Ocular Biometry

Chapter 7 Ocular Biometry



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Introduction


Cataract surgery and intraocular lens (IOL) implantation are currently evolving into a refractive procedure. The precision of biometry is crucial for meeting expectations of patients undergoing cataract surgery.1 Moreover, the optimal results for new IOLs being developed, such as toric, multifocal, accommodative, and aspheric, all depend on the accuracy of biometry measurements. To meet these expectations, attention to accurate biometry measurements, particularly axial length (AL), is critical.2 The fundamental points for accurate biometry include the AL measurements, corneal power calculation, IOL position (effective lens position [ELP]), the selection of the most appropriate formula, and its clinical application.


The measurement of axial eye length is one of the most important steps for IOL lens power calculation. An error in AL measurement of 1 mm can cause an error in IOL power of 2.5D (approximately). AL can be measured with a laser interferometer based system (IOL Master®) or with an ultrasound based system.



IOL Master®



Instrumentation and methods


Non-contact partial coherence laser interferometry (Zeiss IOL Master®, Carl Zeiss AG, Oberkochen, Germany) is used routinely by ophthalmologists worldwide to estimate IOL power before cataract surgery. It was developed to increase the accuracy of biometry measurements and has been shown to be more accurate and reproducible than ultrasound, using contact techniques.3 It is a non-contact and operator-independent method that emits an infrared beam, which is reflected back from the retinal pigment epithelium. The patient is asked to fixate on an internal light source to ensure coaxial alignment with the fovea. The reflected light beam is captured and the AL is calculated by the interferometer.


Because optical coherence biometry uses a partially coherent light source of a much shorter wavelength than ultrasound, AL can be more accurately obtained (reproducible accuracy of 0.01 mm).4 The IOL Master® also provides measurements of corneal power and anterior chamber depth, enabling the device to perform IOL calculations using newer generation IOL calculation formulas.4 As the patient must look directly at a small red fixation light during measurements with the IOLMaster®, AL measurements will be made to the center of the macula giving the refractive AL rather than the anatomic AL. For eyes with extreme myopia or posterior staphyloma, being able to measure to the fovea with the IOL Master® is an enormous advantage over conventional A-scan ultrasonography.5







Biometric A-scan ultrasound


An A-scan is widely used for biometric calculations.image See Clip 7.1 It should be remembered that ultrasonic AL measurement is actually determined by calculation. The ultrasonic biometer measures the transit time of the ultrasound pulse and, using estimated velocities through the various media (cornea, aqueous, lens, and vitreous), calculates the distance.1 In some cases the precision of the measurements can be optimized by use of B-scan, so we will discuss some of those clinical scenarios. Clinical decisions can be made during dynamic examinations. A-scan biometry includes two main techniques: contact method and immersion technique.



Contact


In the contact (applanation) method, the ultrasound probe directly touches the cornea. The contact technique is completely examiner dependent because it requires direct contact and anterior compression of the cornea.image See Clip 7.2 Previous studies have demonstrated a mean shortening of AL by 0.1 to 0.33 mm using the contact technique compared with immersion technique.710 In the echogram for the axial eye length measurement, the first spike represents the probe tip placed on the cornea, followed by the anterior lens capsule, posterior lens capsule, vitreous cavity, retina, sclera, and orbital tissue echoes (Figure 7.1). The corneal spike is a double-peaked echo to represent the anterior and posterior surfaces of the cornea. The retinal spike is generated from the anterior surface of the retina. This echo needs to be highly reflective with a sharp 90° take-off from the baseline. The scleral spike is another highly reflective spike just posterior to the retinal spike. The orbital spikes are low reflective spikes behind the scleral spike.





Settings


Most instruments offer the choice of either a manual or automatic (“pattern recognition”) measurement mode. To obtain an AL measurement in manual mode, the examiner determines the A-scan to be measured and depresses a foot pedal to take the measurement. This is the preferred methodology because the examiner can examine the spikes and make sure that they are properly aligned and appropriately gated. In the automatic mode, the machine is programmed to recognize spikes that occur within a preset range from the probe. When the display has a series of spikes which the software recognizes, the instrument will record that measurement. When the automatic mode is in use, the instrument is prone to making errors and giving inadequate measurements. There is also an option of a contact or immersion measurement mode. Make sure this setting is correct, otherwise your measurements will be erroneous. Biometers also have phakic, pseudophakic, and aphakic settings that can be chosen based on the lens status of the patient. Some instruments also have a dense cataract setting. The dense cataract setting is used when the examiner is having difficulty displaying a distinct, high spike from the posterior lens capsule and the retina.11





Troubleshooting


Errors in an AL measurement are due to improper technique yielding shorter or longer measurements.15,16 One of the commonest errors is misalignment of the ultrasound probe with the visual axis or macular surface. When the retinal, lenticular or corneal spikes are of high amplitude and steeply rising without sloping or spikes, the ultrasound beam is most likely on axis. The scleral echo should easily be identified and the orbital fat echoes should descend quickly and at a steep angle. The retinal spike will be present and of high amplitude and can even appear steeply rising, but, if the scleral spike is not as high in amplitude as the retina, the sound beam is misaligned along the nerve. No sclera is present at the optic nerve: If there are no scleral or orbital fat echoes visible, the ultrasound beam is most likely aligned with the optic nerve rather than the macula.17image See Clip 7.4 Biometry units that are not equipped with an oscilloscope or a screen that displays the actual scan have a high error rate and are definitely not recommended.14




Measuring specific conditions – challenging eyes




Pseudophakic


IOLs occasionally need to be exchanged after cataract surgery because of surgical complications or postoperative refractive surprise.18 Patients who had received older-generation IOLs might request IOL exchange for restoration of more visual function by, for example, correction of presbyopia, astigmatism, glare, etc. Indication for IOL exchange aiming to reduce residual refractive errors increased from 13.9% in the late 1980s to 30–40% in the early 2000s.18 Ocular biometry in pseudophakic eyes is thus even more important than previously expected.6 During the measurement of pseudophakic eyes, the first spike represents the lens implant, followed by multiple signals. IOL implantation causes multiple echoes within the vitreous cavity (Figure 7.3). The first spike (IOL echo) should also be aligned along the visual axis and should be of maximum height. Adjustments should be made according to the ultrasonic velocity of the IOL material. Nevertheless, the identification of retinal spikes can be difficult in some cases because of the proximity of the multiple echoes to the retina spike. In these cases, the examiner should decrease the gain for better identification of the retina spike. Holladay and Prager described a conversion factor to improve the accuracy of the AL measurements in pseudophakic eyes.19 They considered the implant composition, the center thickness, and the amount of vitreous and aqueous crossed by the ultrasonic beam. The conversion factor was obtained by multiplying the center thickness of the IOL by a factor related to the implant’s ultrasonic velocity.





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Mar 5, 2016 | Posted by in ULTRASONOGRAPHY | Comments Off on Ocular Biometry
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