OA Corner Part 10: Explaining spectacle prescriptions Part 2

In last month’s article, we looked at the prescription shown below, and how the optical assistant (OA) could explain this prescription to a patient. This month, we are going to look at the 2 Base Up and the +2.00 Add.

2 Base Up relates to the amount and direction of the required prism this patient needs. I find the best way to start is by explaining that we have two eyes, and this results in two images being produced. However, the brain will fuse the two images together to produce a single image (otherwise we would see double).

Sometimes the two images do not fuse exactly, which can result in double vision that is either permanent or intermittent. Sometimes, the patient will report headaches, or maybe eyestrain or difficulty changing focus from near to distance and visa versa. The prism will bring the two images into line, thus creating a single, clear image.

A prism can be illustrated by a triangle, with the thick part of the prism being the base and the thin part of the triangle being the apex. With the prescription 2 Base Up, the thicker part of the prism will be the base, which is at the top of the lens as the direction is Up. The greater the prism, the thicker the lens will be.

If the prism is Base Down, the thicker edge will be at the lower part of the lens. If is Base In, the thicker part will be on the nasal edge, and the thicker edge for Base Out will be on the temporal side of the lens. Often, prism is split between the two eyes to balance the weight and thickness of the two lenses. Prism splitting is an important topic and one you could discuss with either an optometrist or a dispensing optician.

The final element of the prescription is +2.00 R&L N5. This is the reading addition (Add), and signifies that the patient is presbyopic. Presbyopia occurs around the age of 42 as the crystalline lens within the eye becomes stiffer and less elastic, and less able to accommodate sufficiently to focus clearly on near objects.

Patients usually notice small print becomes increasingly difficult to see, especially in dim lighting, and find they need to hold thigs further away to focus clearly. Patients often joke their arms are no longer long enough. By adding plus power in the from of a reading Add, this replaces the lost accommodation and restores the closer reading position.

To obtain the reading prescription, you add the reading Add to the spherical part of the distance prescription. For the right eye in the prescription above, we have -2.50DS in the distance, and a +2.00DS reading Add, so we sum -2.50 and +2.00, which results in -0.50DS.

The cyl doesn’t alter, so this remains at -0.75 x 90. For the left eye we add -1.50 and +2.00, which is +0.50DS. This gives us a reading prescription of:
• RE -0.50/ -0.75 x 90
• LE +0.50DS 2 Base Up LE

Hopefully, this has given you a better understanding of how to explain a prescription to a patient should they ask you to do so.

OA Corner Part 1: What makes a good OA
OA Corner Part 2: Communications
OA Corner Part 3: A question of strategies
OA Corner Part 4: Understanding bifocals
OA Corner Part 5: Our amazing eyes
OA Corner Part 6: Frame fittings basics
OA Corner Part 7: Frame styling factors
OA Corner Part 8: Single vision lens basics
OA Corner Part 9: Understanding spectacle prescriptions Part 1