A-scan ultrasonography biometry

2008-09-13
HIGH RISK EYE SURGERY Direct the patient to look downward, toward his feet; then lift the patients upper eye lid and insert the flared rim underneath the lid (the upper portion of the Shell will make contact with the sclera while the lower part of the Shell will be held away from the eye); ask the patient to look straight ahead with the uncovered eye, toward the fixation light. Pull the patients lower eyelid down and gently pivot the lower portion of the Shell into the lower fornix, making sure by close inspection that it is in the fornix and not sitting atop a fold in the conjunctiva. This pivotal motion avoids contact with the cornea and insures centration of the device around the limbus. The goal is to put minimal pressure on the eye. In fact, it is quite instructive for the biometrist to be the patient (at least once) and learn firsthand the benefits of a light touch. Note the Luer filler port is facing temporally. The left hand/palm is resting on the forehead (given the biometry instrument is to your left), and is used to reduce Shell pressure on the eye. Try to keep the A-scan instrument in your direct line of sight. It is important to position the biometry screen so that it easily can be seen during the procedure. Moreover, the palm acts as the fulcrum or pivot point for the Shell. Sometimes the Shell can be stabilized with the right hand to make micro-movements. With practice, most practitioners usually will hold the Shell with just the hand resting on the forehead. The right hand is free to make instrument adjustments, if necessary. A facial tissue can be placed on the temporal canthus to catch any excess saline. To make a measurement, pick up the BSS bottle/syringe from its place on the patients shoulder or keep the BSS bottle in your hand and slowly inject the saline into the Shell. As soon as the liquid fills the Shell sufficiently to reach the tip of the probe (about 2 cc), the characteristic waveforms of immersion biometry will be seen on the screen. Note when using tubing with a check valve, the BSS bottle may pinch slightly until the tubing is removed from the bottle. The user may wish to review the waveforms by toggling through the list on the screen and deleting those that are less than perfect. By maintaining the Shell in the patients eye during this review, any measurement that is deleted will be immediately replaced with a new reading, which may in turn be either accepted or deleted. Optionally you can manually begin saving acceptable scans. To remove the Shell from the eye, pivot the Shell upward, directing the patient to continue to look straight ahead. Then pull away from the eye without contacting the cornea. Upon the initial release, the remaining contents of the Shell (1-2 cc of liquid) will spill down the patients cheek. Be prepared with a towel or facial tissue. Biometry by Immersion Although the Shell completely eliminates corneal compression as a complicating factor, and greatly assists in the alignment of the probe with the macula, it is still necessary to review and analyze waveforms to insure a perfect reading. Be sure to accept only steeply rising retinal spikes. The corneal, anterior lens and retinal spikes should be of equal height. If the spikes demonstrate a downward trend or are stair stepped, this suggests that the scan is off axis. With dense cataracts, the tendency is to increase the gain, thereby elevating the spikes. If the tops of the spikes appear flattened, this may indicate that the amplifiers are saturated resulting in an inaccurate reading. With very long eyes such as in those with staphylomas, the macula may be located on the sloping portion of the staphyloma and the retinal spike may not rise to the same height as the corneal spikes. In normal eyes however, the retinal/scleral spikes equal the height of the corneal spikes. Detection of orbital fat spikes is a requirement. A normal scan has a series of orbital fat echoes with descending amplitudes. If they are absent; or markedly attenuated, the probe may be misaligned and the biometrist may have directed the sound beam to the optic nerve instead of the fovea. The learning-to-learn curve requires just a few patients and the immersion technique is easily mastered, but to gain confidence in your measurements follow the additional suggestions and be familiar with the sources of error.
A-scan biometry ultrasonography