Lhermitte’s sign is a distinctive, transient neurological sensation often described as an electric shock. This sudden burst of energy typically begins in the neck and rapidly travels down the spine, often radiating into the arms, legs, or torso. The unique characteristic of this phenomenon is its specific trigger: the forward flexion of the neck, such as bending the chin toward the chest. The sensation is usually brief, lasting only a few seconds, but it can be intense. Lhermitte’s sign is not a disease itself but rather a symptom, indicating an underlying issue affecting the spinal cord.
How the Sign Occurs: Understanding Nerve Hypersensitivity
The root cause of Lhermitte’s sign lies in damage to the sensory pathways within the cervical spinal cord, specifically the dorsal columns. The dorsal columns transmit sensory information like vibration and proprioception (the sense of the relative position of body parts). These nerve fibers are normally protected and insulated by the myelin sheath, a fatty layer that allows electrical signals to travel quickly and efficiently.
When the myelin sheath is damaged, demyelination occurs, leaving the underlying nerve fibers exposed and hypersensitive, similar to a frayed electrical wire. Bending the neck forward causes a physical stretching of the spinal cord, placing mechanical stress on these damaged nerve tracts. This mechanical irritation causes the hyperexcitable nerves to fire an abnormal electrical impulse, which the brain interprets as the characteristic shock-like sensation traveling down the body.
Multiple Sclerosis: The Most Common Underlying Cause
Lhermitte’s sign is most commonly associated with Multiple Sclerosis (MS), a chronic disease of the central nervous system. In MS, the immune system mistakenly attacks and degrades the myelin sheath surrounding nerve fibers. This immune-mediated damage often results in demyelinating lesions, or plaques, forming in the cervical spinal cord and directly impacting the dorsal columns.
The presence of Lhermitte’s sign is a significant indicator for clinicians, often being one of the earliest symptoms experienced by individuals with MS. Approximately one-third of people diagnosed with MS will experience this symptom at least once. Its appearance strongly correlates with neuroradiological abnormalities, as a high percentage of patients show a demyelinating lesion on a cervical spine MRI.
The sign in MS is frequently transient, meaning it may come and go, or resolve completely after several months to a year. This temporary nature occurs because the nerve may partially repair the myelin or the inflammation causing the lesion subsides. Factors such as fatigue, stress, or heat exposure can temporarily increase nerve hyperexcitability, making the symptom more frequent or intense. For individuals without a prior MS diagnosis, the appearance of Lhermitte’s sign prompts further neurological investigation.
Other Conditions That Trigger Lhermitte’s Sign
Any medical condition that causes damage or compression to the cervical spinal cord can provoke Lhermitte’s sign. These causes operate on the same principle: creating an area of irritated or demyelinated nerve tissue sensitive to the stretching caused by neck flexion.
Nutritional Deficiencies and Treatment Side Effects
A severe deficiency in Vitamin B12 is a notable cause, as B12 is necessary for healthy myelin production and maintenance. A prolonged lack of this nutrient can lead to subacute combined degeneration of the spinal cord, which includes demyelination in the dorsal columns. Radiation Myelopathy is damage to the spinal cord that can occur months after radiation therapy for cancers of the head, neck, or chest. The radiation exposure causes delayed tissue injury, leading to nerve hyperexcitability.
Physical Compression and Trauma
Physical compression of the spinal cord can also trigger the electric sensation. Cervical Spondylosis, involving degenerative changes in the neck vertebrae and discs, can narrow the spinal canal and place direct pressure on the cord. Similarly, physical trauma like whiplash or a direct neck injury can cause temporary irritation or inflammation. In all these instances, the final common pathway is the mechanical irritation of compromised nerve fibers in the neck, causing the electric shock sensation.