Multiple sclerosis (MS) is a debilitating disease characterized by demyelination of axons in the central nervous system (CNS). It is caused by an immune-mediated attack on neurons in the CNS, which damages the myelin sheath that surrounds and protects nerve axons. This results in the formation of scar tissue, or sclerosis, that disrupts neurotransmission.
Motor dysfunction and muscle weakness constitute the major symptoms of MS; however many MS patients also struggle with heat intolerance. An epidemiological survey of MS patients by Simmons et al. in 2004 showed that almost 70% of patients experience episodes of heat intolerance.
Heat intolerance is more common in those with relapsing-remitting multiple sclerosis (RRMS). Patients are unable to tolerate extremes of temperature, which ultimately leads to a deterioration of their quality of life.
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Mechanisms Responsible for Heat Intolerance
An increase in the heat of the surroundings can influence the propagation of the action potential along demyelinated axons, and may also increase the refractory period of the nerve fiber. This, in turn, decreases the conduction velocity and ultimately leads to conduction block. Development of lesions in the hypothalamus, the region of the brain which controls thermoregulation, has also been associated with the phenomenon of heat intolerance. This may be correlated with the reduced sweating observed in MS patients.
Heat intolerance may cause symptoms such as impaired vision (Uhthoff's phenomenon), loss of concentration/memory, tiredness and loss of balance. The effects on physical and cognitive functions greatly impact the day-to-day activities of the patients and thereby decrease the overall quality of life. Worsening of the symptoms is often seen with an increase in metabolism (exercise) or with passive exposure to heat.
Heat Sensitivity and Multiple Sclerosis
Numerous studies have demonstrated the importance of physical activity in MS patients to prevent complications and comorbidities. However, patients with heat intolerance are unable to tolerate any form of physical activity or exercise due to the development of hyperthermia. A very practical measure to counteract this problem is the use of a cooling vest or neck wrap during exercise. Such garments help reduce the elevation in body temperature and thus improve exercise capacity.
The fatigue which accompanies heat intolerance can also lead to decreased motor function with serious consequences such as accidental falls in patients. The reduced muscular function and impaired attention observed in MS patients further complicate the problem.
MS patients tend to avoid exposure to the sun due to heat sensitivity and are thus at a greater risk of developing vitamin D deficiency. This can have serious implications for bone health. The immunomodulatory effects of Vitamin D also play a pivotal role in delaying disease progression in MS, and hence its levels should be measured so as to avoid deficiency in MS patients.
Measures to Tackle Heat Intolerance
Even though extremely unpleasant, the symptoms of heat intolerance are temporary. The symptoms quickly reverse once the body temperature normalizes, and there is no permanent damage to the nerve cells. Some common measures are used to decrease the symptoms of heat intolerance. These include non-pharmacological approaches as well as medication.
Non-pharmacological strategies include pre-cooling and cooling techniques, staying in an air-conditioned environment, wearing breathable, loose clothing, cooling vests, neck wraps, and drinking icy drinks and popsicles.
Drugs such as 4-aminopyridine have been prescribed by physicians to MS patients presenting with heat intolerance. The drug blocks potassium channels and hence increases the propagation of action potentials in demyelinated neurons. An increase in the speed of walking has also been observed with the use of 4-aminopyridine, and this may help preserve mobility in MS patients.
Heat intolerance presents a significant problem to MS patients and has been associated with fatigue and decreased cognitive properties. Heat intolerance intensifies the disease symptoms and impacts the quality of life of the sufferers. Even though studies have demonstrated heat intolerance to be a key clinical symptom of MS, further investigations into the exact mechanism are necessary. Gaining patient perspective for the analysis is also warranted. Finally, issues relating to the clinical treatment of this particular condition should also be addressed. New pharmacologic approaches for this debilitating condition need to be explored urgently.