You are in: eMedicine Specialties > Clinical Procedures > Radiology A-Scan BiometryArticle Last Updated: Jan 8, 2007AUTHOR AND EDITOR INFORMATIONSection 1 of 11
 
Author: Rhonda G Waldron, MMSc, COMT, CRA, ROUB, RDMS, Senior Associate in Ophthalmology, Diagnostic Echographer, Senior Technologist, Department of Ophthalmology, Emory University School of Medicine Rhonda G Waldron is a member of the following medical societies: Association for Technical Personnel in Ophthalmology, International Society for Ophthalmic Ultrasound, and Ophthalmic Photographer's Society Coauthor(s): Thomas M Aaberg, Jr, MD, Clinical Assistant Professor, Department of Surgery, Michigan State University College of Human Medicine; Consulting Staff, Department of Ophthalmology, Associated Retinal Consultants Editors: Richard W Allinson, MD, Associate Professor, Department of Ophthalmology, Texas A&M University Health Science Center, Scott and White Clinic; Simon K Law, MD, PharmD, Assistant Professor of Ophthalmology, Jules Stein Eye Institute; Chief of Section of Ophthalmology Surgical Services, Department of Veterans Affairs Healthcare Center, West Los Angeles; J James Rowsey, MD, Former Director of Corneal Services, St Luke's Cataract and Laser Institute, Florida; Lance L Brown, OD, MD, Ophthalmologist, Affiliated With Freeman Hospital and St John's Hospital, Regional Eye Center, Joplin, Missouri; Hampton Roy Sr, MD, Associate Clinical Professor, Department of Ophthalmology, University of Arkansas for Medical Sciences Author and Editor Disclosure Synonyms and related keywords: A-scan ultrasound, intraocular lens calculations, IOL calculations, axial length measurements ULTRASOUND PRINCIPLESSection 2 of 11
 
Sound is defined as a vibratory disturbance within a solid or liquid that travels in a wave pattern. When the sound frequency is between 20 hertz (Hz) and 20,000 Hz, the sound is audible to the human ear. To be considered ultrasound, sound waves must have a frequency of greater than 20,000 Hz (20 KHz), rendering them too high in frequency to be audible to the human ear. In ophthalmology, most A-scan and B-scan ultrasound probes use a frequency of approximately 10 million Hz (10 MHz) that is predesigned by the manufacturer. This extremely high frequency allows for not only restricted depth of penetration of the sound into the body but also excellent resolution of small structures. This meets unique needs, because, at times, the probe is placed directly on the organ to be examined, and its structures are quite small, requiring excellent resolution. The velocity of sound is determined completely by the density of the medium through which it passes. Sound travels faster through solids than through liquids, an important principle to understand because the eye is composed of both. In A-scan biometry, the sound travels through the solid cornea, the liquid aqueous, the solid lens, the liquid vitreous, the solid retina, choroid, sclera, and then orbital tissue; therefore, it continually changes velocity. The known sound velocity through the cornea and the lens (average lens velocity for the cataract age group, ie, approximately 50-65 y) is 1641 meters/second (m/s), and the velocity through the aqueous and vitreous is 1532 m/s. The average sound velocity through the phakic eye is 1550 m/s. The sound velocity through the aphakic eye is 1532 m/s, and the velocity through the pseudophakic eye is 1532 m/s plus the correction factor for the intraocular lens (IOL) material. The cornea is not routinely factored in because of its thinness. If one were to consider 1641 m/s at about 0.5 mm, only 0.04 mm would need to be added to the total eye length, which in no way alters the IOL calculation. In A-scan biometry, one thin, parallel sound beam is emitted from the probe tip at its given frequency of approximately 10 MHz, with an echo bouncing back into the probe tip as the sound beam strikes each interface. An interface is the junction between any two media of different densities and velocities, which, in the eye, include the anterior corneal surface, the aqueous/anterior lens surface, the posterior lens capsule/anterior vitreous, the posterior vitreous/retinal surface, and the choroid/anterior scleral surface. The echoes received back into the probe from each of these interfaces are converted by the biometer to spikes arising from baseline (see Image 1). The greater the difference in the two media at each interface, the stronger the echo and the higher the spike. If the difference at an interface is not great, the echo is weak and the displayed spike is short (eg, vitreous floaters, posterior vitreous detachments). No echoes are produced if the sound travels through media of identical densities and velocities, eg, young, normal vitreous or the nucleus of a noncataractous lens, in which the A-scan display goes down to baseline. Spike height is not only affected by the difference in density as it travels through the eye but also by the angle of incidence, which is determined by the probe orientation along the visual axis. If the probe is held in a perpendicular manner to the visual axis, it is in the proper position to receive the echoes back into the probe tip so they can be converted to spikes. Because sound waves can be reflected and refracted the same as light rays, if the probe is held in a nonperpendicular manner, part of the echo is diverted at an angle away from the probe tip, and, therefore, is not received by the machine. The more nonperpendicular the angle of incidence, the weaker the signal and the shorter the spike amplitude (see Image 2). The shape and smoothness of each interface also affects spike quality. An irregularity in the surface of an interface causes reflection and refraction of the returning sound waves away from the probe tip and, therefore, weaker echoes. That is why it is important to know whether macular pathology is present that could adversely affect spike quality. A perfect high, steeply rising retinal spike may be impossible when macular pathology is present (eg, macular edema, macular degeneration, epiretinal membranes, posterior staphylomas). See Image 3. In addition, sound is absorbed by everything through which it passes before it travels on to the next interface. The greater the density of the structure it is passing through, the greater the amount of absorption. This principle explains why retinal spike quality is reduced in the case of an extremely dense cataract; the lens absorbs much of the sound and less sound actually reaches the retinal surface. ULTRASOUND BIOMETRY INSTRUMENTATIONSection 3 of 11
Ultrasound biometers use a pulse system, pulsing electricity to the probe tip where a crystal element vibrates and emits the sound beam at its given frequency. Then, a pause of a few microseconds occurs, so the returning echoes can be received by the probe tip and converted to spikes on the display. The gain setting on biometers is measured in decibels and affects amplification and resolution of the displayed spikes. When on highest gain, the spike height and the sensitivity of the display screen are maximized, enabling visualization of the weaker signals, but the resolution is affected adversely. When gain is lowered, the spike amplitude and the display sensitivity are decreased, which eliminates the weaker signals but improves the resolution. Resolution is defined as the ability of the machine to display two interfaces that lie in close proximity, one directly behind the other, as separate echoes or spikes (eg, retinal and scleral interfaces). When the gain is too high, the retina and sclera appear as one thickened spike with a wide, flattened peak (see Image 4). The examiner should reduce the gain until the retinal and scleral surfaces are seen as separate spikes to the right of the display. The density of the cataract determines the need for changing the gain setting due to absorption of the sound. The more dense the cataract, the higher the necessary gain. Patients who are aphakic require less gain to prevent merging of the retinal and scleral spikes. Therefore, the gain setting may vary not only from patient to patient but from one eye to the next in the same patient, depending on cataract density. Gates are electronic calipers on the display screen that measure between two points. Biometers are designed so that between each pair of gates a measurement is rendered (see Image 5). Biometers vary in the appearance of these gates, with some units not displaying them at all. Gates should be readily visible for accurate editing of the scans, because if any one of them is aligned along an incorrect spike the entire eye length measurement will be erroneous. The biometer automatically places a gate on what it believes to be the corneal spike, the anterior lens spike, the posterior lens spike, and the retinal spike, and it is programmed to measure the distance between each pair of gates at a given velocity. Ultrasound is measured based on how long it takes the sound to travel from one point to the next at a given velocity. The formula, distance = velocity X time, is programmed into biometers to calculate the distance between each pair of gates. Then, the formula is divided by 2 because the sound also must echo back into the probe tip. By selecting the eye type in the measurement mode (phakic, aphakic, or pseudophakic), the equipment is instructed to use this distance formula with the proper velocities between each gate pair for that particular eye type. For example, in the phakic mode, the machine has been programmed to measure distance between the first pair of gates using a velocity of 1532 m/s, the velocity through the anterior chamber. The velocity of 1641 m/s is used between the second and third gates because this is the velocity through the lens. The velocity of 1532 m/s is once again used in the formula between the third and fourth gates because this is the velocity through the vitreous cavity. The most accurate machines measure each of these 3 sections of the eye individually at the proper sound velocity and then add them together for the total eye length. If any one gate is incorrectly placed, the machine will calculate the 2 sections involved using the incorrect velocity and time, which will make the total length erroneous. Proper gate placement is on the ascending edge of each appropriate spike. If a gate has been placed incorrectly, move it to the appropriate spike before it is stored and used in the calculation. Equipment varies greatly in the ability to see and move gates; refer to the manual for each specific unit to determine if it is possible to move the gates, and, if so, the steps required to do so. If the biometer does not allow for movement of gates, scans must be repeated until they automatically align properly. When the machine is set for phakic average, only 2 gates are present, measuring the total eye at its average velocity of 1550 m/s. The 2 gates should align along the corneal surface and the retinal surface, respectively. The disadvantages to this setting are that the anterior chamber depth and the lens thickness cannot be monitored and that the use of average sound velocity is simply not as accurate. The average sound velocity of 1550 m/s is only accurate through an average length eye. In eyes that are shorter or longer than average, this method of measuring produces an innate error. When setting the measurement mode to aphakic, 2 gates will be present (on the respective corneal and retinal surfaces), and the biometer will calculate the distance at a velocity of 1532 m/s, the correct velocity for the aqueous and vitreous. When setting the measurement mode to pseudophakic, depending on how many pseudophakic options the equipment possesses, the eye length is calculated using 1532 m/s for the aqueous and vitreous, then the correction factor for the given implant material is added. If only one pseudophakic mode option is available, it only will be accurate for polymethyl methacrylate (PMMA) IOLs. Routinely using automatic mode on most equipment increases the risk of error because every biometer will capture poor quality scans. Biometers are programmed to capture any scans with spikes that are of high amplitude within their given area. However, they often cannot determine if the spike arose steeply from baseline or if a slope or step is present in the spike origin. Manual mode is sometimes preferable, in which the examiner presses a foot switch to capture the scan when it is seen to be of high quality. Equipment varies greatly with some manufacturers only using a 4-gate system on automatic mode, which means that anterior chamber depth can be monitored only in automatic mode. If this is the case, automatic mode is preferable, but the examiner must carefully edit the scans stored by the machine. ACCURACY AND STANDARD DIMENSIONSSection 4 of 11
It is critical that the examiner use methods that are standard of care in performing biometry. A 0.1 mm error will result in a 0.25-0.3 diopter (D) postoperative refractive error in an average or long eye. Therefore, an error of 0.5 mm will result in approximately 1.25 D refractive error, and an error of 1.0 mm will result in approximately 2.5-3.0 D postoperative refractive error. If the eye is small, even more postoperative error occurs from inaccurate measurements, as much as 0.75 D per 0.1 mm. If the error is in measuring the eye erroneously small, such as is common from corneal compression, the postoperative refractive error will be in the myopic direction. Conversely, if the examiner measures the eye erroneously long, which is common when the sound beam is not perpendicular to the retinal surface, the postoperative refractive error will be in the hyperopic direction. A good biometrist must be able to recognize when readings appear abnormal; therefore, one must first know standard dimensions of the eye. The average axial eye length is 23.5 mm, with a range of 22.0-24.5 mm. In general, the smaller the eye, the more hyperopic the refractive error. The longer the eye, the more myopic the refractive error. Of note, a patient can be myopic because of steep corneal curvature rather than long axial length, and a patient can be hyperopic because of flat corneal curvature rather than short axial length. The average anterior chamber depth is 3.24 mm but varies greatly. If the biometrist is documenting a shallow anterior chamber depth, examine the medical chart for clinical correlation of this finding. The average lens thickness is 4.63 mm but this also varies, and, with cataractous changes, the lens will increase in thickness to as much as 7.0 mm in extremely dense cases. The average keratometry (K) reading is 43.0-44.0 D, with one eye typically within a diopter of each other. Check these readings against the refractive error of the patient for accuracy. If one eye is found to differ from the other by more than 1 D, immediately begin researching the cause and alert the physician. For instance, if the patient had refractive surgery, corneal transplantation, an injury with a resultant corneal scar, or has keratoconus, the K readings may vary between the eyes. It is rare for the patient to have disparate K readings biologically. If any of the above eye measurements is found to be unusual, another technician should recheck the measurements and immediately alert the physician. Just as precise keratometry and biometry are critical for good surgical outcomes, correct IOL placement by the surgeon is essential. A 0.19 D postoperative refractive error occurs for every 0.1 mm posterior chamber intraocular lens (PCIOL) displacement. A 0.12 D postoperative refractive error occurs for every 0.1 mm anterior chamber intraocular lens (ACIOL) displacement. Lens displacement can also be caused by the patient's ciliary body pushing the lens out of position rather than by the surgeon's placement of the lens. Using the correct IOL calculation formula is extremely important in good surgical outcomes. Third- and fourth-generation formulas are the most accurate; these formulas include the Hoffer Q, SRK/T, Holladay I, Haigis, and Holladay II IOL Consultant formulas. For average length eyes with average K's, these formulas give almost identical calculations. However, when the eye is small, formula selection is more critical. In eyes that are less than 22.0 mm in length, the Hoffer Q and the Holladay II IOL Consultant formulas are the most accurate. For long eyes, the SRK/T and the Holladay II IOL Consultant formulas are the most accurate. The Holladay II IOL Consultant formula is considered by most experts to be the best no matter the axial length. The Holladay II IOL Consultant formula is also the only formula that calculates for piggyback IOL procedures (ie, when 2 IOLs are implanted, which may be necessary when the eye is so small that 1 implant does not contain enough converging power, or when a piggyback lens is inserted to correct a postoperative surprise that results from the first implant). The Haigis IOL Consultant formula is also proving to be accurate no matter the axial length but only if optimized, which requires 200-300 eyes, preferably. CONTACT AND IMMERSION TECHNIQUESSection 5 of 11
Historically, the contact (or applanation method) of biometry was accomplished by gently placing the probe on the corneal vertex and directing the sound beam through the visual axis. This handheld method was most easily and accurately performed with the patient in a reclined position with the patient's head placed in front of the display screen of the biometer. The examiner was seated on an adjustable stool to the other side of the patient, resting his or her arm on the patient's shoulder and the side of his or her hand on the patient's cheek. The patient was instructed to look at a target affixed to the ceiling. Using a gentle on-and-off technique allowed for less corneal compression since the examiner's hand was braced more firmly. It was also easier for the patient to brace the head against the headrest in this reclined position and for the examiner to simultaneously monitor both the display screen and the patient's fixation. Because of the indentation of the globe caused by the contact (or applanation method) of biometry as well as the current patient expectations, this method is being completely abandoned in favor of more accurate noncontact instrumentation. The immersion technique of biometry is accomplished by placing a small scleral shell between the patient's lids, filling it with saline, and immersing the probe into the fluid, being careful to avoid contact with the cornea. This method is more accurate than the contact method because corneal compression is avoided. Eyes measured with the immersion method are, on average, 0.1-0.3 mm longer than when measured by the contact method since no indentation of the globe occurs. The display screen will exhibit 6 (rather than 5) spikes in the phakic patient because the probe and the cornea are no longer in contact with each other, thus appearing separate. Of note, some machines have the probe spike shifted so far to the left in the immersion mode that it does not appear on the display screen; therefore, the corneal spike will be seen first. The correct axial pattern past the probe spike consists of 5 tall spikes that represent the cornea, the anterior lens, the posterior lens, the retina, and the sclera. Perpendicularity is achieved when all spikes are of high amplitude and the retinal spike is steeply rising from the baseline. Another advantage of the immersion technique is that the corneal spike will have two peaks corresponding to the epithelium and endothelium. If both of these peaks are not equally high, the sound beam is not directed through the corneal vertex and, therefore, is not aligned along the visual axis (see Image 6). Care should be taken to keep the gain low enough to appreciate and resolve these two peaks. If the gain is set too high, poor resolution of these two interfaces will occur and the corneal peak will appear wide and flattened. Other advantages of the immersion technique are that it is a faster method than the contact technique and that it reduces technician dependency. When using the contact technique, axial lengths will vary on subsequent scans by the same technician, as well as between technicians depending on the amount of corneal compression. When using the immersion technique, as long as the spikes are of high quality, the axial lengths will not vary from one scan to the next or from one biometrist to the next. Any practice that changes from the contact technique to the immersion technique must repersonalize their IOL constants in their IOL calculations, since they are achieving true rather than slightly shortened eye length measurements. A study should be completed of at least 20 eyes with the new technique, the same surgeon, and the same IOL to determine the correct personalization of the constant. Another current method for highly accurate axial length measurements uses optical coherent light rather than ultrasound. In this method, optical coherent light passes through the visual axis and reflects back from the retinal pigment epithelium. Interferometry is then used to measure the distance from the cornea to the retina. Because this is a noncontact method, as is immersion ultrasound, its accuracy is superior to contact ultrasound biometry. However, this noncontact method cannot be used in the event of media opacity (eg, dense cataracts), corneal or vitreal opacity due to absorption of the light, or an inability of the patient to fixate on the target. Optical coherence cannot measure anterior chamber depth or lens thickness; however, a slit beam is in place for anterior chamber depth measurements. For highly accurate anterior chamber depth and lens thickness measurements, immersion ultrasound should be used. For those practices using optical coherence, immersion ultrasound will be necessary for patients who cannot be measured by optical coherence to ensure the same high level of accuracy. BIOMETRY THROUGH VARIOUS INTRAOCULAR LENS MATERIALSSection 6 of 11
Biometry through an already pseudophakic eye most commonly is performed to compare to the fellow phakic eye for accuracy. Other reasons to measure the pseudophakic eye include patients who are scheduled to undergo an IOL exchange or checking an unwanted postoperative refractive error. Measurement through the pseudophakic eye will result in multiple reverberation echoes in the vitreous cavity that tend to decrease in amplitude from left to right. The number and strength of these reverberations is dependent on the IOL material (see Image 7). Decreasing the gain in the pseudophakic eye is helpful, so that spike amplification of these artifacts is reduced, reinforcing correct gate placement on the retinal spike. To obtain accurate measurements through the pseudophakic eye, knowledge of the implant material is essential. Most IOLs are currently made of PMMA, acrylic, or silicone. The velocity of sound through each of these materials is different because of their differing densities and if measured with the wrong modality can result in significant error. If an eye with an acrylic IOL is measured on pseudophakic PMMA mode, a 0.2 mm error will occur. If an eye with some silicone IOLs is measured on PMMA mode, a 1.2 mm error will occur. The velocity of sound through the pseudophakic eye is 1532 m/s plus the correction factor for the implant material. The velocity through PMMA is 2718 m/s, through acrylic is 2120 m/s, and through silicone is 980-1107 m/s, depending on the silicone used. (Because acrylic and silicone lenses are foldable, they are not as dense as PMMA and, therefore, have a slower velocity.) If a biometer only has one pseudophakic setting, it will be accurate for PMMA only because it was manufactured when PMMA was the only implant material being used, and some biometers do not have upgrades for the various IOL materials. If this is the case, the way to achieve accurate measurements is to use the aphakic setting, which uses a sound velocity of 1532 m/s. Then, the examiner should manually add the correction factor for the IOL material to the results obtained on aphakic mode. The correction factor is +0.4 mm for PMMA, +0.2 mm for acrylic, and -0.4 mm to -0.8 mm for silicone, depending on the silicone velocity. Therefore, if an eye measured 23.32 mm on aphakic mode and the IOL is made of PMMA, the correct axial length is 23.72 mm. If the IOL is acrylic, the correct axial length is 23.52 mm. If it is low-velocity silicone, the correct length is 22.52 mm. When any new implant material is produced, the correction factor can be calculated using the CT of the IOL and the sound velocity of the material at body temperature (35°C), which must be supplied by the manufacturer. The formula for this calculation is the CT multiplied by 1 minus 1532 divided by the velocity of that material, or CT X (1-1532/vel). For example, if the IOL has a CT of 0.8 mm, and the sound velocity of the material is found to be 1040 m/s, then 0.8 X (1-1532/1040) = 0.8 X -0.473 = -0.378. Therefore, the correction factor for this eye is -0.378 from the length obtained on aphakic setting. Another problem arises when the implant material is unknown. If the patient has a wallet card showing the implant used, the manufacturer may need to be called to determine implant material if the model is unfamiliar to the examiner. If the patient does not have a wallet card, contact the surgeon's office to determine the implant used. If the patient cannot recall the surgeon's name, it may be necessary to contact a family member in the case of an IOL exchange. However, the implant reverberation pattern may prove helpful because PMMA has a longer chain of reverberation echoes, followed by acrylic and then silicone. COMMON ERRORS AND CHALLENGING SITUATIONSSection 7 of 11
A problem should be suspected if a difference between the 2 eyes of more than 0.3 mm is present or if a difference in consecutive measurements occurs on the same eye of more than 0.1 mm. In these instances, consult the patient's history to see if a medical reason exists for a difference or macular pathology that could explain the variation on the same eye (eg, posterior staphyloma). The most common error in the contact technique is corneal compression. This inevitably occurs because the eye is pliable and the cornea is indented even with minimal pressure from the probe tip. The lower the intraocular pressure the softer the eye and the more significant the corneal compression. Therefore, the amount of compression can vary even with the same technician. If the contact technique must be used, the anterior chamber depth must be monitored and the shallower anterior chamber depths deleted even if the spikes appear to be of high quality (see Image 8). Of course, the immersion method completely avoids corneal compression, which is why the contact method is becoming obsolete. The second most common error is misalignment, either by not obtaining perpendicularity to the macular surface or by not directing the sound beam through the visual axis. Perpendicularity to the macular surface is achieved when the retinal spike and scleral spike are of high amplitude, and the retinal spike arises steeply from baseline. No sloping of the retinal spike should be present and no jags, humps, or steps should be present on the ascending edge of that spike (see Image 9). If either the posterior or anterior lens spike are not of high amplitude, the sound beam could be misaligned at an angle through the lens and, therefore, not through the visual axis (see Image 10). The posterior lens spike may be slightly shorter than the anterior lens spike because the convex curvature of the posterior lens is steeper than the convex curvature of the anterior lens surface, allowing for reflection of the echoes away from the probe tip. Also, if a dense nuclear sclerotic cataract is present, more sound absorption could occur within the lens, causing the posterior lens spike to be shorter. In these instances, the gain can be increased to obtain better posterior lens and retinal/scleral spikes. Misalignment along the optic nerve is an error that is easily recognized, since the scleral spike will be absent (see Image 11). 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; the sound beam is passing through the nerve cord with only short amplitude echoes present, because the sound beam is striking blood vessels within the nerve cord. In the normal eye, there will generally not be a great difference in axial length when aligned along the optic nerve, but, in cases of a full optic disc, papilledema, or optic disc drusen, this will result in an erroneously short axial length measurement. In cases of optic nerve cupping, as seen in glaucomatous eyes, this error will result in an erroneously long axial length measurement. Another possible error in the contact method is a fluid meniscus between the probe tip and the cornea caused from ointment use, methylcellulose in the eye from previous examinations, or abnormally thick tear film. If either of these is suspected, rinse the eye with sterile saline prior to biometry. Extremely dense cataracts can be a challenge because of absorption of the sound beam as it passes through the lens. A higher gain setting may be necessary to achieve high-amplitude spikes from the retina and sclera. Improper gate placement also can occur easily, because a dense cataract produces multiple spikes within the lens. The posterior lens gate may erroneously align along one of the echoes within the lens nucleus, resulting in an erroneously thin lens thickness and erroneously long vitreous length, which results in an error in the total length of the eye. In this case, manually realign the gate to the correct posterior lens spike, and if the equipment does not allow for manual gate placement, repeat scans until the gates automatically align properly. Posterior staphylomas are among the greatest biometry challenges. These occur primarily in high myopes, where the globes are so elongated and thin that the posterior uvea bulges outward into the sclera, most commonly in the posterior pole. This causes the macula to be sloped in configuration, which in turn causes reflection of the sound beam away from the probe tip and a poor retinal spike. It is impossible to obtain perpendicularity to a macular surface that is sloped; thus, it is impossible to obtain a proper retinal spike. Also because of the sloped surface, the measurements will be not only long but extremely variable. In these cases, a B-scan examination is necessary, with a horizontal macular scan performed and the vitreous length measured from the B-scan. The proper B-scan probe position for this measurement is to have the patient in primary gaze with the B-scan probe face centered on the corneal vertex and the probe marker aimed nasally. (The probe marker is either a line or a dot on one side of the probe, near the probe face.) When this probe position is achieved, the B-scan display will demonstrate the posterior lens curve centered to the left, and the optic nerve void just above the center to the right. The macula will lie centered on the right, about 4.5 mm below the center of the optic disc. Then, place calipers on the vertex of the posterior lens surface and on the macula to measure the vitreous cavity at its sound velocity of 1532 m/s. Compare this vitreous length measurement to the various biometry measurements, and use the measurement that has the most comparable vitreous length in the IOL calculation (see Image 12). Optical coherence biometry has been shown to be beneficial in these cases, but again, it will not be usable if the patient has a dense cataract or other media opacity or the inability to fixate. Patients must be alerted that because their eye is misshapen, they have a higher risk of the postoperative result not being as accurate as a patient with a normally shaped, round globe. Known macular retinal detachments are yet another challenging situation. In retina practices, the physician may decide to remove the cataract while the patient is undergoing retinal detachment repair and will need accurate IOL calculations. In these cases, the retinal spike will appear further to the left in the vitreous cavity, depending on the elevation of the macula. In these instances, the retinal gate should be moved from the detached retina to the next more posterior spike, since the retina should lie back into this position once repaired. The examiner should inquire if the surgeon plans to place a scleral buckle around the globe to repair the detachment, and if so manually add another 0.5-1.0 mm to the total eye length to account for lengthening of the globe by the buckle postoperatively. IOL CALCULATIONS FOLLOWING REFRACTIVE SURGERYSection 8 of 11
When cataract surgery is performed on a patient who has had a previous refractive procedure, accurate keratometry readings are not obtainable by standard manual or automated keratometers. Because refractive procedures flatten the central cornea, keratometer mires are spread over a larger area and now measure approximately the central 4.5 mm of the cornea rather than the central 3.0 mm. Therefore, use of standard corneal measurements for IOL calculations results in hyperopic postoperative refractive errors. Topography also is not accurate for postrefractive corneal curvature measurements. While this section is in no way a comprehensive listing of the methods suggested by the experts for calculating corneal curvature following refractive surgery, the more common methods will be described. One of the most accurate methods for determining corneal curvature after refractive surgery is the history method. The data needed in this method include the spherical equivalent refractive error prior to refractive surgery, the average keratometry reading prior to the refractive procedure, and the spherical equivalent refractive error postrefractive procedure. (The examiner should use caution not to use the current refractive error as the patient may have a myopic shift due to the presence of the cataract.) The formula for this method for patients who are myopic and undergoing refractive surgery is as follows: Because any change in refractive error from a refractive procedure is due to corneal flattening, the change in refractive error from the refractive procedure subtracted from the average K prior to the procedure is the current central corneal curvature. For example, if a patient had an average K of 45.75 D and a refractive error of -8.25 D prior to refractive surgery with a plano postoperative result, that is a change of 8.25 D of corneal flattening. The K reading to use in the IOL calculation is 37.5 D (ie, 45.75 D - 8.25 D = 37.5 D). The examiner should not be concerned with the resultant low corneal curvatures. As high myopes, many patients underwent extensive corneal flattening from the refractive procedure. If the patient underwent hyperopic refractive surgery, the history method can still be used; however, rather than subtracting the amount of correction, it should be added because the cornea was in effect steepened rather than flattened by the procedure. If the patient's previous medical records and measurements are unobtainable, the contact lens method of determining corneal curvature should be used. This method can also be used for comparison. The formula for this method is, as follows: First, refract the patient without a contact lens and note the spherical equivalent. Then, place a hard contact lens (PMMA) of known base curve and plano power on the eye, and refract the patient again. The difference in refractive error is algebraically added to the known base curve of the contact lens. For example, if a patient refracts to a -1.5 D without a contact lens, and a plano contact lens with a base curve of 40.0 D is placed on the eye and the patient now refracts to a -4.25 D, that is a difference in refraction of 3.75 D in the minus direction. The formula K = 40.0 D + (-3.75) = 36.25 D is used as the K reading in the IOL calculation. Care must be taken to note the direction of the change in refraction. For example, if another patient refracts to a -2.0 D without a contact lens and then +0.25 D with the same 40.0 D contact lens, the difference in refractive error is 2.25 D in the plus direction, so K = 40.0 D + (+2.25) = 42.25 D to be used in the calculation. Regardless of the base curve of the contact lens used for the refraction, this formula results in the same K power because the difference in base curve of the contact lens causes the refraction with it in place to vary. Many experts suggest using a consensus approach for determining corneal curvature after refractive surgery, rather than one method alone. Other methods currently being used include the corneal power correction equation by Shammas. This equation involves measuring the K's now and then averaging them into one number. So, the corrected K = 1.14 (average K) - 6.8. Of interest, the modified Maloney method is similar. The corrected K = 1.1 (average K) - 6.1. Another method, referred to as the topography method, as published by Wang et al, involves measuring the eye with topography, multiplying that number by 1.114, and then subtracting the correction factor of 6.1. On a final note, using a formula where the patient's actual preoperative anterior chamber depth is entered into the calculation is best (ie, the optimized Haigis IOL Consultant formula, the Holladay II IOL Consultant formula). Formulas that only take into account axial length and keratometry falsely assume that these patients with such flat K's must also have shallow anterior chamber depths, resulting in incorrect IOL powers. Also, the history method has been shown to be not as accurate in the postradial keratotomy patient, since the postoperative refraction never completely stabilizes. VELOCITY CONVERSIONSection 9 of 11
The velocity conversion equation is helpful in many biometric circumstances, including cases of silicone oil in the vitreous, using an incorrect velocity setting on the biometer or measuring an eye filled with silicone oil. The equation is as follows: In the event of an incorrect eye type setting, this equation is quite simple to use and will preclude the need for the patient to return for repeat measurements. For example, an aphakic eye was measured incorrectly with the phakic average setting. The correct velocity for this eye is 1532 m/s. The velocity used was 1550 m/s. If the eye length obtained was 24.1 mm on the phakic average setting, then 1532/1550 X 24.1 = 23.82 mm = true eye length. For eyes that have silicone oil in the vitreous cavity, this formula is used to determine the true vitreous length. Silicone oil is used surgically to replace the vitreous in some cases of recurrent retinal detachment and macular hole. The oil is removed several months later, but while in the eye, it causes a cataract, often requiring the removal of the cataract at the time of oil removal. The velocity conversion equation is necessary because the velocity through silicone oil is only 980-1040 m/s, much slower than the 1532 m/s the biometer uses in determining vitreous length. Therefore, the biometer measures the vitreous erroneously long, and, consequently, the total length also is erroneously long. In a 4-gate system with silicone oil, the ACD and lens thickness are accurate, so they should be subtracted from the total length to isolate the erroneous apparent vitreous length. The velocity of the silicone oil must also be determined. For 1000 cs oil, the velocity is 980 m/s. For 5000 cs oil, the velocity is 1040 m/s. Then, the formula used is as follows, in a case with oil velocity of 980 m/s: The corrected vitreous length is now added back to the anterior chamber depth and lens thickness for an accurate total eye length (see Image 13). It is best to perform biometry with these patients sitting upright so the bubble will not separate from the retinal surface, causing a spike to arise at the back of the bubble, which can be confused easily with the retinal spike. If an aphakic patient has silicone oil in the eye, it must be determined whether or not the oil is in the anterior chamber or only the posterior chamber. If the oil is only in the posterior chamber, the ACD should be subtracted from the total length, the vitreous length corrected using the velocity conversion equation, then added back to the ACD. If the oil reaches anteriorly to the back of the corneal endothelium, the entire eye length should be corrected using the following equation: If oil is in the vitreous cavity of the pseudophakic patient, the ACD should be subtracted to isolate the posterior chamber, using the following equation: Then, add back to the ACD along with the correction factor for the IOL in place. If the velocity through the oil is 1040 m/s, these same formulas are used substituting 1040 m/s wherever 980 m/s is used above. Some newer ultrasound units have adjustable velocities or silicone filled eye types. In these cases, the vitreous velocity is simply changed to the appropriate velocity through the silicone oil, and the measurement is taken while the patient is sitting upright. Also, optical coherence biometry has been shown to be effective in measuring these eyes, although the cataract must be mild enough for it to be used and the patient must have the ability to fixate. If the silicone oil is to be left in the eye permanently, power must be added to account for the index of refraction of the silicone oil. Consulting the Holladay II IOL Consultant formula for this calculation is best. Also, a silicone IOL should not be implanted in these eyes, because it will adhere to the silicone oil. MULTIMEDIASection 10 of 11
REFERENCESSection 11 of 11
Article Last Updated: Jan 8, 2007 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
