When you already have MS, it’s easy to attribute new or worsening symptoms to MS, which could cause you to overlook signs of early RA. Here’s how these symptoms overlap and some subtle differences, depending on their root causes.
Fatigue
About 4 in 5 people with MS have fatigue, making it one of the most common symptoms of MS. It’s also present in many inflammatory autoimmune diseases, including RA.
In people with MS and RA, fatigue is often caused by chronic inflammation, says Carlson.
Muscle and Joint Pain
Widespread pain and central pain sensitization are also common in both diseases.
“Joint pain and swelling are hallmark features of RA; MS can also present with musculoskeletal pain and spasticity,” says Carlson.
In RA, joint pain comes directly from inflammation in and around the joints, whereas muscle and joint pain in MS is typically caused by spasticity or triggered by gait changes that overload certain joints.
Muscle Weakness
Weakness is another shared symptom, says Daniel Kurz Jr., MD, assistant professor neurology and neurologist at UChicago Medicine in Illinois.
“I often see weakness as a symptom in my MS patients, though like fatigue, it’s a multifactorial symptom. Weakness can occur in the setting of pain, which is something that I hear from our RA patients as well,” he says.
Limited range of motion, or how much an extremity can move, can be a sign or symptom of weakness, says Dr. Kurz.
MS weakness can be related to nerve damage as well other MS symptoms that cause deconditioning of the muscles. RA weakness tends to arise from pain, inflammation, or muscle loss due to reduced activity during flares.
A neurological exam can help distinguish between weakness that has a neurological cause and weakness that is due to joint pain and muscle loss, which would be more RA related, says Kurz.
Numbness and Tingling
Neuropathic symptoms can include feelings of lost or diminished sensation; a pins and needles sensation in the arms, legs, and face; and intense, burning pain. These symptoms happen as a result of nerve damage.
While less common, some people with RA may experience their joint pain in a way that can feel similar to the burning neuropathic pain seen in people with MS, says Kurz. For example, nerves can be compressed due to swollen joints.
“It becomes a question of whether or not it’s inflammation from the central nervous system or the peripheral nervous system or more localized to the joint itself, which would mean it’s related to rheumatoid arthritis,” says Kurz.
Eye Inflammation
Both MS and RA can cause eye inflammation — but for different reasons.
In MS, eye problems usually stem from inflammation of the optic nerve (optic neuritis), which damages the myelin coating around the nerve fibers that carry visual information to the brain. This can cause pain with eye movement and temporary vision loss in one eye.
In RA, eye involvement is not nerve-based; it’s driven by inflammation. Any inflammatory condition that involves collagen, which is the main structural protein in the joints that makes up connective tissue like cartilage, tendons, and ligaments, also affects the eye, according to the Arthritis Foundation. That’s because the sclera (the white of the eye) and the cornea (the lens cap) are primarily made of collagen.
Because untreated inflammation can damage the cornea or other eye structures, new eye symptoms should be evaluated by an eye doctor right away.