Multiple Sclerosis (MS) is a neurological disorder that presents with myriad of symptoms. The disease causes physical as well as emotional changes in the patients. One peculiar symptom seen in people with MS is their sensitivity to heat. While heat sensitivity is a symptom of many other conditions as well, the exacerbation of the other symptoms, when the core body temperature rises, is a disturbing and unfortunate feature that affects people with MS.

What causes the symptoms in MS?

Multiple Sclerosis is an autoimmune disorder. This auto-immunity, the tendency of the body’s immune system to offend its own tissues or organs, results in nerve damage in MS patients. The nerve fibers of the central nervous system have a protective coating called the myelin sheath. The sheath also covers many nerves of the peripheral nervous system. Immune reactions are initiated against the main component, myelin, of this protective sheath which eventually leads to nerve damage.

Most of the symptoms seen in multiple sclerosis patients are due to the damage caused to the specific nerves. The symptoms and signs depend on the part of the nervous system affected and the extent of damage caused to the nerves.

Heat sensitivity in people with MS

Studies report that about 60 to 80% of people diagnosed with MS show excessive sensitivity to heat. People with this neurological disorder experience a temporary exacerbation of their existing symptoms and also new disturbing symptoms when they are exposed to elevated temperatures. They are sensitive to even a slight increase in their core body temperature (0.25°C to 0.5°C) that may be due to physical exercise or a warmer environment.

In many people with MS, the disease presents with intermittent periods of relapses and remissions. But the heat sensitivity causing exacerbation of symptoms is different from relapses. The problem is actually a ‘pseudo-exacerbation’ of the symptoms. Increase in temperature as such does not cause any nerve damage. Most people with MS can actually figure out their intolerance to heat and they are aware of the fact that their symptoms get worse with increasing temperature.

But what causes this heat sensitivity? Scientists, who initially attributed vascular and hormonal causes, currently propose that the reason is a disturbance or block in the normal physiological nerve conduction mechanisms.

Recent research data suggest that demyelination not only causes slowing of nerve impulse conduction along the affected nerve fibers but also linked to a phenomenon called Frequency Dependent Conduction Block. Demyelinated nerves can conduct only single frequency or low frequency impulses. They are not capable of effectively conducting the high frequency nerve impulses. This being the case, it is also found that a slight increase in temperature can slow down or completely block the action potentials in the demyelinated nerve fibers.

Increase in temperature worsens the symptoms of MS

Most of the symptoms of MS are due to the impaired nerve impulse conduction and with an increase in temperature the symptoms worsen in the affected people. Among the various symptoms of MS, fatigue, limb weakness, visual problems, pain and numbness and cognitive dysfunctions are commonly worsened when the core body temperature increases.

• Wilhelm Uhthoff in 1890 described the peculiar phenomenon of ‘temporary worsening of symptoms with exercise’ in optic neuritis patients. Optic neuritis is a condition affecting the eyes. It is a common problem in many people with MS. Uhthoff noticed that visual symptoms were aggravated when people with MS performed exercise. While he attributed exercise to be the etiology of this problem, it was later realized that any action or condition that increases the core body temperature can worsen the symptoms in MS patients. This is called the Uhthoff’s phenomenon or Uhthoff’s sign.
• Fatigue is a common symptom and it is seen in nearly 70% of people with MS. Premature fatigue occurs in people with MS when they are exposed to even a slight increase in temperature. Weakness, especially affecting the limbs is also a symptom that is perceived during or aggravated by a temperature rise.
• Central pain is another common symptom that worsens with an increase in temperature. Studies propose that the reason behind this may be the damage caused to the thalamus and the spinothalamic-cortical pathways leading to thermo-regulatory dysfunction. Numbness is another symptom that worsens with rising core body temperature.
• Cognitive functions in MS patients are also sensitive to heat. Memory problems, judgment difficulties, concentration difficulties and problems with other cognitive skills like language comprehension are more pronounced with the increase in body temperature. A recent study pointed out that people with MS demonstrated worsening of cognitive functions in warmer days.

The influence of temperature on MS patients was established as early as the late 19^th century. Heat sensitivity has been realized as an important reason for the disabling symptoms of the disease. Though the pseudo-exacerbation of the symptoms is a temporary phenomenon, they are quite disturbing and severe enough to restrict the activities of MS patients, especially during warmer seasons.

References

Davis SL, Wilson TE, White AT, Frohman EM. Thermoregulation in multiple sclerosis. J Applied Physiology. 2010 Nov; 109(5): 1531-1537. doi: 10.1152/japplphysiol.00460.2010

Flensner G, Ek AC, Söderhamn O, & Landtblom AM (2011). Sensitivity to heat in MS patients: a factor strongly influencing symptomology–an explorative survey. BMC neurology, 11 PMID: 21352533

Giuliodori MJ, & DiCarlo SE (2004). Myelinated vs. unmyelinated nerve conduction: a novel way of understanding the mechanisms. Advances in physiology education, 28 (1-4), 80-1 PMID: 15149966

Leavitt VM, Wylie G, Chiaravalloti N, DeLuca J, & Sumowski JF (2014). Warmer outdoor temperature is associated with task-related increased BOLD activation in patients with multiple sclerosis. Brain imaging and behavior, 8 (1), 128-32 PMID: 24146082

Marino FE (2009). Heat reactions in multiple sclerosis: an overlooked paradigm in the study of comparative fatigue. International journal of hyperthermia : the official journal of European Society for Hyperthermic Oncology, North American Hyperthermia Group, 25 (1), 34-40 PMID: 19219698

Rasminsky M, & Sears TA (1972). Internodal conduction in undissected demyelinated nerve fibres. The Journal of physiology, 227 (2), 323-50 PMID: 4647244

Rasminsky M (1973). The effects of temperature on conduction in demyelinated single nerve fibers. Archives of neurology, 28 (5), 287-92 PMID: 4696011

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