Multiple Sclerosis (MS) is a chronic condition affecting the central nervous system, including the brain and spinal cord. Reflexes are involuntary actions performed instantly in response to a stimulus, relying on intact neural pathways. Changes in these automatic responses are frequently observed in individuals with MS, providing a physical sign of nervous system involvement.
The Neurological Basis of Reflex Changes
MS causes damage within the central nervous system through inflammation and demyelination. This process strips away the protective myelin sheath, forming lesions that disrupt the flow of electrical signals governing motor control. Damage often occurs in the corticospinal tract, which contains upper motor neurons (UMNs) descending from the brain. UMNs normally exert an inhibitory influence on the spinal cord’s reflex circuits. When MS lesions damage the UMNs, this control is lost, resulting in the disinhibition and hyperexcitability of the spinal reflex arcs, which causes overactive reflexes.
Specific Types of Reflex Alterations
The loss of descending inhibitory signals manifests as hyperreflexia, an increased or overactive reflex response. Hyperreflexia is observed during the testing of deep tendon reflexes (DTRs), such as the knee-jerk or ankle-jerk reflex, where the muscle contracts with excessive force and speed.
A more severe manifestation of hyperexcitability is clonus, which frequently accompanies hyperreflexia. Clonus involves rhythmic, involuntary muscle contractions and relaxations triggered by a sudden muscle stretch. Sustained or continuous clonus is a definitive sign of UMN damage.
Babinski Sign
Another specific finding is the Babinski sign, or extensor plantar response. This pathological reflex is typically only present in infants. While a normal adult response to stroking the foot’s sole is for the toes to curl downward, a positive Babinski sign occurs when the big toe moves upward and the other toes fan out. Its presence in an adult clearly indicates damage to the UMN pathways affected by MS.
Clinical Assessment and Diagnostic Utility
Reflex testing is a routine part of the physical examination used by neurologists to assess the integrity of the central nervous system. Deep tendon reflexes are tested using a reflex hammer on various tendons, including the biceps, triceps, patellar, and Achilles. Clinicians look for symmetry and the level of response, which helps the neurologist pinpoint where damage has occurred.
Reflex responses are graded on a scale ranging from 0 to 4+. A grade of 2+ is considered a normal response. Hyperreflexia is scored as 3+, representing a brisk response, while 4+ signifies a hyperactive reflex with sustained clonus.
The presence of reflex changes, specifically hyperreflexia and a positive Babinski sign, contributes to the evidence required for an MS diagnosis. These findings support the requirement that the disease has affected multiple areas of the nervous system over time, known as dissemination in space. Monitoring reflex status during follow-up examinations can also help track disease progression or indicate new lesion formation.
Managing Symptoms Related to Altered Reflexes
Overactive reflexes and muscle hyperexcitability often lead to spasticity, a common symptom in MS. Spasticity is defined as a velocity-dependent increase in muscle tone that causes stiffness, tightness, and involuntary muscle spasms. This muscle stiffness can interfere with daily activities such as walking, transferring, and performing self-care.
Management strategies are designed to reduce this involuntary muscle overactivity and improve comfort and function. Physical and occupational therapy are foundational, utilizing stretching and specialized exercises to maintain muscle length and joint mobility. Orthoses and splints are sometimes used to help maintain proper posture and positioning.
Pharmacological interventions are also employed to reduce the hyperexcitability of the spinal cord’s motor pathways. Medications such as baclofen, tizanidine, and diazepam are commonly prescribed muscle relaxants that work by calming the nerves. For more severe, localized spasticity, botulinum toxin injections can temporarily weaken specific muscles, and in refractory cases, an intrathecal baclofen pump may be surgically implanted.