There does appear to be a link between rigorous
exercise in heat and the BBB permeability. See below, second link, it also suggests that good fluid intake during exercise limits this. Though if this would make someone more likely to relapse is a different issue, it definitely wouldn't seem a good idea to take the chance if you are already having a relapse!
However, up to now there doesn't look like much research is specifically geared towards this vis a vis MS. See below first link
Excercise and heat are thought to cause-sometimes- a temporary worsening of symptoms (not contributing to new ones but awakening old ones, just as fatigue can do) because of a 'conduction block' that results from becoming overheated, nothing seems to relate this to BBB allowing immune cells through.
Studies in this 'conduction block' theory seem more abundant when related to people with mild or moderate MS and it seems to say that the benefits outway the negatives. I guess it all means to take care you don't overheat but otherwise it could be more beneficial than harmful. But I can definitely understand why you are so cautious as there is very limited information on this. Manchester
Exercise and multiple sclerosis: physiological, psychological, and quality of life issues.Sutherland G, Andersen MB.
School of Human Movement, Recreation and Performance, Victoria University of Technology, Melbourne, Australia. email@example.com
The case for the benefits of physical activity has been well documented in healthy individuals, and the potential for reducing the risk of mental and physical ill health is substantial. Yet, individuals with multiple sclerosis (MS) have long been advised to avoid participation in exercise in order to minimise the risk of exacerbations and symptoms of fatigue. There is, however, increasing interest in how acute and chronic exercise affect physiological and psychological functioning in MS. Much of the research has examined physiological tolerance to exercise and focused on responses in terms of heart rate, blood pressure, cardiorespiratory fitness, muscle function, and symptom stability. Little research has focused on understanding how exercise affects psychosocial functioning and brings about changes in depression, affect, mood, well-being, and quality of life. This paper provides a summary of the research exploring the efficacy of physical activity for people with MS. In addition, the key issues that face clinical practice are examined, and considerations for research are discussed.
Exercise in the heat: effect of fluid ingestion on blood-brain barrier permeability.Watson P, Black KE, Clark SC, Maughan RJ.
School of Sport and Exercise Sciences, Loughborough University, Leicestershire, United Kingdom. firstname.lastname@example.org
INTRODUCTION: This study examined changes in serum S100beta concentration, a peripheral marker of BBB permeability, in response to exercise in the heat with and without fluid ingestion. METHODS: Eight physically active males completed up to 90 min of intermittent exercise at a power output corresponding to 55% VO2peak in a warm environment (35 degrees C, 56% rh). Trials were completed with (F trial) and without (NF trial) the replacement of sweat losses. During the fluid trial, an aliquot of plain water was ingested at 15-min intervals to match the volume of sweat lost during the previous period of exercise. RESULTS: Exercise time was 80.7 +/- 13.0 min in the NF trial and 85.1 +/- 9.5 min in the F trial (P = 0.107). Fluid ingestion resulted in a smaller rise in core temperature (P = 0.050) and heart rate (P = 0.027) during the latter stages of exercise. Serum S100beta concentrations were 0.08 +/- 0.02 microg.L at rest, increasing to 0.20 +/- 0.06 microg.L at the end of exercise in the NF trial, with this response attenuated by the ingestion of fluid (0.13 +/- 0.03 microg.L; P = 0.046). Both serum sodium concentration (P < 0.001) and serum osmolality (P = 0.003) were significantly lower at the end of exercise in the F trial than in the NF trial. CONCLUSION: The results of this study demonstrate that water ingestion can limit exercise-induced increases in serum S100beta, consistent with the preservation of BBB integrity. It is possible that this response was mediated through the maintenance of lower extracellular osmolality late in exercise, thus potentially limiting the osmotically driven movement of fluid across the BBB.