Franklin on the Prescribing of Prism


Benjamin Franklin is widely recognized for his original insight into bifocals, but here I’d like to share with you some insights from Andrew Franklin about prescribing prism. An optometrist from across the pond in the UK, Franklin wrote A Guide to Fixation Disparity several years ago for the superb CE series published by Optician magazine. In it, Franklin makes the following observations:

In general, it is not worth prescribing to a patient who is asymptomatic, but we need to be aware what could qualify as a ‘symptom’ in this context. It could include:

  • The classic asthenopic symptoms (headache, ‘eyestrain’ , tiredness). These are more likely in adults and older children who are also more likely to persist with difficult tasks.
  • Poor concentration or attention span, especially in the younger patient, who may not persist in a task which is not easy to perform. School performance may be affected.
  • Intermittent diplopia is a clear giveaway.
  • Closure of one eye to prevent any diplopia is another.
  • Intermittent blurring.
  • Spasm of the near reflex.
  • Reduced stereopsis and low-grade suppression.
  • Slow acquisition of reading skills. Any child suspected of dyslexia should be thoroughly examined for binocular anomalies before any other assumptions are made.

Franklin adds several other salient points that dovetail nicely with an article that I wrote last year on The Art of Prescribing Small Amounts of Prism.

  • “Fear of over-prescribing prism, and the patient soaking up progressively larger prismatic corrections is widespread but, in the author’s experience, illusory.” – Having said that, Franklin notes that one should develop a personalized system of prescribing the minimum amount of prism to stabilize binocular vision, and he employs the Mallet Unit and relies on the associated phoria heavily in that regard.
  • “If routinely prescribing a reduced relieving prism did not work at all, it would probably have died of natural causes long ago but there is the danger that we could be converting an uncompensated phoria into, um, another uncompensated phoria. In this case, assuming the patient returns, we will have to increase the prism so that eventually we reach the level that will relieve the symptoms properly. To the timid prescriber, this could look rather like the patient soaking up the prism, whereas in fact it is just the opposite.” – Franklin is advising that when one prescribes the minimum amount of prism at the outset, it is neither surprising nor undesirable to make a small increase in prism. I interpret this as the analogy to lens Rx-ing for latent hyperopia, and the need to delicately titrate the power. In due time, one can often step the prism back down as the patient is able to recalibrate.
  • “Finally, should we prescribe the prism unilaterally or split it equally between the eyes? Mallett advocated prescribing according to what was found by the test and in general that seems to work well for the small degrees of prism we usually need.”  – Franklin’s experience is that when there is an obvious imbalance in disparity, one can be confident in Rx-ing asymmetrically rather than routinely splitting the prism evenly between the two eyes.

By the way, I think you’ll enjoy perusing the Archives of other CE presentations available through the Optician’s website. Here are some of my personal favorites:

  1. Health and well-being in eye care practice: Making the difference in your patient’s life
  2. Lifestyle eye care 1: Overview of lifestyle on systemic and eye health
  3. Fundamentals of ophthalmic dispensing 14: Prismatic effects and binocular vision – part 3
  4. Fundamentals of ophthalmic dispensing part 13: Eye movements
  5. Rock and roll in progressive non-adaptation
  6. AI in eye care; an introduction to how neural networks work
  7. Prismatic effect and binocular vision basics – part 1
  8. A holistic approach to eyecare part 1: Understanding the importance of a healthy gut to our wellbeing
  9. Eye care for children with learning disabilities 2: Assessment of vision and visual needs
  10. Binocular vision Part 4 – Stereopsis

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