Ocular Biometry

Ocular biometry is a diagnostic tool used for taking anatomical measurements of the eye. Human eyes are spherically shaped and placed in bony sockets called orbits. Knowing the details of anatomical measurement of eyes is crucial before any ocular surgeries. The axial length of an eyeball is one of the major determinants of visual disorders. Ocular biometry allows accurate measurements of the axial length of an eyeball and corneal power.

What Is Ocular Biometry?

Ocular biometry is a device that combines A-scan and keratometry. A-scan, also known as an amplitude scan, is a type of ultrasound sonography of the eye that measures the axial length of the eyeball. In addition to this feature, ocular biometry involves keratometry that can be used to determine corneal power by measuring the anterior curvature of the cornea.

A-scan (amplitude scan):

A-scan (amplitude scan) can be performed using three different techniques: contact method, immersion method, and automatic method. During A-scan, characteristics of the spikes generated by the machine are used to determine the axial length, anterior chamber depth, and lens thickness.


Keratometry, on the other hand, helps in measuring the corneal curvature which is the major determinant of corneal power. It is done with a keratometer or an ophthalmometer and is useful in determining the measurements for contact lens fitting. It is also one of the most preferred tools to diagnose and monitor corneal astigmatism. While performing ocular biometry, keratometry reading to measure the anterior surface is done prior to the ultrasonographic measurements of an axial length of the eyeball. It is because no drops should be instilled in the cornea before performing keratometry as they may alter the readings of the anterior curvature of the eye.

What Is the Purpose of Performing Ocular Biometry?

Ocular biometry with the combination of A-scan and keratometry provides broad data and has a wide range of indications. Although it is mainly used before cataract surgery to analyze the parameters of intraocular lens, it also helps in the diagnosis of several other conditions as well. Some of the indications of ocular biometry are as follows:  

  • Estimation of intraocular lens (IOL) power
  • Pseudo exophthalmos
  • Phthisis bulbi
  • Congenital glaucoma
  • Posterior coloboma
  • Nanophthalmos
  • Myopia
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What Can You Expect While Undergoing Ocular Biometry?

Slit-lamp examination is performed on a patient to rule out dense cataracts or other gross problems affecting biometry readings. One can expect keratometry before A-scan while going for ocular biometry as normal saline or anesthetic drops used during the ultrasound may alter the true corneal curvature of the cornea.

1. Keratometry can be done either by manual or an automated keratometer. An automated keratometer is used in recent days as a part of biometry. The cornea, the anterior surface of an eyeball, forms a circular image projected by a keratometer. 
2. The device analyzes the dimension of the image and gives an estimate of the anterior surface of the eyeball. It is also crucial for a contact lens user to remove lenses a few weeks prior to performing keratometry to prevent error readings. Hard lens users need to hold the use of the lens 4 to 8 weeks prior while soft lens users may remove them 2 to 4 weeks prior.
3. A-scan, as mentioned above, can be done either with a contact technique or an immersion technique. Modern ocular biometry uses immersion techniques as it provides more favorable data with minimal error. Immersion techniques are also found to be better fitted in cases like farsightedness. 
4. A-scan (amplitude scan) is a form of ultrasound sonography performed with a handheld probe attached to a machine. The display unit of the machine shows readings in the form of spikes. The participant will be asked to either maintain a sitting or supine position in case of contact technique; in the immersion technique, it can only be performed in a supine position. Normal saline is used in the immersion technique, as an interface between the cornea and the probe to reduce possible applanation.  
5. Although it is an easy and hassle-free test, do alert your examiner if you experience any discomfort during the examination.
What Are the Advantages of Ocular Biometry?

Ocular biometry is an advanced tool providing accurate measurements. It helps ophthalmologists to operate on patients with predetermined estimated anatomical measurements. Some of the advantages of ocular biometry are as follows:

Accurate and reliable data
Free of radiation hazard
Easy to perform in any settings

What Are the Limitations of Ocular Biometry?

Ocular biometry may need special considerations in some of the cases. Although it provides accurate data, there are several instances that result in the error output. Some of those instances are noted below:

  • Posterior Staphyloma
  • Inadequate patient fixation
  • High Hyperopia dense cataract
  • Macular and vitreous ailments
  • Pediatric biometry

The Takeaway

Ocular biometry is a diagnostic modality used for taking anatomical measurements of the eye including determination of the anterior curvature of the cornea by keratometry, followed by an A-scan ultrasound for taking accurate measurements prior to intraocular lens placement. 

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