Optical and Ultrasound Biometry

Published on
December 4, 2023
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Optical and Ultrasound Biometry

An ocular biometer measures the size and shape of the eye, specifically axial lengths (distance from the front to the back of the eye implicated in refractive error), anterior chamber depth (important for glaucoma) and keratometry (curvature of the cornea, the clear structure at the front of the eye). Each of these measurements aids in the management of diseases like cataract, and the monitoring of progressive myopia. Both our Adelaide and Henley Beach branches are equipped with either optical or ultrasound biometers like the IOL Master, and most recently the Pentacam AXLWave.

Myopia control

Optical biometry is crucial for our myopia patients. Axial myopia, or short-sightedness, is a condition that causes the eyeball to elongate, which focusses light in front of the now displaced retina causing blurred distance vision. Axial lengths of 24mm are normal, and anything over 26.5mm increases the risk for retinal detachment due to the stretching of retinal tissue. The optical biometer measures axial length to monitor myopia progression and better manage myopic patients to reduce the risk of complications. It’s also useful for assessing the impact of orthokeratology on slowing myopia progression.

Two images of eyes, with several numbers taken during ocular biometry measurements. Importantly, it displays the axial length of this eye, 26mm.
Figure 1. Axial length biometry measurement for monitoring axial length in a progressive myope. 26mm is considered high myopia, carrying a greater risk of complication.

Cataract

The main function of an optical biometer is to calculate the power of the intraocular lens (IOL) for patients who are undergoing cataract surgery. Cataract is the opacification of the crystalline lens in the eye, which causes blurry and cloudy vision. With the addition of the Pentacam AXLWave, we can now also objectively quantify the opacification by densitometry. The opacified lens is replaced during surgery with an artificial IOL, the optical power of which is decided beforehand for clear vision at a chosen distance. Optical biometry also helps us to determine how dense the opacified lens is, which is highly useful for deciding on when to refer for cataract surgery.

Anterior chamber depth

The health of our patients’ eyes is of the utmost importance to us. While we have a range of equipment that tests for a multitude of eye conditions, optical biometry is particularly critical for patients who have been diagnosed with or are suspects of glaucoma, who often need their anterior chamber depth measured. Angle-closure glaucoma occurs when the angle of the anterior chamber becomes too narrow, increasing the pressure inside the eye which can potentially damage the optic nerve. With optical biometry at our disposal, we can better identify the signs of glaucoma and manage accordingly.

Pentacam display of a Scheimpflug image, displaying the anterior chamber depth
Figure 2. Scheimpflug image taken with Pentacam AXL wave, demonstrating the anterior chamber of the eye, and the distance between posterior cornea and the anterior surface of the lens.

Keratometry

Astigmatism is a refractive condition that causes blurred vision at all distances due to variations in the optical power of the eye as a result of abnormal corneal curvature. The degree of astigmatism is measured using an optical biometer, a measurement known as keratometry (K). This is particularly crucial for our patients who are spectacle or contact lens wearers who need accurate readings to ensure their prescriptions areas precise as possible.

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