Yoked Prism and Binasal Occlusion for Management of Traumatic Brain Injury-Induced Midline Shift Sydrome

ARTICLE                                                                                     .

The management of traumatic brain injury-induced visual midline shift syndrome and post trauma vision syndrome through the use of yoked prism and binasal occlusion.

 The majority of individuals who have suffered a traumatic brain injury experience some level of physiological, visual and/or behavioural change that is associated with the injury. The two most common findings are post trauma vision syndrome and visual midline shift syndrome(8).

 Post trauma vision syndrome was first described by Padula in 1994 who noted that the symptoms associated with the syndrome varied from diplopia and vertigo to difficulties reading and memory loss(6,7). He discovered that certain presenting signs were a common denominator amongst affected individuals, including a large exophoria or exotropia, reduced convergence and poor eye tracking. Padula also described visual midline shift syndrome in 1994 as “an unusual phenomenon that often occurs following a neurological event, that the ambient visual process changes its orientation to concept of midline.”(6)

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 Many visual disfunctions that are diagnosed after a traumatic brain injury frequently share a direct link to a problem within the ambient system’s capacity to process and organise spatial information. However, various studies have now demonstrated how yoked prisms are capable of shifting the concept of the visual midline in affected individuals, allowing them to regain their posture by realigning their weight to the affected side(1,2,7). Furthermore, binasal occlusion has been shown to provide the brain with an opportunity to process information in an alternate way by altering the visual input. This alteration has been shown to relieve visual symptoms as well as improve visual function for post-traumatic brain injury patients as demonstrated in this case report(3).

 Visual complications as a result of a traumatic brain injury are often misdiagnosed and therefore mismanaged due to normal findings on neuroimaging and of the ocular structures. As such, affected individuals are often incorrectly diagnosed as having isolated binocular vision issues, including exotropia and exophorias, convergence insufficiency, accommodative dysfunction and dyslexia(5).

A sixty-two year old Caucasian female was referred to a Specsavers practice in New Zealand because she was experiencing dizziness and poor balance. She noticed that when she stood up she felt as though she would fall over but denied any nausea. In crowded spaces, she struggled to walk when there were other people surrounding her. These symptoms were present 5 months post-traumatic brain injury.

The referring hospital noted that the patient had difficulty maintaining balance and concluded that the patient would benefit from a neuro-optometric evaluation. A review of her medical history revealed that she had been involved in a minor car crash causing whiplash and temporary paralysis of the right side of the body. 5 years prior, the patient had suffered a concussion after falling backwards and hitting her occipital lobe. After multiple medical evaluations, including neuroimaging, no cause for her visual symptoms was found. The patient was concurrently under chiropractic care for neck pain as well as a physiotherapist and occupational therapist to try help improve balance and posture.

The initial examination showed 6/9, 6/7.5 and 6/6 distance acuity RE, LE and BE, respectively and near acuity was N5 BE with her habitual spectacles in place (plano/-0.25x140 RE and +0.25/-0.25 x 155 LE with a +1.50 add). Pupils were unremarkable and her confrontations and motilities were both full. 

The manifest refraction showed hyperopia, astigmatism and presbyopia that was marginally different from her habitual spectacles. Her near phoria with manifest refraction was

8Δ 

and distance phoria was 12Δ exophoria. Visual midline shift testing by passing a wand laterally before the patient determined that she had a shift in her concept of visual midline to the right. When walking, she leaned to the right and began to drift to the right.  Based on the measurement, horizontal yoked prism (1Δ BO RE and 1Δ BI LE) was trialled with the new refractive findings to shift the midline back to the centre. The patient was immediately able to bear weight on her left side while walking in a straight line and maintain her balance and posture. The assessment of ocular health was unremarkable. Updated spectacle lenses with the yoked prism (+0.25/-0.75 x 150 1Δ BO RE and +0.50/-0.50 x 160 1Δ BI LE and +2.00 add) was prescribed and she was scheduled for additional testing.

Further evaluation was then conducted to manage the disequilibrium experienced by the patient. Balance board testing was conducted whilst trialling various tinted filters in combination with the prescribed prescription to aid the patient. She found that Cerium C3 tint in combination with the yoked prism and the new prescription increased her comfort and sense of balance when walking around.

Two weeks after this initial visit, binasal occlusion was attempted. Scotch tape was applied to the nasal portion of the back surface of the patient’s lenses. She was then instructed to walk down the store hallway. She appeared more confident walking past others and moving through doorways. She also reported feeling less unbalanced. The patient was instructed to use the binasals during daily activities.

The following week, the patient reported that the binasals drastically helped improve her mobility and comfort when walking. She will continue to be seen yearly as vision therapy proved successful.

 Divergence excess as an isolated binocular vision disorder was ruled out as a differential diagnosis on the basis that the patient hadn’t reported experiencing dizziness or diplopia prior to the brain injury. Divergence excess is defined as an exophoria or tropia that is larger at distance than at near. Although the patient met this criteria, the final diagnosis was post trauma vision syndrome because they exhibited concurrent symptoms including vertigo and a visual midline shift that were not present prior to the brain trauma.

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 Similarly, Ménière's disease was ruled out as a potential diagnosis. Ménière's is a disorder of the inner ear that is characterised by vertigo, tinnitus and nystagmus. Onset typically occurs between the ages of 40-60, with a higher prevalence found among females. Whilst the patient fit the disease demographic, she didn’t complain of any hearing loss or ringing in her ears and her motilities were full with no signs of nystagmus and as such the condition was ruled out.

 Visual midline shift syndrome was first documented by Padula in 1996 who noted how changes to the ambient visual process following a traumatic brain injury altered the affected individual’s concept of the midline(6).

 The visual system is comprised of two separate processes, focal and ambient(5). The focal process is neurologically involved in the functioning of the central vision through the macula. The ambient process utilizes the peripheral vision as an overall spatial orientation system, allowing individuals to orient themselves in space whilst providing general information that is used for balance, coordination, movement and posture(4,5). However, given a neurological accident such as a traumatic brain injury, the ambient visual process can change its concept of the visual midline if its ability to combine information from various parts of the sensory-motor feedback loop is diminished(8). Normally, sensory information from both sides of the body has to be paired through the kinaesthetic and proprioceptive systems with both ambient and vestibular information(4). However, in the case of a traumatic brain injury, information from one side of the body is altered. Since the ambient process functions in a relative capacity, it attempts to create a sense of balance when unilateral interference of information occurs by expanding its concept of space to one side. In doing so, an apparent expansion of space occurs on that side and an apparent compression of space occurs on the contralateral side. This ultimately leads to an imbalance when walking in affected individuals as the phenomena causes a shift in the concept of the visual midline, typically to the neurologically affected side(4,5).

 Prisms are said to be yoked when the bases of both prisms point in the same direction in front of the eyes and have the same power.  A key scientific study conducted by Sheedy and Parsons in 1987 observed the postural changes and perceptual shifts induced by yoked prisms (2). They demonstrated that yoked prisms were able to counteract the amplification and compression of space that occurs in the ambient visual process after neurological trauma, causing the midline to shift to a more centred position. It was noted that yoked prisms enabled the affected individuals to shift their weight and increase the bearing weight on the affected side almost immediately.              

 When yoked prisms are introduced before the eyes of a patient experiencing visual midline shift syndrome, it forces the eyes to reorientate to look at the image of the object in a new position. This readjustment in the motor system sends information to the cortex that states that the sensory component must re-adapt itself to this new position in space. This in turn causes a re-orientation of the motor and sensory organisation in the cortex, effectively countering the visual distortion experienced by the individual (1,7).

Base left and right yoked prisms should be used in an attempt to re-establish lateral midline concepts. However, it is important to note that when prescribing yoked prisms, they should be used for 2-4 hours during the day. This doesn’t allow for total adaptation to the prism and will influence the to re-organise their sensory and motor function when the prism is not in place(2,8).

 Binasal occlusion acts as “a persistent change in the patient’s central visual spacing,” forcing the patient to change the way that they experience the world, pertinently by making the patient more dependent on peripheral cues(10). Binasal occlusion involves occluding the binocular nasal field, thereby affecting binocular integration. Since this part of the visual field overlaps, it requires intense neurological demand and as such requires the maximum level of binocular integration in order to function appropriately(9). However, when a traumatic brain injury occurs, the integration of the binocular nasal field is the most challenging for the individual to produce a single, clear and stable image. As such, when the input from this portion is altered through occlusion, it often significantly relieves visual stress experienced by the patient by alleviating the confusion of attempting to organise this part of the visual field whilst simultaneously allowing the patient to process information from the peripheral visual field.  

 Furthermore, Gallup proposed that binasal occlusion works effectively in combination with yoked prisms as it allows the peripheral visual fields to become more stimulated, causing greater awareness of space and further stimulation of the ambient visual processing system(3). Since binasals act as a constant and consistent reminder interjected between he affected individual and their visual environment, even when not consciously noticed, it further helps to organise visual input and relieve visual input.

 Post trauma vision syndrome and visual midline shift syndrome have been found to have a high prevalence among individuals who have suffered a traumatic brain injury. Many individuals who have the conditions are severely affected by their inability to match information from their visual and motor centres to achieve normal posture and balance. As such, both post trauma vision syndrome and visual midline shift syndrome require the ambient visual process to be re-organised.

 The value of using yoked prism and binasal occlusion together to treat individuals with post-traumatic brain injury has been demonstrated on an individual basis. After a traumatic brain injury, many associated visual dysfunctions experienced by the affected individual are related to the ambient system’s inability to process and organise spatial information. However, incorporating yoked prism provides the patient with the opportunity to shift the concept of the visual midline, helping them to regain their posture and sense of position in space. Similarly, using binasal occlusion as part of the therapeutic regimen provides the individual with the opportunity to experience a change in their visual environment that creates the potential for improved symptoms. Both yoked prism and binasal occlusion should be considered when managing post-traumatic brain injury patients to help them interact with their world in a productive and effective manner.

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