I am often asked why traumatic brain injury (TBI)? Why the brain? You are an ophthalmologist, an eye guy.
Well, here is a little, but important, lesson in neuroanatomy. The brain stem lies at the base of the brain and is the interface of the spinal cord and the brain. It is a critical part of the brain, and strokes and tumors of this region can have devastating consequences. It is compact and full of important neurologic consequence.
There are twelve pairs of cranial nerves which originate from the brain stem- the brain stem consists of the midbrain, pons, and medulla. These are direct connections to the brain that bypass the spinal cord. As one might guess, these connections are critical to life. So what do they have to do with the eyes?
The nerves are given numbers 1-12. Number one, olfactory nerve (sense of smell) has nothing to do with vision. Number two, ophthalmic, the nerve that directly conducts all visual information from the eyes to the brain. This serves a critical function and is a huge nerve, not microscopic, thick like a pencil. If the optic nerve is injured, the patient becomes blind.
Number three, oculomotor, another critical eye nerve. This nerve controls the up, down, and inward movement of the eyeball. It also plays a critical role in constriction of the pupil and elevation of the eyelid. If this nerve is injured the pupil often becomes dilated, the lid droops, and the patient can develop double vision. It is commonly damaged in head trauma.
Number four, trochlear nerve, also effects eye movement. It’s sole function is to control a small eye muscle, the “superior oblique,” which is most important in the torquing movement of the eye. If a person tilts the right side of their head to their right shoulder, the eye must turn in the opposite direction to maintain the stability of the image. Again another important function, if the fourth nerve is damaged (which is common in head trauma), one often experiences vertical double vision.
Number five, trigeminal, provides multiple functions, but most importantly, sensation to the face. This includes sensation to the surface of the eye. If the cornea (the front surface of the eye) becomes numb, the cornea does not do well and vision can become impaired.
Number six, abducens nerve, also relates to eye movement. The abducens nerve supplies a small eye muscle called the lateral rectus. This muscle pulls the eyeball outward or laterally. If the sixth nerve is injured (common in head injury), the patient experiences horizontal double vision, the eyes turn outward.
Number seven, the facial nerve, has multiple functions. One critical function is to close the eyelid or blink. A “Bell’s palsy,” which is a paralysis of the facial muscle, is a relatively common condition.
The face on one side droops but the patient cannot fully close the eyelid. If one can’t close the eyelid, the surface of the eye dries out and bad things happen.
I will stop here, I’ve covered seven of the twelve cranial nerves and six of those seven critical to visual function. I think I’ve made my point by being detailed but not to technical. The brain and the eye could not be more closely related- the anatomy of the brain stem tells the story.
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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.