Uhthoff's Phenomenon in Multiple Sclerosis
The sign you're describing is Uhthoff's phenomenon (or Uhthoff's sign), not Lhermitte's sign. Uhthoff's phenomenon refers to the temporary worsening of MS symptoms with heat exposure, affecting an estimated 60-80% of MS patients 1.
Key Distinction Between These Two Signs
Uhthoff's Phenomenon
- Trigger: Heat exposure (hot weather, exercise, hot baths, fever) 1
- Mechanism: Increased temperature impairs action potential propagation in demyelinated axons, causing conduction slowing and/or block 1
- Symptoms: Temporary worsening of any existing MS symptoms (vision changes, weakness, fatigue, sensory disturbances) that resolve with cooling 1, 2
- Prevalence: 60-80% of MS patients experience this 1
Lhermitte's Sign (What This Is NOT)
- Trigger: Neck flexion or movement 3, 4
- Sensation: Electric shock-like sensation traveling down the spine, arms, legs, or trunk 3, 4
- Mechanism: Ectopic excitation in demyelinated plaques in cervical/thoracic spinal cord affecting posterior columns 5
- Prevalence: 41% of MS patients report this at some point 3
- MRI correlation: Strongly associated with cervical cord abnormalities (17 of 18 patients with Lhermitte's sign had cervical MRI abnormalities) 3
Clinical Significance of Uhthoff's Phenomenon
The heat sensitivity in MS is purely physiological and reversible 1. This distinguishes it from true disease progression or relapse, which is critical for clinical management 2. The phenomenon occurs because:
- Demyelinated axons have impaired saltatory conduction 1
- Elevated temperature further slows conduction velocity in these vulnerable axons 1
- Complete conduction block can occur with sufficient temperature elevation 1
- Symptoms resolve completely once body temperature normalizes 1, 2
Management Approach
Cooling strategies are the primary intervention for patients experiencing Uhthoff's phenomenon 1. This includes:
- Pre-cooling before activities 1
- Active cooling during heat exposure 1
- Avoiding hot environments, hot baths, and excessive exercise in warm conditions 1
- Using cooling vests or other cooling devices 1
The key clinical pitfall is mistaking Uhthoff's phenomenon for an MS relapse, which could lead to unnecessary corticosteroid treatment 2. The reversibility with cooling and absence of new MRI lesions helps distinguish this benign phenomenon from true disease activity 2.