Multiple Sclerosis (MS) is a chronic disease of the central nervous system (CNS), which includes the brain, spinal cord, and optic nerves. The immune system mistakenly attacks the myelin sheath, a protective layer that insulates nerve fibers and allows for rapid signal transmission. This damage, known as demyelination, disrupts communication between the brain and the rest of the body. The resulting scar tissue, or lesions, are scattered throughout the CNS and cause a wide variety of neurological symptoms.
The Characteristic Pattern of MS Symptoms
MS symptoms are highly variable, depending on the location and extent of the nerve damage. When symptoms first appear or during a relapse, they are most commonly asymmetrical, affecting only one side of the body (unilateral). This presentation is seen in initial events like optic neuritis, which typically causes vision loss in a single eye. Sensory disturbances, such as numbness or tingling, often begin on one side before spreading.
While a single episode often presents unilaterally, the disease can cause bilateral symptoms. As lesions accumulate across the CNS, symptoms can affect both sides simultaneously. For example, a person may initially experience weakness in one leg but later develop bilateral leg weakness or sensory symptoms in both arms and legs. The overall pattern is focal, asymmetrical events evolving into widespread, bilateral issues.
How CNS Lesions Determine Symptom Location
The location of a demyelinating lesion directly dictates the corresponding physical symptom because the CNS controls specific body functions. Lesions in the optic nerve cause vision problems, while spinal cord lesions typically lead to issues with movement, sensation, and bladder function in the torso and limbs. The dispersed nature of these lesions explains the unpredictable variety of symptoms seen in MS.
Understanding symptom location requires knowing about the crossing of nerve signaling paths within the brainstem and spinal cord. For many motor and sensory pathways, nerve fibers originating on one side of the brain cross over to control the opposite side of the body. Therefore, a lesion on the right side of the brain often results in weakness or sensory changes on the left side, contributing to the typical asymmetrical presentation of a relapse. Spinal cord lesions can also produce symptoms on the same side as the damage, depending on which nerve tracts are affected.
Symptoms That Commonly Affect Both Sides
Even when motor or sensory deficits are unilateral, certain symptoms of MS are inherently bilateral or systemic. Fatigue is one of the most common symptoms, described as an overwhelming exhaustion that is both mental and physical and does not improve with rest. This generalized fatigue is not localized to one side but affects the person’s overall energy and function globally.
Cognitive impairment (“brain fog”) affects both sides by impacting global brain function. It involves difficulties with memory, attention span, and information processing, reflecting widespread dysfunction. Balance and gait issues are common in MS and require the coordinated effort of both sides, meaning they are experienced bilaterally. Problems with bladder and bowel control are also frequent, stemming from damage to central pathways in the spinal cord that regulate these functions symmetrically.