The eye is often described as the window to the soul. And the pupil is the window to the eye. What is the pupil? People know it as a black circle in the middle of the eye which responds to light. It is actually a virtual space, a void. The surrounding tissue, the iris, is the colored portion of the eye (blue, green, and brown) which closes down when light is bright and opens up in the dark. This self-adjusting structure is critical in understanding the optics of the eye, but I will focus on how the pupil is important in Traumatic Brain Injury (TBI).
The initial evaluation of the pupils is critical in the emergency room in evaluating any patients with head trauma. The “blown pupil“, a fixed dilated pupil next to a normal-size reacting pupil is an ominous neurological sign and strongly suggests elevated intracranial pressure and a need for immediate intervention and scanning of the brain. Something bad is probably going on.
This blog is not meant to address acute neurosurgical emergencies. Pupillary function is interesting but quite complicated. We generally look at the eye as a one-way street, light goes in and is processed by the eye and brain to create a picture of the world (afferent pathway). But the brain also sends signals back to the eye in much smaller numbers (efferent pathway).
The basic manner in which an observer knows this is true is when he/she shines a bright penlight into a persons eye. The pupil immediately constricts to reduce the amount of incoming light. This is actually a complicated task. The brain must first receive this bright burst of light and send a signal back to eye (actually both eyes, they work as a unit in this case) and say
“this is to much light, I want to close the shade and reduce incoming light ASAP.”
Humans are not consciously aware of this reflex but it happens rapidly. A brain signal must be sent back to the tiny constricting muscles in the eye and cause them to contract. Again, we think of the eye receiving signals and sending them to the brain, but we don’t always think of the brain sending messages back.
For all this to take place normally, one needs an intact afferent and efferent pathway to exist. In moderate or severe TBI a lot of things can go wrong with this finely balanced system and pupils can react abnormally. What about in mild TBI (mTBI)? Do pupils respond abnormally- the answer seems to be yes.
We live in an increasing quantitative world, where the reactions of the pupil can be captured microsecond by microsecond on a computer screen. There are now quantitative pupil measuring devices which plot a detailed diagram of pupil movement. This is similar to the advanced technology used in tracking eye movements (previously discussed in another blog post). Studies are showing that the pupils in mild TBI cases tend to respond more sluggishly than normal pupils and may not constrict or dilate as much as before a head injury. This might prove very helpful in athletes where a baseline pupillary response curve can be established in the pre-season and then quickly compared to a sideline test after a head injury. Such tracings might also prove quite helpful in tracking the recovery of a mild TBI after an automobile accident. The value is this test is that it is objective, it’s hard to fake a pupillary response.
A researcher can also vary lights of different wavelengths (colors) to see if differences emerge. Also, if the background light is medium, dim, or dark can effect pupillary response. Children are often poor historians and cannot verbalize complaints- the pupil tracings might be very useful. There are simple pupil tests I always perform in my office to evaluate all patients and in particular TBI patients.
Direct injuries to the eye and orbit (tissue around the eye) can effect pupillary measurement. A direct blow to the eye can cause damage to the pupil and optic nerve independent of the brain- this is common. Only a well-trained ophthalmologist can distinguish between these different types of damage. Once again, ophthalmologists must play a critical role in evaluating the TBI. I’ve been looking carefully at pupils for over 30 years- subtle abnormalities are becoming more important in reaching a correct diagnosis.
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Steven H. Rauchman, M.D. is an eye physician and surgeon who has been in private practice for 30 years. He has served as a Traumatic Brain Injury (TBI) medical/legal expert for the last 6 years specializing in the area of personal injury and related traumatic brain injuries.