OPTICAL PRESCRIPTION Flashcards
Optical Prescriptions
Optical prescriptions are very important. They tell us a lot about the refractive errors of a particular patient. And patients use the optical prescription given to them to get glasses or contacts to help them see.
There are several components to an optical prescription. Based on the prescription, we should be able to answer all of the following questions
Is the patient myopic or hyperopic? How myopic or hyperopic are they? Do they have astigmatism? If so, how much and at what axis? Do they have prism in their glasses? Do they need help with reading?
THE LAYOUT AND INFORMATION ON AN OPTICAL PRESCRIPTION IS BASICALLY ALWAYS THE SAME. WE WILL TALK ABOUT SPHERE, CYLINDER, AND AXIS TODAY. THE OTHER COMPONENTS WILL BE COVERED LATER ON IN THE SEMESTER and next semester.
If a patient has ONLY myopia or hyperopia and their cornea has the same curvature in all meridians what does that mean?
How can we correct that?
(NO ASTIGMATISM), we can correct this with a spherical lens.
Spherical lenses
have a spherical curved surface and, therefore, the same power in all meridians. This makes sense since we are using a spherical lens for a “spherical” cornea.
Since the power is the same in every meridian of a spherical lens, all light rays will converge or diverge equally.
Because light rays in spherical lenses either converge or diverge equally, images produced by spherical lenses form a point focus (as opposed to a line focus, which we will discuss later) at the focal point of the lens.
Minus lense is to corerct what
Myopia
plus lense is to correct what?
Hyperopia
Myopic eye has too much plus or minus power?
what type of lense?
Remember… a myopic eye has excessive plus power that needs to be decreased. We accomplish this with a minus spherical lens.
Whats the prescription for mypia?
For myopia, you may see a prescription anywhere from -0.25 to -19.00 or possibly even greater. A “high myope” describes someone who has a prescription of -6.00 D or more.
FOR EXAMPLE:
OD: -6.00
OS: -5.50
THIS PRESCRIPTION SHOWS THAT THIS PATIENT IS MYOPIC IN EACH EYE. HOWEVER, THIS PRESCRIPTION IS NOT COMPLETE! IT DOES NOT TELL US WHETHER OR NOT THE PATIENT HAS ASTIGMATISM.
If you have a patient who is ONLY myopic or hyperopic, then their prescription is said to be
“spherical”. This MUST be included in the Rx and NOT assumed. Sph is usually used as an abbreviation for sphere.
EXAMPLE:
OD: -6.00 SPH
OS: -5.50 SPH
THIS IS THE DEFINITIVE WAY TO state that SOMEONE DOES NOT HAVE ASTIGMATISM. IF SPH IS NOT INCLUDED IN THE RX, IT COULD BE POSSIBLE THAT THE ASTIGMATISM POWER WAS omitted accidentally.
For hyperopia, you may see a prescription from
+0.25 to +15.00 or possibly higher.
FOR EXAMPLE:
OD: +2.50 SPH
OS: +3.25 SPH
For Spherical lenses how many parameters?
So remember, spherical lenses only have one parameter which is power (D).
Spherical lenses can be plus or minus and will display characteristics of those lenses (image movement, magnification, central or peripheral thickness, convergence or divergence, etc).
But what do we do when someone has astigmatism?
What can we all to the lenses for regular astigmatism
We can add cylindrical lenses to a prescription to correct for any type of regular astigmatism. This is usually shortened to cylinder or cyl
Similar to a spherical lens
Similar to a spherical lens, a cylindrical lens is measured in diopters. A higher dioptric power means a person has more astigmatism. Cylinder can be anywhere from 0.25 D to 6.00 D or more
which one are more complex? Spherical or Cynlindrical
explain both?
Cylindrical lenses are slightly more complex than spherical lenses.
Unlike spherical lenses which refract light rays equally in all meridians, cylindrical lenses refract light only along one axis. This is because cylindrical lenses have 2 optical surfaces – a flat one and a curved one.
That should make sense when we think about what astigmatism is (a cornea with a steeper curve or more power in one meridian and a flatter curve in another meridian).
reAD
The plane surface of a cylindrical lens has what?
has no curvature and no power. It is called the cylinder axis and it is 90 degrees away from the curved surface.
The curved surface has what?
what is it called?
the image forms a focal point? what does it form and which shape does it apply to?
has curvature and power;
it is called the power meridian.
Its image does not form a point focus (focal point) like a spherical lens,
but rather a line focus
(focal line) along the
cylinder axis.
Therefore, if a cylinder lens is placed with its axis at 90, its full power is provided at axis 180. The cylinder is oriented so that its axis is along the steeper meridian of the cornea. It effectively increases the power of the flatter meridian so that it equals that of the steeper meridian. This neutralizes the effect of the astigmatism.
Therefore, if a cylinder lens is placed with its axis at 90, its full power is provided at axis 180. The cylinder is oriented so that its axis is along the steeper meridian of the cornea. It effectively increases the power of the flatter meridian so that it equals that of the steeper meridian. This neutralizes the effect of the astigmatism.
AT WHAT AXIS WILL A CYLINDER LENS OF THE APPROPRIATE POWER BE ORIENTED TO PROVIDE CORRECTION ALONG AN AXIS OF 65 DEGREES?
CYLINDER POWER REQUIRED AT AXIS: 65
CYLINDER SHOULD BE ORIENTED 90 DEGREES AWAY:
65 + 90 = 155
AT WHAT AXIS WILL A CYLINDER LENS BE ORIENTED TO PROVIDE CORRECTION ALONG AN AXIS OF 120?
CYLINDER POWER REQUIRED AT AXIS: 120
CYLINDER SHOULD BE ORIENTED 90 DEGREES AWAY:
120-90 = 30 DEGREES
How many cylindrical lenses have?
Remember, cylindrical lenses have 2 parameters, power and axis, which are 90 degrees away from each other. There must always be an axis if there is cylinder power!
Cylindrical lenses can be recorded either as a plus lens or a minus lens.
++++++++++++++++++++++
They are referred to as plus cylinder or minus cylinder.
OD: +0.75 +1.50 X 075
OD: +1.25 +0.50 X 120
OD: +1.00 -1.00 X 090
OS: +0.25 -1.00 X 090
Let’s review astigmatism one more time.
The cornea has one meridian with a flatter curve and one with a steeper curve. Either the steeper curve has too much power, or you could say the flatter curve has too little power. Thus, the light rays are not coming to a single focal point
A minus cylinder can be used, oriented in the opposite direction as the steeper curve. This would reduce the power 90 degrees away from that (at the steeper curve). Or, a plus cylinder can be used. A plus cylinder is oriented in the same meridian as the steeper curve. This would increase the power of the flatter meridian. The end result would be the same.
A minus cylinder can be used, oriented in the opposite direction as the steeper curve. This would reduce the power 90 degrees away from that (at the steeper curve). Or, a plus cylinder can be used. A plus cylinder is oriented in the same meridian as the steeper curve. This would increase the power of the flatter meridian. The end result would be the same.
Optometrists tend to write optical prescriptions in
in minus cylinder form.
Ophthalmologists write their prescriptions in
plus cylinder form.
Lenses are generally made with
a minus cylinder ground into the lens.
Cylinder lenses are used to correct
astigmatic errors in which only one meridian needs to be corrected and the other is plano (pl). Plano is the same as “zero” spherical power. Cyl is used to correct simple myopic and simple hyperopic astigmatism
If a patient has simple hyperopic astigmatism in both eyes their prescription may look like this…
OD: pl +1.00 x 045
OS: PL +0.75 X 120
The PL Indicates THAT THERE IS NO (ZERO) SPHERICAL POWER!
PLANO
IS ZERO
A MYOPIC PRESCRIPTION
WOULD BE A SINGLE POINT ON THE NEGATIVE SIDE OF THE NUMBER LINE (A MINUS PRESCRIPTION IS NEGATIVE).
A HYPEROPIC PRESCRIPTION
IS A SINGLE POINT ON THE POSITIVE SIDE OF THE NUMBER LINE (A PLUS PRESCRIPTION IS POSITIVE).
IF A PATIENT HAS ASTIGMATISM, THERE WILL BE
2 POINTS ON THE NUMBER LINE, ONE FOR THE SPHERICAL POWER AND ONE FOR THE CYLINDER POWER. THESE POINTS CAN FALL ANYWHERE ON THE NUMBER LINE. IF IT IS A SIMPLE ASTIGMATISM IN PLUS CYLINDER, ONE OF THE POINTS ON THE NUMBER LINE MUST BE ZERO AS THERE IS ZERO SPHERICAL POWER!
How many powers the cornea has
2 different powers
THE CORNEA HAS 2 DIFFERENT POWERS. THE AMOUNT OF CYLINDER IS ACTUALLY THE DIFFFERENCE BETWEEN THESE 2 POWERS.
THE AMOUNT OF CYLINDER (IN DIOPTERS) TELLS US HOW FAR WE WOULD TRAVEL ON THE NUMBER LINE TO GET TO THE 2ND POWER READING. YOU CAN THEN DETERMINE WHAT KIND OF ASTIGMATISM THE PATIENT HAS.
IN OUR EXAMPLE OF SIMPLE HYPEROPIC ASTIGMATISM, WE STARTED AT 0 D IN THE RIGHT EYE. WE WILL THEN TRAVEL IN THE PLUS DIRECTION (AS INDICATED BY THE + IN FRONT OF THE CYLINDER) ON THE NUMBER LINE BY 1.00 D. WE END UP ON +1.00. FOR THE LEFT EYE, WE STARTED AT 0 D. WE MOVE IN THE PLUS DIRECTION 0.75 D. WE END ON +0.75 D.
OD: pl +1.00 x 045
OS: PL +0.75 X 120
Since our first number was “0”,
and we ended on the positive side of the number line, that tells us that this prescription is correcting simple hyperopic astigmatism.
What happens when a patient is myopic or hyperopic WITH astigmatism?
spherocylindrical
In that case, a spherocylindrical lens is used. As the name implies, it is a combination of a spherical lens and cylindrical lens. Thus, spherocylindrical lenses have 2 meridians, one of maximum power and the other of minimum power.
Spherocylindrical lenses
are used to correct compound myopic and compound hyperopic astigmatism as well as mixed astigmatism.
IMAGINE A SPHEROCYLINDRICAL PRESCRIPTION TO BE 2 POINTS ON A NUMBER LINE
ONE POINT IS FOR THE SPHERICAL POWER, AND THE OTHER POINT IS FOR THE CYLINDRICAL POWER. NEITHER POINT CAN BE ZERO.
IF BOTH POINTS FALL ON THE NEGATIVE SIDE OF THE NUMBER LINE, THAT WOULD BE COMPOUND MYOPIC ASTIGMATISM.
IF BOTH POINTS FALL ON THE POSITIVE SIDE OF THE NUMBER LINE, THAT WOULD BE COMPOUND HYPEROPIC ASTIGMATISM.
IF ONE POINT IS ON THE NEGATIVE SIDE, AND ONE IS ONE THE POSITIVE SIDE, THAT WOULD BE MIXED ASTIGMATISM.
If a patient has compound myopic astigmatism, their prescription may look like this…
OD: -2.50 +1.75 X 002
OS: -4.25 +0.25 X 178
If a patient has compound hyperopic astigmatism, their prescription may look like this…
OD: +2.00 +1.00 X 095
OS: +1.75 +1.25 X 085
If a patient has mixed astigmatism, their prescription may look like this…
OD: -2.00 +3.00 X 075
OS: -4.00 +5.00 X 070
QUICK REFERENCE GUIDE FOR TYPE OF ASTIGMATISM
Aside from the “normal” changes a patient may have in their prescription, there are several conditions that can cause a shift in an optical prescription.
SOME EXAMPLES INCLUDE…
- CATARACTS
- RETINAL PATHOLOGY
- DIABETES
- CORNEAL PROBLEMS
- PREGNANCY
Cataracts can cause a myopic shift in a prescription. The hardening of the lens centrally can increase the dioptric power. Remember, higher power means a shorter focal length. The focal point is shifted closer to the lens.
RETINAL PATHOLOGY
IN ADDITION TO DECREASED VISION, RETINAL PATHOLOGY CAN CAUSE A HYPEROPIC SHIFT IN THE PRESCRIPTION OF AN EYE. IF A PATIENT HAS MACULAR SWELLING OR FLUID, THE RETINA IS PUSHED FORWARD. THIS ELEVATION DECREASES THE AXIAL LENGTH OF THE EYE. AND A SHORTER EYE IS OF COURSE MORE HYPEROPIC
DIABETES
PATIENTS WITH UNCONTROLLED DIABETES MAY EXPERIENCE A MYOPIC SHIFT IN THEIR PRESCRIPTIONIN ADDITION TO A NUMBER OF OTHER EYE PROBLEMS (SUCH AS DIABETIC
RETINOPATHY). THERE IS A COMPLEX
PROCESS INVOLVED, BUT HIGH BLOOD
SUGARS CAN LEAD TO SWELLING OF
THE FIBERS INSIDE THE LENS OF THE
EYE. THIS CAUSES AN INCREASE IN
THE POWER OF THE LENS AND
THUS A MYOPIC SHIFT.
CORNEAL PROBLEMS
ANYTHING THAT DISRUPTS THE SURFACE OF THE CORNEA CAN AFFECT THE PRESCRIPTION OF THE EYE. THIS INCLUDES SCARRING FROM INJURY OR CORNEAL DISEASE, PTERYIGUMS, DRY EYES, ETC. SHIFTS CAN BE MYOPIC, HYPEROPIC, OR INDUCE ASTIGMATISM.
PREGNANCY
PREGNANCY CAN ALSO AFFECT THE PRESCRIPTION OF THE EYES. THIS COULD BE THE RESULT OF ANY NUMBER OF THINGS SUCH AS HORMONE CHANGES, FLUID RETENTION, CHANGES IN CIRCULATION OR METABOLISM. IN THE CASE OF FLUID RENTENTION, INCREASED FLUID IN THE CORNEA AND/OR LENS CAN CHANGE THE THICKNESS AND CURVATURE. THIS TYPICALLY RESULTS IN A MYOPIC SHIFT BUT HYPEROPIA AND ASTIGMATISM CAN ALSO OCCUR.
these chages can be temporary
These are just a few examples of situations where a change in an optical prescription can occur. It is important to consider the possible cause of a refractive change in a patient because some of these changes could be temporary!
Can Astigmatism be in both eyes
Remember, astigmatism can occur by itself or with myopia or hyperopia. It can be corrected with a cylindrical lens or a spherocylindrical lens. And a patient does NOT always have astigmatism in both eyes or the same amount (D) or axis in both eyes!
