measured total distance walked as well as the distance walked every minute to assess any slowing over time. Previous research has demonstrated that people with MS often slow down during the 6MWT, suggesting evidence of fatigability., Stellman suggested that the 6MWT provided greater ecological validity than shorter walking tests for people with MS. Participants also completed the visual analog scale to measure subjective perceptions of fatigue (VAS-F) before and after the 6MWT.
Data Analysis
All analyses were conducted using IBM’s SPSS, version 24.0. The normality of the data was examined using the Kolmogorov-Smirnov test and visually with quantile-quantile plots. The 6MWT and VAS-C scores within each condition were normally distributed. Thus, we used parametric statistics. The differences in VAS-C (Δ VAS-C) from before and immediately after wearing the cooling vest for 30 minutes, and from before to after the 6MWT, were analyzed with a 1-sample t-test comparing the mean change to zero. Distance walked during the 6MWT was analyzed with a 2-way within-subjects analysis of variance comparing cumulative distance walked during each minute of the 6MWT between the UC and CC conditions. The differences in VAS-F scores before and after the 6MWT (Δ VAS-F) were compared between the UC and CC conditions using a within-subjects t test. We set α to .05 and used a 2-tailed statistical approach.
Sample Size
We used the G*Power software to calculate the sample size using the effect size from a previous walking study in people with MS with a similar research design and outcome measures.16 Using the effect size value of ηp2 = .42, an α of .05, and a desired power of .80, our a priori power analysis showed that a total sample size of 14 would yield sufficient power to detect significant differences in the analysis proposed.
Results
The final sample included 9 women and 6 men, ranging in age from 44 to 71 years (mean [M] = 61.1 years). Years since diagnosis ranged from 5 to 40 years (M = 16.8 years). EDSS scores ranged from 3 to 5 (median [mdn] = 4), FSS scores ranged from 27 to 63 (mdn = 50), and MSIS-29 converted scores ranged from 18% to 99% (mdn = 44.8%). Information about participant characteristics is included in the Table. Eight participants underwent the CC first, followed by the UC, and 7 participants completed the study in the opposite order. Independent t tests showed no testing order effects on outcome measures (all P ≤ .320).
VAS-C
Figure 1 shows that the cooling vest had a significant positive effect on participants’ VAS-C. Their perception of cold significantly increased (M = 20.6, SD = +21.2) after wearing the vest for 30 minutes (t[14] = 3.77, P= .002, d = .97). However, participants felt significantly warmer (M = –12.4, SD = +18.8) after performing the 6MWT (t[14] = –2.55, P = .023, d = .66). Consequently, participants’ perception of cold after the 6MWT (M = 9.80, SD = +23.8) was similar to how it was when they arrived for the session (t[14] = 1.60, P = .133, d = .41).
6MWT
As shown in Figure 2, wearing the cooling vest for 30 minutes prior to the 6MWT showed a significant improvement over time compared with when participants did not wear the cooling vest (F(5, 70) = 5.50, P < .000, ηp2 = .28). Post hoc analyses revealed that during the CC, participants accumulated farther distance at minutes 5 (M = 200.5 meters [m], SD = +138.5) and
6 (M = 242.5 m, SD = +166.5) compared with the UC (M =
186.8 m, SD = +137.9 at minute 5 and M = 228.4 m, SD = +167.7 at minute 6, all P ≤ .016).
VAS-F
Figure 3 shows participants’ change in subjective fatigue from pre- to post 6MWT. The average increase in fatigue in the CC (M = 16.80, SD = +20.31) compared with the UC (M = 22.93, SD = +20.54) was not significantly different (t[14] = –.97, P = .348, d = .25).
Discussion
We tested an inexpensive, readily available cooling device on walking in people with MS, hypothesizing that the addition would result in improved walking endurance compared with UC. Our results support this hypothesis, as participants in a CC walked longer distances than those in the UC. This finding provides evidence that cooling can improve performance in longer walks, such as a 6-minute walk, in people with MS with the single application of a cooling vest.
Up to 90% of people with MS complain of difficulty with walking.2 This difficulty is multifactorial, and possible limitations are due to impairments in strength, flexibility, sensation, motor control, and fatigue. The heterogeneity of walking dysfunction in MS makes the application of standardized intervention strategies problematic. However, an overarching issue that limits walking in people with MS is fatigability, defined as the magnitude or rate of change in performance over a given time. Pragmatically, fatigability in MS refers to the inability to walk for sufficient periods, and it can restrict functional tasks. Our results provide evidence that cooling can impact fatigability, as evidenced by improvement in the 6MWT, suggesting that cooling may improve walking in people with MS regardless of disease expression. Participants not only walked longer distances overall in the CC, but the majority increased their distances later in the 6MWT test (at minutes 5 and 6). This might suggest that participants were able to maintain their pace in the CC, providing further evidence that cooling decreases fatigability in MS.
Using the 6MWT as a primary outcome measure was essential for our study. Although shorter walking tests commonly used in MS (eg, 10-meter or 2-minute walk tests) may provide useful information regarding walking short distances, they lack the ecological validity of longer walking and may not be of sufficient duration to provoke fatigue. Since the purpose of this study was to examine the impact of cooling on walking fatigue in MS, a measure that was specifically provocative of MS fatigue was indicated. The 6MWT may also be more representative of typical functional walking than shorter measures. Although the psychometric properties of shorter walking tests are robust, they may not provide useful information for MS clinical decisions. An additional advantage of the 6MWT is that it allows measurement of specific time intervals. The slowing of walking over the last 2 minutes in the UC relative to the CC would not have been obtainable using other tests.
Several studies have provided evidence of the effectiveness of cooling in improving mobility in people with MS. However, many modalities may not be feasible (eg, cold water immersion) or too costly (eg, technologically advanced cooling suits) for regular clinical practice. In this study, we intentionally examined an inexpensive and easily obtainable device that is often provided for free in the United States by MS advocacy groups. This represents a means by which MS mobility deficits can be effectively managed at minimal cost.
Despite our positive findings, our study has several limitations. Although diverse, our sample was relatively small. Our study was unblinded, so participants may have simply thought the CC was better for walking. However, since there was no significant difference in perceptions of fatigue between the 2 conditions, unblinding may not have been an issue. This absence of difference may be attributable to objective and subjective fatigue being separate phenomena, or it may represent a limitation in using a self-report fatigue measure. Moreover, unlike previous MS cooling studies, we did not attempt to use sham cooling. Whether sham cooling would have served as an appropriate control condition is questionable—a sham vest could have acted as a layer of insulation, preventing heat loss and elevating core temperature in already thermosensitive people. Therefore, we believed that comparing the cooling vest with an uncooled condition or standard of care resulted in a more valid testing of our hypothesis.
We did not use heat sensitivity as an inclusion criterion, as it is measured exclusively by self-report. It is not known whether self-reports of heat sensitivity actually represent a physiologic limitation in function associated with a rise in temperature, and therefore, it would be unclear as to whether our results would have different effects on those who do and do not report heat sensitivity. Saying that one is heat sensitive is not the same thing as having a worsening performance as body temperature increases. A future research topic may be whether people with MS who do report heat sensitivity perform differently with cooling than those who do not. Thermosensitivity in MS may be a similar phenomenon to MS fatigue, as it may have both subjective and objective components.
Conclusions
This study provides evidence that the use of cooling vests can be effective in limiting the impact of fatigue during walking in people with MS. Treating clinicians should consider the use of cooling vests to improve their patients’ walking endurance.
