Ocular Biometry
Ocular biometry is used to diagnose certain eye diseases after analyzing the characteristics and measurements of the eyeball.
General Description
Ocular biometry is used to measure various parameters of the eye, such as the dimensions of the eyeball, axial length, anterior chamber depth, lens thickness, and corneal curvature. This test is part of the preoperative assessment of patients undergoing cataract or refractive surgery, as it helps determine the most appropriate procedure and the power of the intraocular lens to be implanted.
Ocular biometry can be classified into two different types depending on the technique used:
- Ultrasound biometry: uses high-frequency ultrasound to obtain measurements, in a manner similar to ultrasonography.
- Optical biometry: this procedure uses a beam of light, usually a laser, and applies interferometry techniques, which exploit the interference between different waveforms. Its main function is to measure the total axial length (distance between the anterior surface of the cornea and the retina) for intraocular lens calculation.
When is it indicated?
Ocular biometry is performed in the following cases:
- Cataract surgery, in which a pseudophakic intraocular lens (replacing the natural lens) will be implanted.
- Myopia surgery in which a phakic intraocular lens (which does not replace the natural lens) or a pseudophakic lens (if the natural lens is damaged) will be used.
- Monitoring the progression of high myopia, since excessive elongation of the eyeball may lead to complications.
Optical biometry is most commonly used, whereas ultrasound biometry is selected in patients in whom the use of light is contraindicated (e.g., cataracts or ocular hemorrhage).
How is it performed?
While the patient maintains fixation on a central point, measurements are obtained:
- Optical biometry: the patient places the chin on a rest and the forehead against a support to maintain a forward position. A laser beam is then activated, which produces interference patterns as it passes through the eye. These patterns are analyzed by a computer to determine the measurements of the ocular structures.
- Ultrasound biometry: an ultrasound beam is directed toward the cornea and passes through the ocular structures to the retina. Each structure has a different composition and therefore produces distinct echoes when exposed to sound waves. The time interval between sound emission and echo detection allows calculation of each segment of the eye (aqueous humor–lens, lens–vitreous, vitreous–retina). The sum of these measurements determines the total axial length of the eyeball.
- Contact method: the transducer is applied directly to the surface of the eye. To obtain reliable results, five measurements must be taken in each eye, slightly prolonging the procedure.
- Immersion method: the probe is placed within a gel-filled cylinder positioned over the eye to obtain measurements.
Risks
Ocular biometry is a safe procedure that does not pose any health risks or cause damage to the eye, regardless of the technique used.
What to expect from ocular biometry
Ocular biometry is an outpatient procedure after which the patient can immediately resume normal activities. As pupil dilation is not required, vision is not affected at any time.
On the day of the test, patients should attend without contact lenses. If ultrasound biometry is used, anesthetic eye drops are administered to prevent discomfort when applying the transducer to the eye.
The procedure lasts approximately five to ten minutes. Once measurements are obtained, the ophthalmologist evaluates the results, which may be provided on the same day of the consultation.
Specialties in which ocular biometry is requested
Ocular biometry is a diagnostic test specific to the field of Ophthalmology.
How to prepare
No specific preparation is required prior to ocular biometry; however, it is recommended not to wear contact lenses for 24–48 hours beforehand.
