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. The
presence of 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). 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 can lead to significant errors
in A-scan axial length measurements. This is 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. Failure to recognize the
characteristic features of a posterior staphyloma on A-scan biometry
may result in a post-operative refractive surprise.
Certain findings 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 OCB can
convert a time consuming, and often difficult measurement into a quick,
nearly routine, procedure.
Formula
