Different Pupil Sizes as a Sign of MS

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Vision problems are common in multiple sclerosis (MS), caused by inflammatory damage to the optic nerve. One of the lesser known symptoms is a phenomenon known as Marcus Gunn pupil in which a pupil fails to constrict (get smaller) as it should when a light is shined into it.

Marcus Gunn pupil can be unnerving or even scary if you don't know what's going on. In some cases, the condition may only be noticed when a doctor or ophthalmologist performs a routine eye exam. At other times, it may be visible when looking into a mirror or noticed by others, especially if you have lighter eyes.

While Marcus Gunn pupil is rarely the sole defining symptom of MS, there are cases where it will appear alongside other vision problems as the first signs of the disease. 

Cause of Marcus Gunn Pupil

Marcus Gunn pupil (also known as the relative afferent pupillary defect, or RAPD) occurs when the nerve pathway from the brain to the affected eye is impaired.

Multiple sclerosis is characterized by an abnormal immune response which causes progressive damage to the protective covering of nerve cells (known as the myelin sheath). When this happens, the exposed nerves can misfire or fail to communicate with each other as they should.

Depending on where the damage occurs, the affected tissues can form lesions (plaques) characteristic of advancing disease. When this occurs on the optic nerve and/or retina, a person may experience symptoms of optic neuritis.

These symptoms can vary depending on the extent or location of the plaques. It can happen to one or both eyes and may include:

  • A dull ache behind your eye
  • Pain when moving your eye
  • Vision impairment or loss
  • Washing out or loss of color vision
  • Reduced peripheral vision
  • Flashing or flickering lights

In addition to the vision itself, the nerve pathway damage can affect eye movement and lead to double vision (diplopia) and involuntary eye movement (nystagmus).

Marcus Gunn pupil is part of a cascade of symptoms associated with optic neuritis. It doesn't usually appear on its own and is more often accompanied by blurred vision, eye pain, and headaches.

Diagnosis of Marcus Gunn Pupil

The swinging light test is the primary exam used to diagnose Marcus Gunn pupil.

Normally, when a light is shined directly into an eye, both pupils constrict at the same time (a response known as the pupillary light reflex). This doesn't happen with Marcus Gunn pupil. Instead, when a light is directed into the affected eye, there will only be mild constriction of both pupils (or, in the worse case, a converse effect).

The severity of the symptom can be classified as follows:

  • Mild RAPD, characterized by weak pupil contraction followed by dilation
  • Moderate RAPD in which the pupil size remains unchanged
  • Severe RAPD in which the affected pupil dilates to a larger size

Treatment of Marcus Gunn Pupil

Depending on the severity of symptoms, doctors can decide which treatment, if any, is needed.

MS-associated optic neuritis is often self-limiting, meaning that it will often occur during a relapse and resolve on its own without treatment. In cases where treatment is needed, disease-modifying drugs may be prescribed.

Steroids are also commonly used to treat optic neuritis, delivered in either intravenously or in pill form. In the event of severe vision loss, plasma exchange therapy (plasmapheresis) may be recommended.

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Article Sources

  • Blazek, P.; Davis, S.; Greenberg, B. et al. "Objective characterization of the relative afferent pupillary defect in MS." J Neuro Sci. 2012; 232(1-2):193-200.