When I started to work in primary care practice I soon came across this dispensing dilemma.

Having completed my examination and handed the patient over to the dispensing optician for spectacle and single vision lens selection, I was asked by the dispensing optician which pupillary distance (PD) to use for the lens optical centres; the patient’s actual PD (66 mm) or the distance between the optical centres on the current spectacles (62 mm). The current spectacles were two years and had been supplied by another practice.

Sometimes the optical centres are set narrower or wider than the actual PD to create prism which alleviates symptoms due to a binocular vision anomaly. Sometimes the optical centres are set narrower or wider than the actual PD in error. My patient was a moderate myope so had been wearing base-in prism successfully since the spectacles were supplied. My examination had not revealed any history of binocular vision anomalies, nor symptoms suggestive of a binocular vision anomaly and while she had a small exophoria at near there was rapid recovery.

Furthermore, her near point of convergence was in the normal range. The patient was pre-presbyopic so was unlikely to be making use of the base-in prism as an aid for reduced accommodative convergence. She had adapted to the clinically inappropriate prism.

The dilemma (or, more accurately, the trilemma) was whether to (i) supply spectacles with the same optical centre distance as in the current spectacles (ii) supply spectacles with the optical centre distance set to the actual patient PD (iii) to set the optical centre distance half-way between 66 and 62 mm, that is at 64 mm, with a view to setting the optical centre distance to the actual PD at a future dispense.

I explained the trilemma to the patient and advised her that my clinical decision was to opt for the second option. The findings, discussion and decision were noted in the clinical records. The spectacles were supplied and did not result in non-tolerance. Where there is a binocular vision anomaly or the optometrist is concerned about a sudden removal of prism then options one or three may be appropriate.