Vision, Balance, and the Horizons of Optometry
Eyes do more than see; ocular-vestibulomotor connections help anchor the body in space, facilitating stable imagery as well as smooth ocular and extraocular movement. Research is revealing that this ambient visual process may hold the key to helping patients maximize potentials beyond present levels of treatment following a cerebrovascular accident or brain injury.
by William V. Padula, OD, SFNAP, FAAO, FNORA
Neuro-optometric rehabilitation after a cerebrovascular accident (CVA) or traumatic brain injury (TBI) — what has come to be my chief area of interest as a clinician, researcher, and inventor — has traditionally emphasized evaluation of two parameters typically compromised by such neurologic events: visual acuity and, to a lesser extent, visual field. Individuals who have had a CVA (such as a stroke) or TBI frequently have visual symptoms that don’t easily fit into the standard optometric model of the eyes. Traditionally, measurement of visual acuity, visual field, and eye muscles — together with a thorough eye health assessment — are undertaken to evaluate post-CVA or TBI patients who have diplopia, reading and balance problems, or difficulty seeing clearly. However, such measurement usually does not help determine why these patients lose their place while reading, perceive print to be moving, function poorly in busy or crowded environments, or experience disorientation and dizziness.
This can present a diagnostic challenge for eyecare providers, who may find no problem with their eyes, and cause anxiety for patients, who are sometimes referred for psychological evaluation.
Functionally, such patients are often able to focus on an object, search for detail, and analyze an image correctly (indicating an intact focal visual system), but they lack a stable neurovisual context for processing the image smoothly (indicating an impaired ambient system).1 The focal and ambient visual systems can be thought of as two processes that need to work together (bi-modally); when they don’t, the entire system becomes dysfunctional. Most research to date, quite naturally, has focused on the focal visual system, as it is the main pathway for what we think of conventionally as vision. But, as I have come to understand in the course of my career, the ambient process is vital.
Research Blind Spots
When I graduated from optometry school, I received a fellowship at Gesell Institute of Child Development in New Haven, CT, where I studied visual processing in childhood development. Later, I was invited by the New Haven Easter Seals to set up a low vision clinic for people with macular degeneration, glaucoma, and other types of eye diseases.
On the first day of the clinic, almost every patient who walked through the door was either a CVA or TBI survivor or was struggling with a neurologic condition such as multiple sclerosis, Parkinson’s disease, or cerebral palsy. The patients presented symptoms and characteristics beyond what is seen in the normal population. I went to the medical literature to find out what research had been done to help these patients, but I found very little: people with neurologic deficits seemed to have been largely passed over in optometry in terms of rehabilitation.
This led me to dig deeper into existing research — and to conduct research of my own — showing that many of their problems could be treated through the use of lenses (which focus light) and prisms (which bend light toward the base).
Two Visual Processing Systems
Research dating back to the early part of the 20th century has documented the existence of these two visual systems. The focal system, with its vast connections to the occipital cortex, serves to analyze visual inputs and search for detail; while the ambient system serves the midbrain as well as the frontal, occipital, and parietal lobes. Via the midbrain, the ambient system provides a spatial format that matches information with spatial, vestibular, kinesthetic, and motor pathways.
In many ways the focal visual process, and the results of that process, are far more interesting than the ambient process; the focal process is literally more colorful! It’s no wonder that most research to date has been focused there. But the ambient process, as a kind of preconscious basis of vision, provides the necessary stability for focused vision and maneuvering in the world. And it is the ambient process that is often affected following a cardiovascular or neurologic event. Dysfunction of the ambient visual process may result in one of two syndromes: post-trauma vision syndrome (PTVS) or visual midline shift syndrome (VMSS).
Process Disorders and Prisms
PTVS is thought to result from a disruption of normal sensorimotor feedback and “feed-forward” mechanisms of the ambient processing system following traumatic injury.2 PTVS is characterized by diplopia, blur, headaches, photophobia, seeing movement, or vertigo, and may be accompanied by exophoria, exotropia, accommodative or focusing dysfunction, convergence insufficiency, or increased myopia.1
Studies using visual evoked potentials (VEP), a noninvasive test that can be used to detect organic dysfunction in the central nervous system, have demonstrated that TBI patients have reduced VEP amplitude compared with uninjured control patients. Further, the use of optometric modalities, in this case base-in prisms and bi-nasal occluders, can increase VEP amplitude — that is, they can actually change the brain processing that creates VEP amplitude.2 Further research is needed to determine whether prisms could one day be used to treat symptoms of PTVS, including headaches, eye strain, difficulties with reading, and problems with attention.
VMSS is a fascinating neurologic disorder in which the perception of visual midline shifts to the side; it may occur following vascular or traumatic injury or result from a chronic neurologic condition such as cerebral palsy, Parkinson’s disease, or multiple sclerosis. Patients with VMSS lean to or shift weight to one side only (or forward or backwards), although they perceive themselves to be standing straight. Not surprisingly, patients with VMSS are at significantly increased risk for falling injuries.3 Fall injuries are a major source of fracture, head trauma, and other serious injury in the elderly population. In the US, an estimated 2.5 million emergency room visits each year are due to falls.4
Research has shown that yoked prism eyeglasses (eg, with binocular base ends of the prism positioned in the same direction for each eye) can help patients with VMSS correct visual midline toward center, improve balance, and reduce risk for falling.3 In one study, post-stroke VMSS patients with hemiparesis were able to even out their weight distribution to include their affected side.3
In Clinical Practice
Optometric prism prescription must be tailored to the individual patient, by assessing posture and balance while standing and moving. This can be challenging for professionals who have not had formal training in assessing posture. To solve this problem, a team of professionals that I am working with is creating a device to assess weight shift and postural imbalance. The instrument, called the NeurOpTrek, then calculates the amount and direction of the yoked prisms necessary to affect the postural and visual-spatial imbalance.
NeurOpTrek extrapolates the exact axis and range of power of prisms needed to correct for visual midline deviations in individual patients. The patient simply has to walk across a “map” on the floor, and the instrument will use the algorithm that we developed to produce the appropriate prism prescription. We are in the process of conducting a series of validation tests on NeurOpTrek and adapting the instrument for use in an eye doctor’s office or a hospital rehabilitation center. It is within one to two years of becoming commercially available.
Our hope with this and other research endeavors in neuro-optometic rehabilitation is that eyecare providers can more confidently and effectively take part in multidisciplinary care for patients with complex central nervous system challenges.
William V. Padula, OD, SFNAP, FAAO, FNORA, is founding president of the Neural Optometric Rehabilitation Association. Dr. Padula is on staff at both the Hospital for Special Care and Gaylord Hospital in Connecticut, and he has a private practice in Guilford, Connecticut.
REFERENCES
1. Padula WV, Argyris S. Post trauma vision syndrome and visual midline shift syndrome. NeuroRehabilitation. 1996;6:165–71.
2. Padula WV, Argyris S, Ray J. Visual evoked potentials (VEP) evaluating treatment for post-trauma vision syndrome (PTVS) in patients with traumatic brain injuries (TBI). Brain Inj. 1994;8:125–33.
3. Padula WV, Subramanian P, Spurling A, et al. Risk of fall (RoF) intervention by affecting visual egocenter through gait analysis and yoked prisms. NeuroRehabilitation. 2015 Oct 20. [Epub ahead of print] PubMed PMID: 26484522.
4. Centers for Disease Control. Important Facts About Falls. Accessed November 4, 2015, at http://www.cdc.gov/homeandrecreationalsafety/falls/adultfalls.html.
