Keratometry CH6

Question 1

The structure of the eye with the most refractive power is the:
a) lens
b) cornea
c) vitreous/aqueous
d) retina

Answer 1

b) While the lens, cornea, and vitreous/aqueous each have refractive power, and in spite of the
fact that the lens can adjust and focus, the cornea is the strongest refracting structure of the eye.

Question 2

Corneal curvature is measured (quantified) using the:
a) Placido’s disk
b) slit lamp
c) vertexometer
d) keratometer

Answer 2

d) The keratometer is the only device listed that measures corneal curvature. Placido’s disk is used to evaluate corneal shape and topography, but it does not give a measurement. The vertexometer is used to measure vertex distance, the distance between a corrective lens and the
front surface of the cornea

Question 3

Corneal curvature can be recorded as:
a) millimeters/diopters
b) milliliters/diopters
c) millimeters/cylinder
d) centimeters/decibels

Answer 3

a) Corneal curvature can be designated in both millimeters and diopters. (Did you notice
that answer b was in microliters?)

Question 4

Unequal corneal curvature where the flattest and steepest curves are 90 degrees apart
is known as:
a) myopia
b) presbyopia
c) astigmatism
d) hyperopia

Answer 4

c) Astigmatism occurs when the cornea is more curved in one direction and less in the other. Think of the back of a spoon or the surface of an eggshell; these curves are almost always 90 degrees from each other

Question 5

The average power of the human cornea is:
a) 30 to 32 D
b) 35 to 36 D
c) 43 to 44 D
d) 45 to 47 D

Answer 5

c) The average refractive power of the cornea is 43 to 44 D

Question 6

Average thickness of the central cornea is:
a) 0.8 mm
b) 0.5 mm
c) 0.2 mm
d) 1.0 cm

Answer 6

b) The average thickness of the central cornea is 0.5 mm. (Note: Be sure to pay attention
to measurement designations: Answer d was in centimeters.)

Question 7

The following prescription, Plano – 2.00 × 180, is an example of:
a) with-the-rule astigmatism
b) against-the-rule astigmatism
c) oblique astigmatism
d) irregular astigmatism

Answer 7

a) In with-the-rule astigmatism, the strongest corneal axis is vertical; this translates to approximately 180 degrees in minus cylinder and approximately 90 degrees in plus cylinder.
Against-the-rule astigmatism is the opposite, with the strongest corneal axis in the horizontal, approximately 90 degrees in minus cylinder and approximately 180 degrees in plus. Irregular astigmatism does not have a single axis because the flattest and steepest meridians are not at 90 degree angles from each other

Question 8

Which of the following is an example of oblique astigmatism?
a) -2.00 + 1.25 × 093
b) -0.75 - 1.25 × 003
c) -4.25 + 2.75 × 046
d) Plano

Answer 8

c) Oblique astigmatism means the axes are not vertical (eg, 093) or horizontal (eg, 003), but
at an angle in between (eg, 046). Note that the axes do not have to be “on the dot” at
90, 180, 45, or 135 to qualify.

Question 9

If the axes of astigmatism are not 90 degrees from each other, this is termed:
a) irregular astigmatism
b) compound astigmatism
c) dry eye syndrome
d) astigmatism of vitreous face

Answer 9

a) Irregular astigmatism occurs when the steepest and flattest curves are not 90 degrees
from each other. Compound astigmatism is a type of regular astigmatism. Answer d is
bogus

Question 10

Of the following, which patient is most likely to have irregular astigmatism?
a) postoperative cataract
b) aphakic
c) surface ocular trauma
d) postoperative LASIK

Answer 10

c) The most common cause of irregular astigmatism is trauma to the ocular surface. While ocular surgery can cause irregular astigmatism, it is not the most likely of those listed.

Question 11

The steepest part of the cornea is the:
a) periphery
b) inferior one-fourth
c) stroma
d) center

Answer 11

d) The cornea is steepest in the center, then flattens out toward the periphery. (Remember,
the stroma is a layer of the corneal tissue.)

Question 12

The diameter of the cornea’s optic zone, or cap, measures:
a) 4.0 mm
b) 0.5 mm
c) 20 mm
d) 3.06 µm

Answer 12

a) By definition, the corneal cap or optic zone is the central 4.0 mm of the cornea. (Did you
notice that answer d was in micrometers?)

Question 13

Manual keratometry would not be the most appropriate method for reliable measurements in which of the following cases?
a) fitting contact lenses
b) monitoring keratoconus
c) calculating intraocular lens (IOL) power
d) evaluating after cataract surgery

Answer 13

b) This was a tough one; did you catch the phrase “most appropriate”? Because the keratometer evaluates only a tiny section of the cornea (3 mm), it would be more accurate to follow keratoconus using corneal topography. This is not to say that one cannot use manual keratometry in this case, but the question asked for “most appropriate.”

Question 14

Keratometry would be useful in evaluating all of the following except:

a) keratoconus
b) preoperative cataract surgery
c) contact lens fitting
d) corneal ulcer

Answer 14

d) The keratometer is definitely useful in measuring the curvature of the central cornea for contact lens fitting and IOL calculations. Keratoconus is better followed by corneal topography although one can use the keratometer. The best answer is d, corneal ulcer, which would cause irregular keratometry mires and is best evaluated at the slit lamp.

Question 15

Label the parts of the keratometer:
*barrel
*chin rest
*chin rest adjustment
*focus adjustment
*lock
*occluder
*eye piece
*forehead adjustment
*vertical barrel adjustment
*vertical mires adjustment
*horizontal mires adjustment

Answer 15

B: barrel
K: chin rest
G: chin rest adjustment
C: eye piece
F: focus adjustment
J: forehead adjustment
E: horizontal mires adjustment
I: lock
A: occluder
H: vertical barrel adjustment
D: vertical mires adjustment

Question 16

Number the following steps of using a keratometer in chronological order:
____occlude the eye not being tested
____turn the drum so that the horizontal plus signs are aligned exactly tip-to-tip
____turn the dials to superimpose the horizontal plus and vertical minus signs
____position the patient
____focus the mires, and center the cross-hairs in the lower right hand circle
____focus the eye piece

Answer 16

Order:
3 occlude the eye not being tested
5 turn the drum so that the horizontal plus signs are aligned exactly tip-to-tip
6 turn the dials to superimpose the horizontal plus and vertical minus signs
2 position the patient
4 focus the mires and center the cross-hairs in the lower right hand circle
1 focus the eye piece
(Note: Steps 5 and 6 may be reversed, but
most examiners prefer to set the axis
before completing alignment. In addition,
some keratometers have plus sign mires at
both horizontal AND vertical, rather than
plus at horizontal and minus at vertical.)

Question 17

Occluding the eye not being measured is helpful because it:

a) reduces reflections
b) aids in fixation
c) eliminates lenticular astigmatism
d) reduces irregular astigmatism

Answer 17

b) The occluder forces the patient to fixate with the eye being measured. This helps hold
that eye steady for an accurate reading

Question 18

You are attempting a K reading and do not see both horizontal plus signs. This might
be due to:

a) a drooped upper lid
b) the patient closing his or her eye
c) a keratometer occluder in the way
d) improper focusing

Answer 18

c) The plus sign is projected on the side, and the main thing that might obstruct the mires
from the side is the occluder on the keratometer. A common vertical obstruction (which would obstruct the minus signs) is the patient’s upper lid.

Question 19

Matching. Select one best answer:
Probable condition
flat cornea
corneal warpage
keratoconus
spherical cornea
steep cornea
dry eye
astigmatism

Keratometer mires appear
A) small
B) round
C) elliptical
D) very large
E) clear, then quickly blur
F) wavy, blurred, discontinuous
G) distorted, small, cannot superimpose

Answer 19

*flat cornea - D) very large
*corneal warpage - F) wavy, blurred, discontinuous
*keratoconus - G) distorted, small, cannot superimpose
*spherical cornea - B) round
*steep cornea - A) small
*dry eye - E) clear, then quickly blur
*astigmatism - C) elliptical

Question 20

If the cross-hairs of the keratometer are not centered during the initial reading for
contact lens fitting:

a) the fit may be inaccurate
b) lens diameter may be incorrect
c) lenses will be difficult to handle
d) the lens will not transmit oxygen

Answer 20

a) If the cross-hairs are not centered, you are not reading the corneal apex. Because the
corneal periphery is flatter, the resulting fit will be too loose when placed on the steeper
corneal apex.

Question 21

K readings in a contact lens-wearing patient may be used to evaluate all of the
following except:

a) progressive corneal steepening
b) lens fit
c) lens coverage
d) corneal warpage

Answer 21

c) The keratometer can be used to evaluate corneal steepening by taking a series of readings over time. Corneal warpage is evident by distorted mires. By using the keratometer to look at a patient while actually wearing his or her contacts, the fit can be evaluated by the appearance of the mires and whether they clear/blur before or after a blink. (Of course, a slit-lamp exam is still required to complete the evaluation.) Lens coverage of the cornea is evaluated at the slit lamp, not with the keratometer.

Question 22

The patient’s keratometry mires are very oval-shaped. This means that there is:
a) no astigmatism
b) high emmetropia
c) significant astigmatism
d) inaccurate alignment

Answer 22

c) Oval mires indicate the presence of significant astigmatism. Actually, this makes the reading easier, as even a tiny turn of barrel or dial makes an obvious difference. An eye with no astigmatism would have very round mires. Emmetropia is lack of refractive error, so it cannot be classified as high, low, or in between. Inaccurate alignment is evident when the cross-hair is not centered in the lower right circle

Question 23

The patient’s keratometry mires are squiggly looking and change when the patient
blinks. You should try:

a) instilling topical anesthetic
b) instilling artificial tears
c) re-explaining the procedure
d) realigning the patient

Answer 23

b) “Squiggly” mires that change when the patient blinks indicate a problem with the tear
film. Instill some artificial tears, have the patient blink a lot, and blot gently with a tissue. Then, try the readings again. There is no need to use an anesthetic.

Question 24

No matter how you rotate the keratometer drum, the “plus signs” remain aligned.
This indicates that there is:

a) irregular astigmatism
b) poor tear film
c) poorly focused eye piece
d) no astigmatism

Answer 24

d) If the corneal surface is spherical (ie, no astigmatism), the “plus signs” will stay aligned
no matter where the barrel is positioned, because the curve is the same in every meridian.

Question 25

Which of the following types of astigmatism is not obvious when measuring with the
keratometer?
a) astigmatism at 180 degrees
b) irregular astigmatism
c) lenticular astigmatism
d) oblique astigmatism

Answer 25

c) Answers a and d are evident by the axis readings on the keratometer. Irregular astigmatism can be detected by the fact that the sections of the mires do not focus together, and there seems to be several “axes,” although a measurement is not possible. Lenticular astigmatism is caused by the crystalline lens, which is not detected with the keratometer, but rather by noting a difference in the amount of astigmatism found on keratometry and that found on refractometry.

Question 26

You are taking a K reading. The power drum is at 52.00 D (the upper limit of the
scale), and the mires are still not aligned. To extend the range of the keratometer, you
should:

a) affix a –1.00 D trial lens over the aperture
b) affix a +1.25 D trial lens over the aperture
c) affix a +2.25 D trial lens over the eye piece
d) it is not possible to extend the range of a keratometer

Answer 26

b) If the keratometer reading is past the upper range (52.00 D), affix a +1.25 D trial lens to
the aperture. (If the reading is below the lower range, then a -1.00 D trial lens is used.)

Question 27

You have taken a K reading by extending the instrument’s range. To get the correct
value, you can:

a) use a conversion chart
b) subtract the power of the extending lens
c) add the power of the extending lens
d) simply record the drum reading

Answer 27

a) If a trial lens is used to extend the range of the keratometer, the new reading is most
conveniently interpreted using a conversion chart. If you are aching to do the math and you have extended the range upward, multiply the reading by 1.1659. If you have extended the range downward, divide by 1.1659.

Question 28

You are adjusting the keratometer eye piece by looking at the occluder through the
ocular. The mires are distorted. You should:

a) calibrate with metal balls
b) spray cleaner into the instrument
c) clean the instrument with compressed air
d) return to the manufacturer for cleaning

Answer 28

d) Distorted mires are caused by problems with the internal filters and lenses. Only the
manufacturer can handle that.

Question 29

When calibrating the keratometer with the metal balls, all of the following are done
except:

a) using chrome balls of known radius
b) placing holder on the headrest
c) setting the eye piece for your refractive error
d) placing the chrome ball in the holder with your fingers

Answer 29

d) Do not handle the chrome balls with your fingers. Use a tissue or magnet. Oils from the
fingers can cause the chrome to corrode, changing the readings

Question 30

If calibration of the keratometer reveals a discrepancy:
a) adjust the drums
b) tighten the headrest
c) loosen the top screw
d) have the manufacturer repair it

Answer 30

d) If you test the keratometer and the readings are not true, the manufacturer will have to
repair it

Question 31

If the keratometer’s occluder is loose:

a) simply remove it
b) rebend or replace the washer
c) tape it up out of the way
d) adjust the headrest appropriately

Answer 31

b) A loose occluder can be rebent, or you can replace the washer. It is important to use the
occluder, however, because it helps keeps the eye centered for the reading.