A relative afferent pupillary defect (RAPD), also known as a Marcus Gunn pupil, is a
medical sign
Signs and symptoms are the observed or detectable signs, and experienced symptoms of an illness, injury, or condition. A sign for example may be a higher or lower temperature than normal, raised or lowered blood pressure or an abnormality showin ...
observed during the
swinging-flashlight test whereupon the patient's
pupils
The pupil is a black hole located in the center of the iris of the eye that allows light to strike the retina.Cassin, B. and Solomon, S. (1990) ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company. It appears black ...
dilate when a bright light is swung from the unaffected eye to the affected eye. The affected eye still senses the light and produces pupillary sphincter constriction to some degree, albeit reduced.
Depending on severity, different symptoms may appear during the swinging flash light test:
Mild RAPD will presents as a weak pupil constriction initially, after which dilation continues to happen.
When RAPD is moderate, pupil size will remain, after which it dilates
When RAPD is severe, the pupil will dilate quickly
Cause
The most common cause of Marcus Gunn pupil is a
lesion
A lesion is any damage or abnormal change in the tissue of an organism, usually caused by disease or trauma. ''Lesion'' is derived from the Latin "injury". Lesions may occur in plants as well as animals.
Types
There is no designated classifi ...
of the
optic nerve
In neuroanatomy, the optic nerve, also known as the second cranial nerve, cranial nerve II, or simply CN II, is a paired cranial nerve that transmits visual information from the retina to the brain. In humans, the optic nerve is derived fro ...
(between the retina and the
optic chiasm
In neuroanatomy, the optic chiasm, or optic chiasma (; , ), is the part of the brain where the optic nerves cross. It is located at the bottom of the brain immediately inferior to the hypothalamus. The optic chiasm is found in all vertebrat ...
) due to glaucoma, or severe
retina
The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...
l disease, or due to multiple sclerosis. It is named after
Scottish ophthalmologist
Ophthalmology ( ) is a surgery, surgical subspecialty within medicine that deals with the diagnosis and treatment of eye disorders.
An ophthalmologist is a physician who undergoes subspecialty training in medical and surgical eye care. Followin ...
Robert Marcus Gunn.
A second common cause of Marcus Gunn pupil is a contralateral
optic tract
In neuroanatomy, the optic tract () is a part of the visual system in the brain. It is a continuation of the optic nerve that relays information from the optic chiasm to the ipsilateral lateral geniculate nucleus (LGN), pretectal nuclei, and su ...
lesion, due to the different contributions of the intact nasal and temporal hemifields.
Diagnosis
The Marcus Gunn pupil is a relative afferent pupillary defect indicating a decreased pupillary response to light in the affected eye.
In the
swinging flashlight test, a light is alternately shone into the left and right eyes. A normal response would be equal constriction of both pupils, regardless of which eye the light is directed at. This indicates an intact direct and consensual
pupillary light reflex
The pupillary light reflex (PLR) or photopupillary reflex is a reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retinal ganglion cells of the retina in the back of the eye, there ...
. When the test is performed in an eye with an afferent pupillary defect, light directed in the affected eye will cause only mild constriction of both pupils (due to decreased response to light from the afferent defect), while light in the unaffected eye will cause a normal constriction of both pupils (due to an intact efferent path, and an intact consensual pupillary reflex). Thus, light shone in the affected eye will produce less pupillary constriction than light shone in the unaffected eye.
Anisocoria is absent. A Marcus Gunn pupil is seen, among other conditions, in
optic neuritis
Optic neuritis describes any condition that causes inflammation of the optic nerve; it may be associated with demyelinating diseases, or infectious or inflammatory processes.
It is also known as optic papillitis (when the head of the optic nerv ...
. It is also common in retrobulbar optic neuritis due to
multiple sclerosis
Multiple (cerebral) sclerosis (MS), also known as encephalomyelitis disseminata or disseminated sclerosis, is the most common demyelinating disease, in which the insulating covers of nerve cells in the brain and spinal cord are damaged. This d ...
but only for 3–4 weeks, until the visual acuity begins to improve in 1–2 weeks and may return to normal.
[Mumenthaler Neurology 4ed, Thieme 2004, page 486 Demyelinating diseases]
A total
CN II
In neuroanatomy, the optic nerve, also known as the second cranial nerve, cranial nerve II, or simply CN II, is a paired cranial nerve that transmits visual information from the retina to the brain. In humans, the optic nerve is derived from ...
lesion, in which the affected eye perceives ''no'' light, is very similar to a Marcus Gunn pupil; to distinguish them, in a CNII total lesion shining the light in the affected eye produces zero dilation nor constriction.
See also
*
Argyll Robertson pupil
Argyll Robertson pupils (AR pupils) are bilateral small pupils that reduce in size on a near object (i.e., they accommodate), but do ''not'' constrict when exposed to bright light (i.e., they do not react). They are a highly specific sign of neu ...
*
Adie syndrome
Adie syndrome, also known as Holmes-Adie syndrome, is a neurological disorder characterized by a tonically dilated pupil that reacts slowly to light but shows a more definite response to accommodation (i.e., light-near dissociation). It is fr ...
*
Cycloplegia
Cycloplegia is paralysis of the ciliary muscle of the eye, resulting in a loss of accommodation. Because of the paralysis of the ciliary muscle, the curvature of the lens can no longer be adjusted to focus on nearby objects. This results in sim ...
*
*
Parinaud's syndrome
Parinaud's syndrome is an inability to move the eyes up and down. It is caused by compression of the vertical gaze center at the rostral interstitial nucleus of medial longitudinal fasciculus (riMLF). The eyes lose the ability to move upward and ...
*
Syphilis
Syphilis () is a sexually transmitted infection caused by the bacterium ''Treponema pallidum'' subspecies ''pallidum''. The signs and symptoms of syphilis vary depending in which of the four stages it presents (primary, secondary, latent, an ...
References
External links
{{Eye pathology
Eye diseases
Human pupil