What does Refractive error mean Flashcards
EMMETROPIA
Emmetropia’ means the absence of a refractive error, so parallel light from a distant source is perfectly focused on the retina
Emmetrope VA
normal distance acuity with no spectacles (uncorrected Snellen acuity of 6/6 or better) ~provided, of course, there is no amblyopia, ocular pathology, or cerebral visual impairment.
AMETROPIA
‘Refractive error’ (ametropia) means that an eye does not allow light from a distant source to be focused perfectly on the retina.
Which is more common? Myopia or hypermetropia
Myopia is just as common as hypermetropia.
What is myopia
‘Myopia (short-sightedness) means that the refractive power of the eye is too great relative to the axial length of the eye; as a result, the image of a distant object lies in front of the retina
Causes of myopia
if the refractive power is too high or if the eye is too long
How is myopia corrected
By a minus (con- cave) lens, which effectively weakens the refractive power to allow the image to be shifted back on to the retina
What is hypermetropia
‘Hypermetropia’ (long-sightedness), also known as hyperopia, means that the refractive power of the eye is too weak relative to the axial length of the eye; as a result, the image of a distant object lies behind the retina
How is hypermetropia corrected
by a plus (convex) lens, which effectively strengthens the refractive power to allow the image to be shifted forwards on to the retina
What is astigmatism
Astigmatism refers to the refractive power of the eye being different in different meridians. Therefore, light from a point of a distant object cannot forma single point of an image
What is Sturm conoid
An eye with astigmatism behaves as a sphero-cylindrical (toric) lens. The principal meridians form separate line foci and between them lies Sturm’s conoid. The dioptric midpoint of Sturm’s conoid is the circle of least confusion, situated at the focal point for the lens spherical equivalent value
Where to place the corneal incision in cataract surgery
on the steepest K meridian, the degree of corneal astigmatism is reduced, which can be beneficial to the patient since astigmatism has no refractive advantage
What is regular astigmatism
This applies when the meridians of maximum and minimum refractive power are perpendicular to each other
Types of regular astigmatism
With the rule
Against the rule
Oblique astigmatism
What is WTR astigmatism
- The steepest meridian is vertical, or within 30 degrees of 090 (060- 120), and the flattest is horizontal
- The maximal refractive power of the cornea acts vertically (the steepest K meridian will be near 090)
- The axis of a correcting plus cylinder will be vertical (090), since its power needs to act horizontally to strengthen the relatively weaker horizontal meridian
Example of WTR astigmatism on PRESCRIPTION
+1.00/ +1.50 x 090. There is a +1.50 dioptre cyl, which has its axis (denoted by x’) at 090. This cyl is superimposed on a +1.00 dioptre sphere. In the absence of any lenticular astigmatism, one would expect the K value in the vertical meridian to be greater than the horizontal meridian.
What is ATR astigmatism
- The steepest meridian is horizontal, or within 30 degrees of 180 (000-030 and 150-180), and the flattest is vertical
- The maximal refractive power of the cornea acts horizontally (the steepest K meridian will be near 180)
- The axis of a correcting plus cylinder will be horizontal (180), since its power needs to act vertically to strengthen the relatively weaker vertical meridian
Example of ATR astigmatism
+2.00 / + 1.75 x 180. There is a +1.50 dioptre cyl with its axis (denoted by ‘x’) at 180. This cyl is superimposed on a +2.00 dioptre sphere. In the absence of any lenticular astigmatism, one would expect the K value in the horizontal meridian to be greater than the vertical meridian.
What is oblique astigmatism
The maximal and minimal meridians are perpendicular to each other but are not acting at or within 30 degrees of the vertical or horizontal plane for example, a maximal meridian at 050 and a minimal meridian at 140.
What is irregular astigmatism
When the meridians of maximum and minimum refractive power are not perpendicular to each other. The most common cause for this is keratoconus (which gives a scissor-like retinoscope reflex that is difficult to neutralise).
Causes of irregular astigmatism
*Keratoconus
*Post penetrating keratoplasty
What is simple astigmatism
When the eye is plano (emmetropic) in one meridian (ie. the rays in this meridian focus on the retina) and either myopic or hypermetropic in another (ie. the rays in this meridian do not focus on the retina).
Example of simple astigmatism
0.00 / +1.50 x 055 implies that no spherical correction is required, but a +1.50 cyl lens is required with an axis at 055 (power acting perpendicularly at 145) to correct the refractive error.
What is compound astigmatism
when both meridians are hypermetropic (i.e. the rays in all meridians come to focus behind the retina) or both are myopic (rays in all meridians come to focus in front of the retina).
Example of compound astigmatism
+1.00 / +2.00 x 090 implies that a +1.00 spherical lens with a + 2.00 cyl lens with axis at 090 (power acting perpendicularly at 180) is required to correct the refractive error.
What is mixed astigmatism
This applies when one meridian is myopic (rays fall in front of the retina) and the other is hypermetropic (rays fall behind the retina).
Example of mixed astigmatism
-1.50 / +2.50 x 040 implies that a -1.50 spherical lens with a + 2.50 cyl lens with axis at 040 (power acting perpendicularly at 130) is required to correct the refractive error.
How does the cyl axis run on a trial frame
From right to left 0 to 180
How to transpose from + to - cyl and vice versa
1) Add cyl to sphere
2) Change cyl sign
3) New axis perpendicular to old axis (+/-90)
Transposition example
Transpose —8.00 / +3.00 x 165.
New sphere = (-8.00) + (+3.00) = -5.00 (adding the cyl to the sphere to give
new sphere)
New cyl =-3.00 (changing the sign of the cyl to give the new cyl)
New axis = 165 - 090 = 075 (the new axis is perpendicular to the old axis)
To give ~5.00 / -3.00 x 075.
How to calculate spherical equivalent
Spherical equivalent =sphere +(cylinder/2)
What is anisometropia
when the difference in refractive prescriptions between the two eyes is sufficiently large to result in troublesome symptoms such as aniseikonia (different image size of single object between eyes) and asthenopia (eye strain —patients are often non-specific, but complain of fatigue, blurred vision, and headache).
What difference in dioptres between the eyes causes anisometropia and the troublesome effects
when the difference in spherical equivalent is more than 1.5 dioptres.
