This is a very common comment that eye care professionals hear from their patients and thus an excellent opportunity to unpack many important issues in ophthalmology and optometry. I am going to break this into several blog posts and part (I) will start with optics. Future blog posts will focus on ophthalmic disease which, by far, is often the reason for this comment, but as you will find, not always. If you are a real optics nerd, like me, check out the greatest book ever on this topic entitled, “The Fine Art of Prescribing Glasses Without Making a Spectacle of Yourself” by
Let’s start with optics and let’s assume that the refraction was perfect. (And please, no jokes about the word “assumption” -as in the line that Walter Matthau delivers in the movie, Bad News Bears).
Now, what can go wrong from the point at which the ‘assumed’ correct prescription goes into the hands of the optician and then to the lab that make the lens?
- The famous “minus” vs. “plus” prescription (transcription) error. A really nutty thing in eyecare is the way prescriptions are written. Prescriptions can be written in “plus” cylinder notation or “minus” cylinder notation and they are mathematically identical. A problem occurs when the lens making lab gets these two confused and the glasses prescription will be very off. This is very uncommon but can happen. As an interesting historical side note, Ophthalmologists typically use the “plus” notation and Optometrists typically work in the “minus” notation. There are several possible reasons for this that date back to the 1800’s when cylinders were ground into the front vs. back of lenses and how Ophthalmologists vs. Optometrists then wrote the prescriptions. However, the notation differences have persisted. My theory is that as surgeons, “plus” cylinder notation helps us think about the surgical correction of astigmatism better. However, the plus (no pun intended) of using “minus” cylinder notation is that it makes it easier to write contact lens prescriptions without having to transpose the prescription.
- Optical center errors. When you have glasses made, the optician marks the exact spot on the lens that corresponds to the center of the patient’s pupil. This then gets transmitted to the lens lab and they will create a lens that has it’s optical center corresponding to the patient’s pupil. It is remarkable to me how many patients have incorrectly positioned optical centers in their glasses. I see this error made all the time by both opticians (MARKING THE LENS) and by the lens maker (MAKING THE LENS). The patients will have a variety of problems with these glasses including poor vision, double vision, fatigue, and general unhappiness. Determining the optical center of the lens is not trivial and often done incorrectly. This error then sends the prescribing doctor and the lens maker in an endless loop of finger pointing and trying to correct the problem. I have been frustrated by this problem for years until now. We (at FishmanVision) recently obtained the Visionix VX-40, a wavefront device that provides a visual topographic map of the power ranges across the lens of the glasses and determines an immense amount of information on the lens including the optical center, lens aberrations, the size and position of the progressive zone. The Visionix VX-40 has allowed us to determine whether the lenses were made correctly. It is a game changer for us and for our patients.
- Aniseikonia (image size difference). In all fairness, this is really not an error by either the doctor or the lens lab, but an iatrogenic issue associated with the magnification that the lens makes on the patient’s retina. This issue is conceptually simple – if each eye has a different prescription (hence magnification), then the image size that gets projected onto the patient’s retina will be different and the patient can experience a huge number of issues that range from dizziness, fatigue (also known as asthenopia), pulling sensation, and overall miserableness. The error really is in not recognizing the problem and then trying various fixes like prisms, “dumbing down one lens” to better equalize the difference, patching, none of which will be satisfactory in my experience. To add insult to injury, the aniseikonia can be optical or can be retinal. I will write an entire blog post on causes of retina aniseikonia, but for now, let’s stick to optical aniseikonia. If the patient has optical aniseikonia, then the lenses can be made to equalize the image magnification. Aniseikonic lenses are one of several ways to solve this problem. Sometimes, the problem is only in the reading zone and another solution is to use a special lens grinding technique used to neutralize the unwanted prism effect when looking down through the reading area (usually a bifocal) of widely differing lens powers.
- Reading add errors. The reading add has a whole host of issues from a PAL zone that is too narrow, to a PAL zone or bifocal that is positioned too low, to a PAL design that doesn’t allow an large enough viewing area for the reading task needed.
- Stress induced optical aberrations. Frames can cause warpage of a plastic lens and the lens will have optical aberrations that can be seen with cross polarizers. This is a simple solution and you have to remake the lens without internal stress.
- Frame wrap angle. Frame wrap angle describes the horizontal angle of the lens plane in front of the eyes. Check out the excellent article in 20/20mag.com on this subject.
- Pantoscopic tilt. The Pantoscopic tilt describes the vertical angle between a wearer’s primary gaze and the intersection of the same with the plane of the lenses. Progressive wearers find even minor amounts of frame-front skew will cause significant visual distress and discomfort. Check out the excellent article in 20/20mag.com on this subject.
- Vertex distance errors – the glasses are made and fitted with a vertex distance that is different from the vertex distance that was in the phoropter when the refraction was performed. Check out the excellent article in 20/20mag.com on this subject.
- Large angle kappa and PALS. Angle Kappa is the angle between the pupillary axis and the visual axis. It is termed positive when the pupillary axis is nasal to the visual axis, and negative when the pupillary axis is temporal to the visual axis. Please see the excellent article in 20/20mag.com for full details.