Dispensing to higher prescriptions needs great care in order to ensure the patient is happy both optically and cosmetically with their new spectacles. In Optical Assistant (OA) Corner Part 24, we look at what constitutes a high prescription, before turning to the guidance that patients should be given when choosing an appropriate frame.
First of all, what constitutes a high prescription? Once a prescription reaches +/-5.00D, a vertex distance is needed – and so this could be considered a high prescription. A complex lens as defined by the GOS voucher constitutes an even higher prescription as this refers to a lens over +/-10.00D in any meridian, or a prism controlled bifocal. Dispensing to higher prescriptions needs great care in order to ensure the patient is happy both optically and cosmetically with their new spectacles.
The right frame is very important, and careful guidance is needed when helping someone with a high prescription choose a new frame. It is a good idea to take the PD of the patient before you start looking at a frame. This is because if you can match the patient’s PD to the box centre of the frame, the lens will not need to be decentered, which means the edge thickness will be kept to a minimum.
The box centre of the frame is the horizontal box lens size plus the distance between the lenses (DBL) and this can be found stamped on the frame. For example, if the eyesize is 48mm and the DBL is 18mm, the box centre distance is 48+18 = 66mm. So if your patient has a PD of 66mm too, this would be ideal. This does of course depend on the patient’s headwidth as the frame can’t be too wide or too narrow, but it is a good starting point and something to bear in mind when helping a patient choose a frame.
Frame material is also very important, as is frame size. Generally, plastic frames are better than metal as they have a thicker rim, which will help to disguise some of the edge thickness. If the temporal lens edge is significant, a wider side can also help to hide the lens thickness. It is best to avoid rimless and supras, as all the edge thickness will be on show, although this does depend on the actual prescription and the lens material chosen.
Choosing a lens with a higher index will reduce the lens thickness, although increasing the refractive index will also increase the lens aberrations, which some patients may not be able to tolerate. Most patients are willing to persevere with this, however, in order to benefit from thinner lenses, and many will adapt.
Aspheric lenses will also reduce lens thickness, and these lenses are especially beneficial to hypermetropic prescriptions. This is because they reduce the centre thickness of the lens and are flatter than non-aspheric lenses and so not as bulbous; cosmetically, they look much better. They are also lighter in weight, but as with the higher index lenses, some patients cannot tolerate them due to the changes in the lens form. Anti-reflective coatings should always be considered with high index and aspheric lenses, as they are more reflective.
For very high prescriptions, there are lenticular lenses. There are different types but, essentially, they are lenses with the prescription in the centre of the lens (called the aperture), and a margin surrounding the aperture, which can have either no prescription (plano) or a very reduced prescription. This results in a much lighter weight lens, and enables a larger frame to be chosen, as there is minimal edge thickness. The major disadvantage of this lens type is the reduced field of view, and care needs to be taken when dispensing these lenses.
There are important measurements that need to be taken when dispensing higher prescriptions, and guidance should be sought to ensure the patient’s vision is not compromised. A knowledge of how to advise a patient on suitable frame and lens type for their prescription will always provide a good starting point when helping a patient choose a new pair of spectacles.
Sue Deal FBDO R is a practising dispensing optician, ABDO College examiner, senior tutor and supervisor for dispensing opticians. She is also a practice visitor and external moderator for ABDO. She was recently awarded the ABDO Medal of Excellence for her outstanding services to the profession.