What is Marcus Gunn Pupil?
Marcus Gunn pupil (MGP) is a term used to describe an unusual eye condition related to certain eye disorders. This term is often used interchangeably with the Marcus Gunn phenomenon or relative afferent pupillary defect (RAPD). Typically, a healthy pupil narrows or constricts when exposed to intense light. However, a Marcus Gunn pupil has a relative weakness in the light reflex response compared to the other eye. This means that when light quickly moves from the healthy eye to the Marcus Gunn pupil, it expands instead of constricting like a regular eye would. This phenomenon was named after Robert Marcus Gunn, a Scottish eye doctor or ophthalmologist. This occurrence was first observed by Hirschberg in cases of unilateral retrobulbar optic neuritis, an inflammation of the optic nerve. The presence of a relative afferent pupil defect is the key sign of a one-sided sensory abnormality or unequal vision loss in both eyes.
To describe this in simpler terms, we observe the following:
1. Both eyes’ pupils narrow when light stimulus is applied to the normal eye
2. Both eyes’ pupils widen when quickly moving the light from the normal eye (after a brief exposure to light) to the affected eye.
What Causes Marcus Gunn Pupil?
A Relative Afferent Pupillary Defect (RAPD) is observed in various health conditions, although not all these causes may result in an easily noticeable RAPD.
Here are some possible factors:
Damage to the Optic Nerve and Vision Path:
* Damage to the optic nerve
* Damage to the optic chiasm, which is the part of the brain where the optic nerves cross
* Damage to the optic tract, which carries visual information from the optic chiasm to the brain
* Damage to the pretectum, the area of the brain that helps control pupil size
* Glaucoma, a condition that damages the optic nerve
* Visual field defect, which affects part of your normal field of vision
Damage to the Retina/Back of the Eye:
* Detachment of the retina, the layer at the back of the eye that senses light
* Central retinal vein occlusion (CRVO), which is a blockage of the main vein in the retina
* Central serous chorioretinopathy (CSCR), a condition that causes fluid build-up under the retina
* Macular degeneration, a condition that causes loss of vision in the center of the visual field
* Retinitis pigmentosa (RP), a genetic disorder that causes loss of vision
* Endophthalmitis, an inflammation of the inside of the eye
RAPD Impacting the Opposite Eye:
* Dense cataract, a clouding of the lens in the eye
* Eye patching, covering one eye
* Dark adaptation, when one eye adjusts to darkness
Other Causes:
* Amblyopia, often known as lazy eye
* Anisocoria, a condition where a person’s two eyes have pupils of different sizes
Damage to the optic tract, pretectum, dense cataract, and eye patching or dark adaptation can cause RAPD in the opposite eye. If anisocoria causes the pupils to differ by more than 2mm in size, RAPD will occur in the smaller pupil.
Risk Factors and Frequency for Marcus Gunn Pupil
The condition known as Marcus Gunn pupil can occur in people of any age or gender. A particular study discovered that 42% of people within a normal health range showed an occurrence of Relative Afferent Pupillary Defect (RAPD), a condition closely associated with Marcus Gunn pupil. This percentage had an RAPD score between 0.08 and 0.22 log units, while a smaller percentage, 6%, showed a score between 0.23 and 0.39 log units. These RAPD occurrences can be explained by inaccurate measurements or an unequal balance in the visual pathway.
In separate health conditions, a high frequency of RAPD is noted. This includes over 90% of acute, one-sided optic nerve inflammation cases, 91% of cases of blockage of the main vein in the retina, over 50% of instances of retinal detachment that incorporate the central region of the retina, as well as 23% of cases of a specific type of glaucoma called primary open-angle glaucoma.
- Marcus Gunn pupil can happen in anyone, regardless of age or gender.
- 42% of a normal health range population have RAPD with a score between 0.08 and 0.22 log units.
- A smaller group, 6%, showed a score between 0.23 and 0.39 log units.
- In separate conditions, RAPD is common – over 90% in one-sided optic nerve inflammation cases, 91% in cases of blocked main retina vein, over 50% in retinal detachment cases involving the macula, and 23% in primary open-angle glaucoma cases.
Signs and Symptoms of Marcus Gunn Pupil
People with RAPD, or Relative Afferent Pupillary Defect, may show normal vision. They might also exhibit signs of a base condition. It’s interesting to note that RAPD can persist even when a person’s vision has remarkably recovered after an incident of optic neuritis. Importantly, RAPD on its own does not cause uneven pupil sizes.
An eye exam for diagnosing RAPD involves several steps such as:
- Checking visual acuity: This could vary from light perception to normal 20/20 vision.
- Assessing ocular alignment: People with long-term lazy eye might show outward deviation of the eye.
- Evaluating the pupils: This may reveal a weaker initial constriction with greater redilatation, an initial delay with greater redilatation, or an immediate enlargement, known as pupillary escape. In fact, pupillary escape is a clear and specific sign of RAPD.
- Examining the front part of the eye: The presence of new blood vessels growing on the iris should make one suspect for a condition called ischemic CRVO.
- Surveying the back of the eye: Helps to identify underlying causes such as inflammation of the optic nerve, ischemic optic nerve issues, detached retina, blocked blood vessels in the retina, glaucoma, and others.
Additional supportive tests such as testing for color and contrast vision, visual field evaluation, and using tools like a visual evoked potential test or an MRI scan in cases of optic neuritis, may be employed for a thorough diagnosis.
Testing for Marcus Gunn Pupil
A relative afferent pupillary defect is a medical condition that usually suggests an issue with the vision pathway leading to your brain. One important method for identifying this condition is through eye exams known as neuro-ophthalmic examinations.
Several tests can be used to identify and measure this condition, including the Gunn test, Kestenbaum-Gunn test, swinging flashlight test, and a procedure called pupillography. Some tools, like neutral density filters and cross-polarized filters, can be used along with these tests to provide more accurate results.
The Modified Gunn Test / Kestenbaum Gunn Test is one way to assess the reaction of your pupils to light. The swinging flashlight test is another method. This involves shining a flashlight into one eye and then quickly shifting the light to the other eye, while observing any changes in the pupil’s size. An eye with this condition will dilate when the light is shone in it because the weak eye does not constrict to the light as well as the healthy eye does.
The swinging flashlight test is scored from Grade I to Grade V, based on the pupil’s response. The higher the grade, the more severe the defect is likely to be.
Sometimes, the condition needs to be identified in special situations. For example, if one pupil is unable to react due to injury, other methods are used to look for the condition. In these cases, doctors focus on the reactions of the healthy pupil.
The Tilt test is another useful method used to detect a subtle relative afferent pupillary defect. For this test, a neutral density filter is placed in front of the weaker eye to lower the light input and make the difference in the reaction of the two eyes more noticeable.
Quantification of this condition can be achieved by using neutral density filters. This involves dimming the light in the healthy eye with these filters and performing the swinging flashlight test until no defects are noted. The density of the filter represents the size of the defect.
One of the limitations of the swinging flashlight test is that results can vary based on several factors including the patient’s cooperation, the lighting conditions, and how experienced the examiner is. Pupillography, a more modern approach that uses a computerized device to simulate the swinging flashlight test, can provide more accurate results. It provides detailed information about the pupil’s reaction to light and helps doctors detect and measure the condition accurately.
Treatment Options for Marcus Gunn Pupil
The treatment is designed to address the root cause of the health issue.
What else can Marcus Gunn Pupil be?
Hippus refers to random changes in the size of the pupil, which can often be observed when the retina is continuously illuminated.
Another condition, known as Alternating Contraction Anisocoria, occurs when the eye’s direct pupillary shortening goes beyond the simultaneous shortening of the opposite pupil. This can happen in either one or both eyes. In certain cases, the pupillary reaction of one eye is always greater than that of the other; this is often confused with a different condition, the relative pupillary afferent defect. It is theorized that this could be caused by a lesion at the back of the space between the eyes and one or both arms of the upper part of the midbrain. This damage could be the result of a combination of issues with both the first and second neurons that relay information from the senses to the brain.
Here are a few key points one must know for diagnosing these conditions:
- If there’s a relative pupillary afferent defect (RAPD) with normal vision, color recognition, or field defects, one should consider a higher-order lesion at the level of the pretectum.
- If RAPD is evident in one eye and the other eye has field defects, specifically temporal field defects, this may signal an optic tract lesion.
- If RAPD and new blood vessels in the iris (NVI) occur together, this may suggest the presence of ischemic CRVO, a type of vein occlusion.
- If RAPD and anisocoria (unequal pupil sizes) are present together, repeating the test after 30 minutes of light adaptation can offer more insights.
- If a cataract and RAPD occur in the same eye, this could indicate a condition in the posterior part of the eye.
- If a cataract and RAPD appear in opposite eyes, this could just be a temporary phenomenon or the result of a condition in the posterior segment of the other eye, such as recovered optic neuritis.
What to expect with Marcus Gunn Pupil
In optic neuropathy, which is a condition that damages the nerves transmitting visual signals from the eye to the brain, the extent of RAPD (short for Relative Afferent Pupillary Defect – a method for spotchecking eye health by looking at pupil’s reactions to light) usually corresponds with the severity of vision loss. Meanwhile, in maculopathy (disease affecting the macula, a part of the retina essential for sharp vision), vision loss tends to be more significant compared to the extent of RAPD.
In the case of glaucoma, a condition that damages the optic nerve over time, RAPD presence acts as concrete evidence of vision field loss. It also signals the earliest signs of optic nerve damage in patients with optic nerve hypertension, a scenario when the pressure inside the eye is high.
In retinal vein occlusion, a situation where a vein in the retina is blocked, an RAPD greater than 0.9 log units is related to severe non-functioning blood vessels and weak vision (Usually less than 3/60). If there is a significant increase in RAPD for a patient with non-ischemic CRVO (Central Retinal Vein Occlusion – a blockage in the main vein of your retina), it typically is an early sign of conversion to ischemic CRVO, a type that is often more severe.
In cases of retinal detachment, a medical emergency that happens when the retina pulls away from its normal position, RAPD usually appears when the condition involves the macula, and vision after surgery is usually lower in cases where the macula was involved.
Lastly, in cataract — a condition characterized by the clouding of the lens of the eye, if a patient has RAPD in the same eye, then the visual improvement after the cataract surgery tends to be unsatisfactory due to related defects in the anterior visual pathway (the part of the eye responsible for transmitting visual information to the brain). This poor visual prognosis should always be explained if the cataract-affected eye shows RAPD.
Possible Complications When Diagnosed with Marcus Gunn Pupil
Not noticing this symptom can lead to further health problems resulting from the initial cause of the issue.
Preventing Marcus Gunn Pupil
Examining a patient for a Relative Afferent Pupillary Defect (RAPD) is a useful tool in predicting if a patient may lose part of their vision due to certain conditions affecting the optic nerve and the retina (the thin layer at the back of your eye that senses light). RAPD is a condition where one pupil reacts differently to light than the other. It’s a reliable test for predicting a lack of blood supply (ischemia) and differentiating between ischemic (where blood supply is cut off) and non-ischemic central retinal vein occlusion (a blockage of the vein carrying blood away from the retina).
This straightforward test for RAPD can provide useful information to help doctors advise their patients about their condition.
