PERRLA and Your Eye Exam

An acronym for pupillary response testing

Illustration by Joshua Seong. © Verywell, 2018. 

Do you wonder what your eye doctor is checking for as she shines a bright light into your eyes? Be assured that this light is not meant to irritate you—it has a purpose and can "shed light" on how well your nervous system is working. 

More specifically, this eye exam, called pupillary response testing, is used to examine the functioning of cranial nerve II (called the optic nerve) and cranial nerve III (called the oculomotor nerve). The optic nerve transmits visual signals from the retina to the brain. Damage to the optic nerve (like from optic neuritis) can cause vision problems. 

The oculomotor nerve controls four of the six eye muscles and also controls constriction of the pupil and the ability of the eye to focus. Damage to the oculomotor nerve (like from a head injury, infection, high blood pressure, or brain tumor) can cause double vision or an enlarged or blown pupil. 


When a physician or nurse is examining a patient's eyes, PERRLA is an acronym used to describe the look and function of the eyes. It stands for:

  • Pupils
  • Equal
  • Round
  • Reactive to
  • Light and
  • Accommodation (which means the ability of the eyes to focus on objects that are close-up and far away)

 An eye exam (described by PERRLA) would be abnormal if the pupils are:

  • Not of equal size 
  • Not round (misshapen)
  • Don't change (not reactive) when a light is shined on them (they should get smaller)
  • Don't change (not accommodating) when looking at something close or far away

Pupillary Response Test Described by PERRLA

To conduct a pupillary response test, a person is usually asked to sit in a dimly lit space or room in order to carry out the following steps:

  • The doctor or nurse will look for uniformity between both pupils and note the shape and size prior to starting the test.
  • A small handheld flashlight is moved back and forth between the eyes every two seconds and used to shine a light into the eyes. This assessment is called the "swinging flashlight test." The doctor or nurse looks for a direct response to the swinging flashlight test, to see if the pupils get smaller (constrict) in response to direct light, regardless of which eye the light is directed at.
  • At the same time, constriction of the other pupil should take place (this is called a consensual response).
  • The doctor or nurse may use a pen or the index finger to see how the eyes fixate on an object. The finger is pulled back, forward and side to side to see whether the eyes are able to focus. Normally, the pupils constrict while they are fixated on an object that is being moved close or far away.

What the PERRLA Test Reveals

A decreased direct response to light in one eye may be a sign of a relative afferent pupillary defect (RAPD). This is also sometimes called a Marcus Gunn pupil.

There are many different conditions which lead to RAPD such as:

  • Severe glaucoma
  • Optic neuritis
  • Optic nerve tumor
  • Ischemic optic neuropathy (also called a "stroke of the optic nerve")
  • Head injuries that damage the optic nerve
  • Surgical damage to the optic nerve (due to damage caused by anesthesia, eye, orbital, sinus, or plastic surgery)
  • Severe retinal diseases

Abnormal accommodation reflex (response to looking at something moving toward the eye) can be caused by a variety of conditions, including:

  • Lesions of the optic nerve
  • Lesions of the oculomotor nerve 

A Word From Verywell

The eye is a complex organ as are the nerves that control it. Remarkably, though, a test as simple as a pupillary response test serves as a window into your central nervous system.

While an abnormal test cannot offer a precise diagnosis, it signals to the doctor that something is not right and that a more thorough medical workup is needed.

As always, be sure to ask your doctor about your symptoms and the results of any assessments having to do with your eye and vision.

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