Concussion injuries to the brain are one cause of such differences, for instance. In other circumstances, a difference in pupil size might be an indicator of certain ocular conditions, neurological disease or a sign of brain injury in the case of head trauma. This is called essential anisocoria, and is not a matter for concern at all, just something to be noted for future reference if necessary, particularly if they are different by more than two millimeters or so. It is not uncommon for people to have small observable differences in pupil size from birth this would be like having one foot slightly larger than the other. Pupil dilating drops, used routinely to give eyecare practitioners a better look into the back of the eye, may cause the pupils to dilate to eight millimeters or more, which is why you need to use dark sunglasses afterwards.Īn eyecare practitioner will usually check the size and shape of the pupils during the early part of a comprehensive vision examination, using either a penlight (see figure 1) in normal room illumination, or with an instrument called a direct ophthalmoscope in a darkened room. In bright daylight outdoors, the pupils should contract to two millimeters or less, while they may dilate in a dim-lit restaurant to six or seven millimeters. Normal pupil size in everyday room illumination is about three to five millimeters in diameter. The iris muscles control the size of the pupil, making it larger in dim light and smaller when the light is bright. The pupil is not actually a structure, but is simply a little window in the center of the colored part of the eye, the iris.
The pupil also contracts when the eye changes focus from far away to close up. Anisocoria is the medical term that simply means the pupils are not equal. Normally, the pupils of the eyes are equal in size, are both round in shape and both react quickly and equally to light and any change of focus.
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