Uhthoff Phenomenon

Etiology

Uhthoff phenomenon is most commonly observed in multiple sclerosis but may occur in other optic neuropathies or disorders of afferent pathways,[6] for example, neuromyelitis optica.[7] In MS, several factors, including the blockade of ion channels, heat shock proteins, circulatory changes, effects of serum calcium, and unidentified humoral substances, have been hypothesized and investigated as a cause of Uhthoff phenomenon. Temperature-sensitive conduction blockade of partially demyelinated axons in the demyelinated plaques is the most widely accepted mechanism.[8] Several other factors, including perimenstrual period, exercise, fever, sun-tanning, hot showers, sauna, psychological stress, and even hot meals and smoking of cigarettes, have been reported in the literature as triggers for Uhthoff phenomenon.[9]

Epidemiology

Between 60% and 80% of patients with MS exhibit Uhthoff phenomenon with heat exposure. In one study, 52% reported experiencing Uhthoff phenomenon, with a follow-up range of 1 to 20 years. Of the patients with MS and Uhthoff phenomenon, 88% experienced nonvisual heat-related phenomena compared with 30% without Uhthoff phenomenon. About 16% of patients experienced complete recovery in 8 weeks, and persistence of the sign beyond 2 months may be a marker of poor remyelination.[1][10]

Pathophysiology

The precise mechanisms of Uhthoff phenomenon are not completely understood. Still, they are likely due to a combination of structural and physiological changes within the demyelinated axons in the central nervous system (CNS) in the setting of the elevated core body temperature. Studies have shown a decrease in conduction velocity in response to increased temperature in MS patients.[11] The temperature-related slowing of conduction velocity can be reversed with cooling, and this has been shown in experiments studying the adduction velocity in patients with internuclear ophthalmoplegia (INO) in MS. Adduction velocity of eye movements in MS-related INO as measured by infrared eye movement recording techniques was reduced by a systematic increase in core body temperature and reversed to baseline with active cooling.[12]

The normal myelinated nerve is a highly specialized structure, with clustering of sodium channels at the nodes of Ranvier. This facilitates saltatory conduction, whereas demyelination worsens the nodal region, leading to the transformation of faster saltatory conduction to slower membrane conduction.[13] Segmental demyelination involves a primary derangement in sodium channel-mediated axonal depolarization and the unmasking of potassium channels, resulting in K efflux and hyperpolarization, surpassing the action potential–generating processes. Newly assembled sodium channels are subsequently inserted within the axonal membrane as an ion channel adaptation, but the newly incorporated sodium channels may exhibit altered physiological properties.[14] 

Temperature escalation of as little as 0.2 °C to 0.5 °C is sufficient to close the axonally-derived sodium channel and terminate the depolarization phase of the action potential. Demyelination reduces the safety factor of axons, which is defined as the ratio of the current available to initiate an action potential to the minimal current required.[15] An increase in temperature further reduces the axon's safety factor. Hence, an increase in temperature (even as little as 0.5 °C) in individuals with MS results in the closure of the voltage-gated sodium channels in demyelinated axons, thereby compromising action potential depolarization and decreasing the safety threshold for high-fidelity nerve transmissions.[16] This can produce abnormalities ranging from delayed conduction to complete conduction block and clinically manifests as worsened MS symptoms (eg, decreased visual acuity or double vision). Almost all the precipitating factors of the Uthoff phenomenon cause elevated core body temperature. 

History and Physical

Events preceding the worsening of neurological symptoms should be analyzed during history taking. Factors including exercise, taking a hot bath or shower, exposure to the sun, menstrual cycle, psychological stress, hot meals, fever, and infection should be addressed as any of these can precipitate worsening of the symptoms in patients with MS. The transient worsening of the symptoms induced by such factors is termed "pseudo exacerbation" or "pseudo-relapse" as opposed to a true relapse or exacerbation in patients with MS.[3][17][3] This worsening typically should last less than 24 hours. Relapse or exacerbation is the hallmark of relapsing MS and is characterized by new focal neurological deficits lasting for at least 24 hours in the absence of fever or infection. Often, a detailed history can differentiate a true relapse from a pseudo-relapse. Examination reveals various neurological deficits of the location of demyelination, including amblyopia, nystagmus, INO, muscular weakness, and abnormal reflexes. 

Evaluation

Episodes of Uhthoff phenomenon are generally considered to be the result of established demyelinating plaques in the setting of thermal stress. The key to diagnosing is a detailed patient history regarding the circumstances in which the symptoms appeared. In patients with pseudo-relapse, care should be taken to rule out common precipitating factors like urinary tract infection,[18] upper respiratory tract infection, or metabolic abnormalities through laboratory tests. Thus, the workup should include a detailed history, physical examination, and laboratory investigations to rule out metabolic, toxic, and infectious derangements, including magnetic resonance imaging (MRI) of the brain and T- and C-spine, with and without contrast to assess for any new contrast-enhancing lesions that could reveal a new clinical attack for the demyelinating condition the patient suffers. For new diagnosis, a lumbar puncture may be indicated to pinpoint the demyelinating or inflammatory condition, including basic cerebral spinal fluid studies, meningoencephalitis panel, oligoclonal bands, immunoglobulin G index, neuromyelitis optical Aquaporin-4 or myelin oligodendrocyte glycoprotein antibodies, flow cytometry, cytology, among others.

Treatment / Management

A fundamental principle in the prevention and treatment of Uhthoff phenomena is to be familiar with the antecedent factors that can result in the elevation of core body temperature and their corresponding impact on the patient's neurological functioning and safety. Patients should be counseled about the stimulating effects of taking hot showers or baths regarding reducing appendicular and core muscle strength leading to profound weakness and thereby placing them in grave danger of drowning. Also, they should be cautioned against saunas, exposure to the sun when the outside temperature is greater than 30 ºC, hydrotherapy with water at high temperatures, short-wave radiotherapy, and paraffin application. Patients should be advised about exercising during early morning and late evening hours when the temperature is cooler.[19][20][21](B3)

Uhthoff phenomenon fully resolves following variable periods of rest (generally ranging from minutes to an hour) under circumstances where heat stressors are removed, or active cooling measures are applied. Simple and convenient strategies such as taking cold showers, applying ice packs, using regional cooling devices, and using cold beverages can also be tried for heat sensitivity. Cooling garments have improved neurological function (motor performance and visual acuity) and perceived subjective benefits (feeling less fatigued) in patients with MS and Uhthoff phenomenon.

Reports show that oral administration of 4-aminopyridine (4-AP) reduces the worsening of visual impairment after increased body temperature in MS patients.[22] 4-AP is a dose-related potassium channel blocker that prolongs action potential duration by reducing potassium efflux, thereby increasing the hyperpolarization threshold. It enhances the fidelity of conduction in segmentally demyelinated nerve fibers. The Food and Drug Administration (FDA) has approved dalfampridine, an extended-release formulation of this agent, to improve the walking capacity of patients with MS.[23][24](A1)

Differential Diagnosis

The differential diagnoses for [title of article] include the following:

• Antiphospholipid antibody syndrome
• Behçet disease
• CNS lupus
• CNS lymphoma
• CNS vasculitis
• Copper deficiency
• HIV
• Human T-lymphotropic virus (HTLV)
• Leukodystrophies
• Lyme disease
• Sarcoidosis [25]
• Sjögren syndrome
• Small vessel disease
• Osmotic demyelination syndrome [26]

Prognosis

The prognosis of this phenomenon is good as long as pseudo-exacerbation triggers are avoided. Treating the underlying neuroimmunological disorder is instrumental in avoiding recurrence.

Complications

There are no known complications related to the treatment of this symptom. Common adverse events related to dalfampridine use include dysuria, hematuria, urinary frequency, and back pain. Less common side effects include formication, memory issues, mood issues, dysarthria, dysphagia, diffuse fatigue, urinary, and bowel incontinence.