Sheard's and Percival's criteria are still taught as the standard formulas for how much prism to prescribe. In practice, applying them formulaically tends to produce more prism than a patient actually needs — and more isn't better here.
What the Formulas Actually Do
Both criteria attempt to calculate a prescription based on the relationship between a measured phoria and the fusional reserve available to compensate for it. The math is precise, which is part of the appeal — it gives a specific number rather than a clinical judgment call. But precision isn't the same as accuracy, and treating a formula-derived number as the correct answer regardless of how the patient actually responds is where this goes wrong.
The "Prism Eating" Problem
Overprescribed prism doesn't just fail to help — it can make a patient more symptomatic over time. The visual system's own resting posture is adaptive: give a patient more prism than they need, and the brain's tonic vergence baseline can reorganize around that higher value, effectively training the visual system toward the prescription rather than resolving the underlying dysfunction. This is part of why "prism eating," where patients seem to need progressively more prism over successive visits, has become a real concern in the field — it's often not that the underlying condition is worsening, it's that the treatment itself is shifting the target.
Why This Happens So Easily
Sheard's and Percival's criteria were developed as general clinical guidelines, not as patient-specific prescribing tools, and applying them without weighing the individual patient's actual symptomatic response treats a population-level formula as though it were personalized medicine. A patient can meet the formula's threshold for a certain amount of prism and genuinely need far less — or, less often, need more — depending entirely on their own fusional capacity and how their system actually responds to a trial.
What I Do Instead
The magnitude of prism that actually helps a given BVD patient is often under 4 prism diopters, arrived at empirically — through how the patient responds while actually wearing trial lenses over time, not through a formula-calculated neutralization of a measured deviation. It takes patience, since the benefit of a well-chosen prism prescription for BVD often isn't apparent in the exam lane the way it is for a high phoria or diplopia; it takes time spent actually wearing the lenses before the relief becomes noticeable. That patience is worth more than mathematical precision that risks overshooting the target.