In a new series of articles, I will discuss the basics of common ophthalmic instruments used for corneal and anterior segment measurements. The topic of the first installment is biometry or axial length (AL) measurement.
Accurate axial length determination is critical for successful cataract surgery. This is even more important now as patients expect perfect results. Fortunately, we are able to achieve more accurate and predictable outcomes. This is mostly due to improved technology: more accurate biometry, newer IOL calculation formulas, smaller wounds, attention to astigmatism correction, and better IOLs (aspheric, toric, and presbyopia-correcting). Although we can more frequently obtain a target refraction very close to predicted, post-operative refractive surprises still occur. The most common cause is an axial length error (0.1 mm error = 0.25-0.3 D surprise). A myopic surprise occurs if the AL measurement is too short, and a hyperopic surprise occurs if the measurement is too long. Therefore, accurate biometry is essential. The surgeon should suspect a problem and double check the AL if there is a difference of > 0.3mm between the eyes, or there is a difference of > 0.1mm among readings in the same eye.
Accurate keratometry readings are also important because each diopter of error in the average K value results in an equivalent error in predicted IOL power. The other parameters used in IOL calculations include A-constant of the IOL, anterior chamber depth, effective lens position, and IOL geometry. Even with accurate biometry and keratometry measurements, a surprise may occur in the setting of unusually high or low values for which most IOL formulas are less accurate. The exceptions to this are the Holladay II formula and the Haigis formula when surgeon optimized, both of which are accurate for all size eyes.
The two types of biometry are ultrasound (A-scan) and optical (IOLMaster). A-scan ultrasonography is performed via two methods: immersion (the gold standard, with accuracy = 0.0126-0.05 mm) or contact (more prone to error, with accuracy = 0.1 mm at best). The A-scan uses reflected sound waves to measure intraocular lengths. A sound beam passes through the eye, strikes various interfaces (media with different densities), and part is reflected back (echo). The “A” stands for amplitude, which is a measure of the reflectivity or strength of the echo (spike height) and depends upon the density of the media, incidence of the sound beam (perpendicularity), and gain. The gain refers to amplification of echoes, which is directly proportional to sensitivity, indirectly proportional to resolution, and adjustable on the machine.
The greater the density difference at an interface, the bigger the echo, and the taller the spike. The frequency of the sound beam is fixed (commonly 10 MHz) whereas the velocity depends on the density of the media (faster through solid: lens = 1641 m/sec, while aqueous/vitreous = 1532 m/sec). Length is calculated by multiplying the velocity of the beam by the time it takes for the beam to traverse the eye. A-scan devices utilize gates or caliper markers that indicate points of measurement. Gates are placed on the spikes to allow accurate length calculation. The best machines use the correct velocity through each segment of the eye (aqueous, lens, vitreous) rather than an average value for the whole eye. When the ultrasound beam is aligned correctly along the visual axis, 5 tall spikes are produced: cornea, anterior lens surface, posterior lens surface, retina, and sclera. These spikes should be equal in height, and the retina spike must rise steeply (i.e., be perpendicular to the baseline without any steps).
As far as technique, immersion vs. contact, the former is more accurate and actually faster to perform but it requires the use of a shell (i.e., Prager or Hansen) and water bath. The contact mode is more prone to errors from misalignment, corneal compression, and fluid meniscus. Common A-scan errors include:
- Misalignment — if the probe (e.g., sound beam) is not perpendicular to the lens (resulting in a short lens spike) or to the macula (resulting in a poor retina spike). Furthermore, misalignment also occurs if the probe is aligned along the optic nerve (resulting in no scleral spike).
- Gain too high — although this increases the sensitivity, it also produces poor resolution of spikes, causes the retina and sclera spikes to merge, combines the anterior and posterior corneal peaks, and cuts off the tops of all spikes (flat-topped rather than pointed spikes).
- Falsely short reading — this type of error may be caused by corneal compression (AL & ACD will both be 0.14-0.36 mm shorter than their true values), the beam not being perpendicular, vitreous opacity or membrane, choroidal thickening/effusion, wrong gate position, or incorrect velocity (too slow).
- Falsely long reading — such an error occurs from a fluid meniscus (between the probe and cornea), posterior staphyloma, measuring to the sclera instead of the retina spike, wrong gate position, or incorrect velocity (too fast).
- Incorrect velocity — if the eye is not phakic, it is important to change the setting so the machine uses the correct velocity for either an aphakic or a pseudophakic eye with the specified IOL material (alternatively for pseudophakes, the aphakic setting can be used with a correction factor). The aphakic scan has 1 less spike (lens spikes absent but spike from capsule or hyaloid face is present), whereas the pseudophakic eye shows multiple spikes in the vitreous due to reverberation artifact from the IOL. Phakic eyes filled with silicone oil also pose a potential for incorrect velocity errors. In this situation, accurate biometry requires using a silicone oil setting (980 or 1040 m/s depending on the type of silicone oil) or a velocity conversion equation for the vitreous portion ((Vc/Vm) x AVL = TVL) (where Vc = velocity (correct), Vm = velocity (measured), AVL = apparent vitreous length, TVL = true vitreous length).
Optical biometry is similar to A-scan but uses reflected light rather than sound to measure axial length. The IOLMaster
(Carl Zeiss Meditec) is such a device and has accuracy equivalent to immersion A-scan (0.02 mm), and may in fact be better since it uses a narrower beam and measures to the RPE rather than the ILM. In addition to AL measurements, the IOLMaster also provides keratometry, anterior chamber depth, white-to-white, and IOL calculations.
The AL display is quite different from the A-scan readout. Rather than 5 spikes, the IOLMaster produces a primary maxima (which appears as a narrow, well-defined, centered peak identified by a circle above it), secondary maxima (which appear as discrete lower peaks, sometimes disappearing into the baseline), and a baseline (which is low and even, but may become high and uneven with decreasing signal-to-noise ratio (SNR)). The SNR is a measure of accuracy and decreases with increasing cataract density. SNR > 2.0 is valid and good if repeatable, SNR between 1.6-2.0 is borderline but usable if repeatable, and SNR < 1.6 is not usable. However, a proper waveform is more important than the SNR value. Occasionally, a measurement has a triple peak configuration. These correspond to the ILM (150-350 μm before the RPE), RPE (middle peak), and choroid (150-250 μm behind the RPE). The secondary maxima should be 800 μm behind the RPE peak.
Measurement errors can occur, such as a falsely short reading from the ILM (or IOL reflections in a pseudophakic eye), a falsely long reading from the choroid, and a poor reading. Limitations of this technology that result in poor readings include movement (patient with tremor), dense anterior segment opacity (i.e., mature cataract, PCO, corneal scar), vitreous hemorrhage, condensed posterior hyaloids face, and RPE atrophy. If possible, measurements should be attempted around an opacity.
With the newest software, very rarely is a cataract so dense that an accurate AL measurement cannot be obtained with the IOLMaster. In addition, other advantages of this technology are: it is noncontact, no water bath is needed, it can measure through glasses, it is accurate for silicone oil and posterior staphyloma, and of course other measurements (keratometry, anterior chamber depth, white-to-white, and IOL calculations (Haigis, Hoffer Q, Holladay, SRK II, SRK/T)) can all be performed at a touch of a button.
In summary, precise and accurate preoperative measurements are critical in achieving successful results. If you are finding variability and surprises in your post-op target refractions, and you are not using an immersion A-scan or IOLMaster instrument, you should seriously consider switching to one of these technologies. Not only will your patients be happier, so will you. Furthermore, if you are planning on using premium IOLs in the future, it is imperative to be spot on with your lens calculations, and the IOLMaster is an invaluable tool in accomplishing this goal. Personally, over the past decade, my practice has used contact A-scan, then immersion A-scan, and now IOLMaster, and with each change my results have improved significantly.
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