Our friends, Linda and Bob Sanet, frequently speak about the importance of “opening up periphery” with regard to expanding fusion abilities. An important component of opening one’s periphery is that it allows the individual to take advantage of Panum’s Fusional Area, providing a healthy binocular panorama. This concept dates way back to 1858, when Panum noted that regions of overlap between the two eyes permitting single vision are wider or greater in the periphery than in the central field.
Clearly optometric vision therapy remains the treatment of choice for many patients with visual difficulties. However, as desirable as optometric vision therapy can be, a significant number of patients don’t have the cognitive ability, maturity, motivation, time, or resources to dedicate themselves in the way that is required for success. For example, in a recent review of intervention for uncompensated vertical deviations in an urban optometric clinic, only 37% of the population completed optometric vision therapy. It would therefore be intriguing if a small amount of vertical prism could “jump start” the patient in a desirable direction, effectively “stretching” Panum’s Fusional Area. If that is what we’re doing, then you should be able to notice that just a small amount of vertical prism, measured by Maddox Rod phoria in free space, fixation disparity, or whatever your preferred methods are, results in an opening of periphery observable through increased horizontal fusion ranges, more accurate SILO, improved stereopsis, reduced postural sway, expanded peripheral awareness, and so forth.
Consider this review article about the prescribing of low amounts of vertical prism by Eric Weigel. In it, he includes a useful set of signs and symptoms of relatively small uncompensated vertical heterophoria.
Dr. Debby Feinberg, about whom we blogged yesterday, formulated a 25 item Vertical Heterophoria Symptom Questionnaire designed to probe the symptoms above using a Likert scale. Here is the version included in Dr. Weigel’s review article:
In essence, the symptom questionnaire extends the 15 item CI Symptom Survey which focuses on reading efficiency and nearpoint comfort to include distance considerations as well as problems involving dizziness and adaptive head positions. It has now been validated as the “BVDQ” or Binocular Vision Dysfunction Questionnaire that inquiring patients take through their website.
Most of the signs and symptoms associated with uncompensated vertical heterophoria are not new, dating back at least as far as 1892, when Hansell reported on a series of 13 successive cases of hyperphoria in Transactions of the American Ophthalmological Society, treated successfully by tenotomy or prism. Nor is the concept that small amounts of uncompensated vertical phoria are associated with symptoms new, ranging from asthenopia, to loss of place while reading, drowsiness, fatigue, postural deficiency, vertigo, nausea, and motion sickness, as documented by Scobee & Bennet in 1950. Bennet and Thompson followed this up by suggesting a systematic protocol or prescribing low amounts of vertical prism to improve symptomology.
As reviewed in Medical Hypothesis, Discovery, and Innovation in Ophthalmology (2015), Matheron and Kapoula hypothesized that uncompensated vertical heterophoria indicates a perturbation of the somaesthetic cues required in the sensorimotor loops involved in postural control and the capacity of the CNS to optimally integrate these cues. Sensorimotor conflict can induce pain and modify sensory perception. Writing in PLOS One (2011), Matheron and Kapoula noted that neutralizing prisms of very small amounts of vertical heterophoria, less than one prism diopter as measured by the Red Maddox Rod, can often stabilize postural control and alleviate chronic discomfort or pain.
It is somewhat ironic, therefore, that Mark Rosner, Debby Feinberg’s husband, related on Gary Gerber’s Power Hour that their discovery of the influence of prescribing vertical prism started with her brother-in-law (Mark’s Brother, an ENT physician) who was given a 10 prism diopter loose vertical prism by his ophthalmologist that she incorporated into his Rx. That ultimately led to a stream of referrals of his patients for unresolved dizziness, headaches. pain, reading problems and so forth, but with Debbie Rxing much lower amounts of prism than what she gave to Mark. As a further irony, Feinberg and Rosner refer to these lenses as “micro-prism“, a term originally coined by our late colleague, Dr. Merrill Bowan. Whereas Bowan primarily championed small amounts of base-in prism, Feinberg and Rosner champion small amounts of vertical prism. In their Power Hour interview, Rosner notes the majority of vertical prism is under 2^, with at least half under 1^.
Insightfully, Gary Gerber asks Debby during the interview if she ever prescribes prism obliquely for a cyclotorsional deviation, and she says no – it is always either 6:00 or 12:00 (base up or base down). At first blush that might seem curious, since head tilts comprise vectors of misalignment rather than purely horizontal or vertical. But we can rationalize this by the analogy to oblique cylinders, which behavioral optometrists often prefer to prescribe at axis 90 or 180 rather than obliquely. I’ll wrap up this discussion in a Part 3 blog during which I’ll relate to my personal approach to prescribing vertical prism.