Temperature sensitivity in multiple sclerosis: An overview of its impact on sensory and cognitive symptoms

Abstract

Multiple sclerosis (MS) is an autoimmune neurodegenerative disease characterized by demyelination of the central nervous system (CNS). The exact cause of MS is still unknown; yet its incidence and prevalence rates are growing worldwide, making MS a significant public health challenge. The heterogeneous distribution of demyelination within and between MS patients translates in a complex and varied array of autonomic, motor, sensory and cognitive symptoms. Yet a unique aspect of MS is the highly prevalent (60-80%) temperature sensitivity of its sufferers, where neurological symptoms are temporarily exacerbated by environmental- or exercise-induced increases (or decreases) in body temperature. MS temperature sensitivity is primarily driven by temperature-dependent slowing or blocking of neural conduction within the CNS due to changes in internal (core) temperature; yet changes in skin temperature could also contribute to symptom exacerbation (e.g. during sunlight and warm ambient exposure). The impact of temperature sensitivity, and particularly of increases in core temperature, on autonomic (e.g. thermoregulatory/cardiovascular function) and motor symptoms (e.g. fatigue) is well described. However, less attention has been given to how increases (and decreases) in core and skin temperature affect sensory and cognitive symptoms. Furthermore, it remains uncertain whether changes in skin temperature alone could also trigger worsening of symptoms. Here we review the impact of temperature sensitivity on MS sensory and cognitive function and discuss additional factors (e.g. changes in skin temperature) that potentially contribute to temperature-induced worsening of symptoms in the absence of alteration in core temperature.

Keywords: Multiple sclerosis; body temperature; cognitive; demyelination; sensation; skin; temperature sensitivity.

Figure 1.

A schematic overview of a normally myelinated neuron and of a demyelinated neuron in MS. While in an unaffected myelinated neuron, saltatory conduction velocities of neural inputs are preserved along the axon, in a demyelinated neuron saltatory conduction between the Nodes of Ranvier is either slowed or blocked. Demyelination-induced conduction slowing/blocking, and related inability for neural signals to be appropriately exchanges between neurons, is at the root of MS-related signs and symptoms.

Figure 2.

Summary of the cognitive, sensory and motor symptoms that have been recorded to occur during the course of MS. Cognitive impairments in MS affect executive functions, such as deficits in information processing, memory and language. Sensory symptoms constitute an uncomfortable subjective experience for the patient when stimuli touch their skin. Altered sensations are perceived, such as numbness, burning, pain and pins and needles sensation (allodynia, paraesthesia, dysesthesia, trigeminal neuralgia); electrical sensation that moves along the spinal cord (Lhermitte’s sign) and decreased vibration experience with the touch of a vibrating object on the skin. Motor symptoms include discoordination, tremor and ataxia in the limbs which subsequently result in regular falls and fatigue; visual impairments and articulation deficits.

Figure 3.

Summary of symptoms exacerbated by increases in body temperature in MS. Rises in body temperature induce: 1) cognitive deficits in attention, concentration and information processing; 2) sensory deficits such as altered sensations, body pain, and decreases in temperature sensitivity of the skin; 3) motor deficits including spasticity, muscle weakness, balance impairments, fatigue and speech difficulties (dysarthria). All the above symptoms have been observed and/or quantified during increases in body temperature in people with Multiple Sclerosis. These symptoms are pseudo-exacerbations of ongoing MS symptoms and thus, they subside as soon as body temperature is restored to its previous normothermic levels.

Figure 4.

Summary of symptoms exacerbated by decreases in body temperature in MS. Drops in body temperature induce: 1) cognitive deficits in memory, language and attention; 2) altered sensations, such as tingling, numbness, burning sensation over the body (paraesthesia), and vertigo sensation that results in body imbalance; 3) motor deficits including body stiffness, tremor in the extremities, and visual difficulties. These cold-induced pseudo-exacerbations of symptoms subside as soon as the patient’s body temperature recovers to its normothermic levels.