Intraocular Lens Power Calculation

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.

1. 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.
2. 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.
3. 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 angles.

Pachymetry

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.

Tear Tests

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.

1. Schirmer’s test 1 – The patient is given anesthetic to test baseline secretions
2. Schirmer’s test 2 – The patient is not given anesthetic to test baseline secretions + reflex secretions. Reflex tears are a response to irritation.

Tear Break-Up Time – A tear break-up is an area of the eye that is caused by lack of tears.

http://www.systane.ca/ca_en/ professional/TearFilm_Breakup.asp

In the image above a patient was given fluorescein. The black spots represent tear break-up this patient may have dry eyes.

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.

Glare Testing

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

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.

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