When a surgeon performs cataract surgery he does not want to put in just any old
lens. He needs to make an educated decision on what lens would cause the light
to converge on the retina just right. To make this decision the physician needs
the following information Refraction – The most current manifest
refraction. Keratometry – The curvature of the eye A-constant – A
constant that comes with the IOL Master Axial length – The distance
between the anterior and posterior poles of the eye. The distance between the
tip of the cornea to the retina.
Anterior Chamber Depth
The Anterior Chamber is the fluid-filled space between the posterior cornea and
the lens. Evaluating anterior chamber depth is important in testing for narrow
angle glaucoma. I use three sets of data when evaluating anterior chamber depth.
Hyperyopia– in some patients, causes the iris to
“bow-out.” Normally an iris should be somewhat flat like a
plate or a disk. When an iris “bows-out” it is shaped more like
a parachute. The more positive the spherical value of their prescription
the more likely they are to “bow-out.” When an iris bows-out it
may cause constricted flow of the aqueous through the trabecular meshwork.
Pen Light– Shine a penlight across the eye. If the side opposite
from the source of light casts a shadow that is a bad thing. that means
that the iris is bowing-out. If the iris is bowed out then it needs to be
checked on a slit lamp.
Slit Lamp– This is the ultimate test. Shine a narrow beam of
light at an angle on the limbus(The area where the iris and the white of
the eye meet). Two beams of light should be seen. The first beam of light
is the reflection off of the cornea and the second beam of light is the
reflection off of the iris. The larger the gap between these two
reflections the better. If there is no gap then the patient may have narrow
A pachymeter is used to test corneal thickness. Most corneas are about 500 micro
meters. The thickness of the cornea is useful when evaluating glaucoma.
Calibrate Biometry Instruments
Biometry instruments must be calibrated. To know how to calibrate a particular
biometry instrument consult the user manual.
Schirmer’s test – This test tests for sufficient tearing. The
test is performed by putting filter paper under the lid. There are two types
of Schirmer’s test.
Schirmer’s test 1 – The patient is given anesthetic to test
Schirmer’s test 2 – The patient is not given anesthetic to test
baseline secretions + reflex secretions. Reflex tears are a response to
Tear Break-Up Time – A tear break-up is an area of the eye that is caused by lack of tears.
Rose Bengal– This test is used on patients with dry eye. It is like
fluorescein, but it binds much tighter to degenerated epithelial cells and not
to living cells. This makes it a better indicator of corneal damage due to dry
eyes. The one disadvantage is that it is more painful than fluorescein.
Patients with cataracts many times complain of glare. A patient may read 20⁄30,
but when light is shining toward the eye the patient may only be able to read
the 20⁄60 line. If there is a large difference in vision due to glare the cause
is most likely are cataract.
Color Vision Testing
In our eye we have two main types of photoreceptors. These are the rods and
cones. Cones are responsible for our color vision. The colors that our cones
respond to are red, green, and blue. Ishihara color plates are used to test for
color blindness. The most common color defects are red-green color deficiency.
Contact A-Scan is a regular test performed before cataract surgery. This
information is needed when determining the IOL (intraocular lens) power the
patient should be given The patient is given local anesthetic then a probe is
placed on the eye. This probe evaluates the axial length of the eye. The axial
length of most eyes should be 23-24mm. One disadvantage to using a contact
A-scan is that the axial length may be smaller than it really is due to the
force of pressing the probe on the eye too hard.
Laser Interferometry (IOL Master)
The IOL master, like the contact A-Scan, can measure axial length. The benefit
of using the IOL master is that it is non contact which means that it does not
have error due to pressing on the eye like the contact A-Scan. The IOL master
uses infrared light to measure the length of the eye. The IOL master can also be
used to find keratometry.