Visual Dysfunction Related to Mild Traumatic Brain Injury
The Visual System involves complex actions and interactions of the eyes and the brain. To simplify this description, the Visual System is being placed into three areas of function: acuity, perception, and eye movement. Any one of these functions can be impaired without impairment to the remaining two functions. Or, all functions may be impaired as the result of MTBI. The extent of injury will depend upon the force to and location of trauma in the brain. Dysfunction in any of these areas may contribute to headaches, fatigue, and/or dizziness.
Acuity or How Sharply You See
The Eye and the Optic Nerve
As light enters the eye it travels through the cornea, lens and retina (the neural part of the eye). At this point, the image of what is being seen is processed, reversed and transmitted along the optic tracts (visual pathways). The image is carried via the optic tracts through the brain to the Occipital Lobe (primary visual cortex) at the back of the brain.
The eye can be injured by a direct blow which may injure the cornea, lens, retina, and/or optic tract. Blurred vision or partial visual loss can result from this injury which may be transient or improve with treatment or may be permanent.
Perception or Interpreting What You See
The Occipital Lobe
This lobe sits at the back of the brain and receives the images transmitted to it from the optic tracts. A blow to the occipital lobe (back of the head) may result in an inability to make sense of what you see (visual agnosia) in your environment or read in a book or newspaper. The worst result would be "cortical blindness", an inability to see anything secondary to impaired interpretation of what is seen. This condition may be permanent or transient.
Control of Eye Movements
The superior colliculus and paramedian pontine reticular formation (brainstem)
Each eye has approximately six muscles. Each muscle independently controls an eye movement. Each muscle is individually controlled by one of three Cranial Nerves: III, IV, and VI. Normal eye movements are synchronized to present reflections onto the retina to result in a single image. If any one or all of the three Cranial Nerves are damaged the eye movement and synchronization are altered and two images may be seen. This is double vision or diplopia. Double vision may exist in all fields of vision or only in certain areas.
Independently activated eye movements involve different areas of the brain:
- Saccades, movement on command, and searching movement (pons - brainstem)
- Slow pursuit or tracking a moving object (occipital lobe)
- Vestibuloocular reflex eye movement (VOR): keeps the eye fixed on an object while the head in moving. (brainstem and vestibular system)
- Vergence eye movements: influence by areas in the occipital lobe to keep both eyes on an object whether near or far
Other vision problems related to MTBI
- Sensitivity (photophobia): an increased sensitivity to light
- Nystagmus: an involuntary rhythmic movement of the eyes with a normal range of duration. Saccade is the fast component of this function. The duration of nystagmus may be abnormally long secondary to injury to the brainstem, vestibular system or cerebellum and/or the interconnection between any two or of all three systems. The result is usually a sensation of motion sickness. Diagnosis is typically made by testing the vestibular system functions
- Visual overstimulation: intolerance to busy environments with changing light patterns, visual movement, or clutter