Friday, October 28, 2011
Biometry in Posterior Staphyloma
Failure to recognize a posterior staphyloma can result in an unpleasant refractive surprise following cataract surgery.
It is well known that the incidence of posterior staphyloma increases with increasing axial length. Uncommon below 26.5 mm, it has been reported that a posterior staphyloma may be found in 70% of eyes with axial lengths above 33.5 mm. In reality, it is likely that nearly all eyes with pathologic myopia have some form of posterior staphyloma.
The presence of a posterior staphyloma should always be considered if there is difficulty obtaining a distinct retinal spike during A-scan ultrasonography in the setting of moderate to high axial myopia. A staphyloma can have a major impact on axial length measurements, as the most posterior portion of the globe (anatomic axial length) may not correspond with the center of macula (refractive axial length).
This can lead to significant errors in A-scan axial length measurements because the anatomic axial length (the distance from the corneal vertex to the posterior pole) may differ from the refractive axial length (the distance from the corneal vertex to the fovea). It is helpful to be aware that this anatomical variation may be present in any axial myope.
Certain findings would suggest the presence of a posterior staphyloma. These include a long axial length with inconsistent axial length readings in both the measured eye and compared to the fellow eye. Most posterior staphylomata are located in the peripapillary region, adjacent to, but not centered at, the macula. When the fovea is situated on the sloping wall of the staphyloma, it may only be possible to display a high quality retinal spike when the sound beam is directed eccentric to the fovea, toward the rounded bottom of the staphyloma. This will result in an erroneously long axial length reading. Paradoxically, if the sound beam is correctly aligned with the refractive axis, measuring to the fovea may result in a poor quality retinal spike and inconsistent axial length measurements.
The simplest method by which to measure axial length, in the setting of a posterior staphyloma, is by optical coherence biometry (OCB) using the Zeiss IOL Master. If the patient's visual acuity is good enough, have him or her look directly at the red fixation light, and the axial length measurement will typically be to the center of the macula.
Immersion vector-A / B-scan
If access to an IOL Master is not possible, an immersion vector-A / B-scan can be used to measure the axial length to the center of the macula. Developed by Holladay and first described in the 1992 textbook, Ultrasound of the Eye and Orbit by Byrne and Green, this approach to axial length measurement in the setting of a posterior staphyloma is as follows:
An immersion echogram through the posterior fundus is obtained using a horizontal axial B-scan. The goal is to center the cornea and lens echoes in the echogram while simultaneously displaying the optic nerve void near or slightly above the center. The A-scan vector is then adjusted so as to pass through the middle of the cornea as well as the anterior and posterior lens echoes. Such alignment assures that the vector will intersect the retina in the region of the fovea. This technique is particularly important when the macula lies on the sloping wall of the staphyloma.
With the void of the optic nerve visualized on B-scan, a simultaneous vector A-scan is directed to the center of the macula, temporal to the edge of the optic nerve. Alternatively, if it is possible to visually identify the center of the macula with a direct ophthalmoscope (often quite difficult in a high myope), the cross hair reticule can be used to measure the distance from the center of the macula to the margin of the optic nerve head. The vector A-scan is then positioned that same distance temporal to the void of the optic nerve on simultaneous B-scan.
Optical Coherence Biometry
By OCB, using the Zeiss IOLMaster, the challenges in measuring eyes with posterior staphylomata are often avoided. As long as the patient can see well enough to look directly at the small fixation light, the axial length measurement will be to the fovea, yielding the refractive, rather than anatomic, axial length.
For eyes with a posterior staphyloma and sufficiently clear ocular media, the use of the IOLMaster can convert a time-consuming, and often difficult measurement, into a quick, nearly routine, procedure.
Posted by Venkataramanan Ramasethu