PERRLA

The “E” in PERRLA means equal pupils. Usually, both pupils have a similar size, which varies depending on how dark or light the environment is. When the room is bright, the pupils constrict to allow less light into the retina, whereas in dark environments, they dilate to let more light in to improve vision.  

A difference in pupil size is known as anisocoria. In some people, one pupil is slightly larger than the other without any underlying disorder. In other cases, anisocoria arises from damage to the autonomic fibers that innervate the constrictor or dilator muscles of the iris, which control the diameter of the pupil. This can happen due to nerve compression from various etiologies including aneurysms, intracranial hemorrhages, or strokes. Other causes of anisocoria include repeated eye infections, trauma, prior intraocular surgery, and certain eye drops when applied to only one eye (e.g., tropicamide, atropine, pilocarpine). 

The “L” in PERRLA means pupillary light reflex, which is the reflex that controls how pupils react in response to light. In a healthy individual, shining a light in one eye causes a constriction of that same pupil, in addition to constriction of the other pupil, called the consensual pupillary light reflex.  

In individuals with damage to the optic nerve, shining a light in the healthy eye causes bilateral pupillary constriction, but when swinging the light in the affected eye there is a sluggish or absent bilateral pupillary constriction. As a result, both eyes will show a dilation when the light is swung from the healthy to the affected, thereby resulting in a relative afferent pupillary defect (RAPD), also known as a Marcus Gunn pupil. 

The “A” in PERRLA means pupillary accommodation reflex, which refers to the change in the shape of the lens of the eye in response to focusing on a near object, similar to how a photographer changes their lens to take pictures on a camera.  

When focusing on close objects, the pupils constrict, and the lens becomes thicker and more rounded. Conversely, when focusing on objects far away, the pupils dilate, and the lens becomes thinner because it is under more tension. The accommodation reflex can be tested by holding the index finger away from the person's face and then gradually moving it closer to the nose. The examiner will see how the pupils constrict as the index finger gets closer to the nose, and the eyes turn inward to focus the image, known as convergence.