Uhthoff Phenomenon in Multiple Sclerosis
The transient worsening of MS symptoms with heat exposure in this patient represents Uhthoff phenomenon (Answer C), a well-characterized manifestation affecting 60-80% of MS patients that results from temperature-dependent conduction block in demyelinated axons.
Pathophysiology of Heat Sensitivity in MS
The mechanism underlying this patient's symptoms involves the detrimental effects of increased temperature on action potential propagation in demyelinated nerve fibers 1. When body temperature rises, even by as little as 0.4°C, demyelinated axons experience:
- Conduction slowing and complete conduction block in partially demyelinated pathways, which is reversible upon cooling 1, 2
- Impaired saltatory conduction due to disrupted myelin, making axons particularly vulnerable to temperature changes 1
- Decreased cold thermosensitivity (approximately 10% reduction) across a wider temperature range than healthy individuals 3
Clinical Characteristics
Uhthoff phenomenon presents with specific features that distinguish it from other MS complications:
- Transient and reversible symptom worsening that resolves with cooling 1, 2
- Triggered by heat exposure including hot baths, exercise, fever, or environmental heat 2
- Affects 60-80% of MS patients, making it an extremely common manifestation 1, 3
- Can produce "new" neurological signs not previously experienced by the patient in 60% of cases during hyperthermia 4
Why Other Options Are Incorrect
MS relapse (Option A) would present with persistent neurological deficits lasting at least 24 hours without fever or infection, not transient symptoms that resolve with cooling 1.
Lhermitte sign (Option B) is an electric shock-like sensation down the spine with neck flexion, unrelated to temperature changes 1.
ADEM (Option D) is a monophasic demyelinating disorder, not a phenomenon occurring in established MS patients 1.
Clinical Management Implications
For patients experiencing Uhthoff phenomenon:
- Avoid heat exposure including hot baths, saunas, and strenuous exercise in warm environments 2
- Use cooling strategies such as air conditioning, fans, and cold water application when symptoms occur 5
- Wear layered clothing that can be adjusted to maintain comfortable body temperature 5
- Recognize that symptoms are temporary and do not represent disease progression 1, 2
Important Clinical Pearls
The presence of heat sensitivity is associated with lack of disease-modifying therapy prescription and greater subjective hand impairment, though interestingly not with objective autonomic or corticospinal dysfunction 6. Approximately 90% of MS patients report worsening symptoms in hot baths, while about half report improvement with cold baths, demonstrating the reversible nature of this phenomenon 2.
The threshold for conduction block is proportional to the degree of demyelination and can be influenced by temperature, pH, electrolytes, and neurotransmitters 2. Some patients may experience paradoxical responses, including symptom improvement after heat exposure ("overshoot phenomenon"), though this is less common 4.