The pupil is the aperture of the eye. It is the black hole in the center of the iris, the colored part of the eye. The pupil allows light to pass through the eye to the retina, which is the tissue that lines the back of the eye.
The average size of the pupil is 2 to 4 mm in diameter in bright light and 4 to 8 mm in the dark. The pupil will constrict in bright light and dilate in dim light. If a light is shown in just one eye, the opposite eye will also constrict. When you look at something up close, your pupils will constrict to focus on this near target. If a pupil fails to constrict in light or when focusing up close or if the pupil fails to dilate in the dark, the pupil is abnormal. During the course of an eye exam, your pupils will be checked for their size, shape and reaction to light.
The following are conditions associated with pupil abnormalities.
Anisocoria is difference in pupil size between the eyes. This is a normal finding in about 20% of patients. In these patients, the difference in size will be equal whether they are in bright or dim light.
Afferent Pupil Defect
A relative afferent pupil defect occurs when the pupils respond differently to bright light shone into one eye at a time. For instance, a pupil may constrict when bright light is shone into it, but it does not constrict when bright light is shone in the opposite eye. If an afferent pupil defect is noted, it commonly indicates the presence of a unilateral optic nerve problem such as the following:
• Optic neuritis, which is associated with multiple sclerosis
• Ischemic optic neuropathies, both arteritic (Giant Cell Arteritis) and non-arteritic
• Asymmetric severe glaucomatous damage
• Traumatic optic neuropathy
• Compressive optic neuropathy
Third Nerve Palsy
The third cranial nerve, oculomotor nerve, is responsible for innervating 4 of the 6 muscles that control the movement of our eye up, down, left and right. It is also responsible for pupil constriction and elevation of the eyelid. Someone with a complete third nerve palsy will present with a dilated pupil, closed lid and an eye that is pointed down and out. This condition requires prompt medical evaluation and imaging because if an aneurysm is present, it is a surgical emergency.