The potential acuity meter (PAM) measures retinal visual acuity behind cataracts or other media opacities by projecting a small beam of light through clear areas of the cataract. It is used to estimate visual outcomes after cataract surgery and other procedures. PAM testing is performed quickly in a dimly lit room after pupil dilation and involves having the patient read letters as the light beam is repositioned. While PAM tends to underestimate potential acuity, it provides a reasonably reliable method for predicting visual results of cataract surgery.
2. PAM
The Guyton-Minkowski Potential Acuity Meter measures retinal visual acuity
behind a cataract or other media opacity.
Introduced in 1983, the PAM is mainly used to estimate visual outcomes
after cataract surgery.
It projects a Snellen eye chart—via a narrow beam of light—that converges
to a 0.1-mm aerial aperture.
This opening is placed onto less dense areas (ie, windows) within the
cataract, allowing the eye chart to be focused onto the retina with minimal
cataract-induced light scattering.
Because the PAM test uses a smaller aperture than the 1-mm opening for
pinhole, it more accurately (1) measures retinal acuity and (2) estimates
postsurgical visual results.
3. PAM PROCEDURE
PAM testing is performed in a dimly lit room, with the PAM mounted on a slit
lamp that is set to the lowest magnification.
Glare may be avoided by turning off the illumination. Other eye charts
should also be turned off or stowed away.
Pupil dilation is preferable, because more windows are made available for
the PAM light beam to pass through.
Additionally, amblyopic patients may do better after patching the good
eye. The eye should not be exposed to bright lights prior to performing the
test.
When the patient is ready, the operator should turn the dioptric setting to
the approximate spherical equivalent of the eye and explain that a light
will appear and letters or numbers will be visible.
4. PAM PROCEDURE
Character clarity may change during the test.
The patient is instructed to avoid head movement, as this will displace the
light beam and delay the procedure.
They should report what characters are visible through clenched teeth,
minimizing head movement.
The basic set-up technique is to focus the beam onto the patient's retina
through the cataract.
The patient is encouraged to read the lines of the chart aloud until no other
smaller legible lines are encountered.
This process is repeated until the examiner is confident that the patient
cannot read any finer lines.
If the patient correctly reads any three characters in a certain line, then
that level of visual acuity is established.
5. PAM PROCEDURE
The resulting potential acuity is the smallest line where the patient was able
to read three characters, even if they lose sight of it in subsequent
retesting.
The light beam should be repositioned in other windows in an effort to
enable the patient to see additional finer lines.
The test takes 5 to 10 minutes per eye.
6. PAM is mostly used for patients about to undergo cataract surgery,
but it may also be used for other ocular media problems including
large refractive errors, corneal/vitreous opacities, partial hyphema,
IOL deposits, posterior capsular opacities, and asteroid hyalosis.
Generally, if any retinal detail is clinically visible, there is an
adequate window for PAM testing, because the PAM light beam is
smaller than the size of the pupil needed to see the retina.
For opaque corneas, mature cataracts, thick pupillary membranes,
dense vitreous hemorrhage, and severe optic nerve or retinal
disease, the patient may report that the PAM light is not visible.
Non ocular conditions that make PAM testing difficult- to-impossible
to conduct, include poor patient posture or mental status, literacy,
nystagmus, and patient fatigue.
7. APPLICATIONS
PAM testing tends to underestimate potential acuity, so postsurgical results
are usually better than predicted.
The accuracy of the PAM test decreases as the density of the cataract
increases or when preoperative visual acuity is poorer.
Patients with these characteristics should not be excluded from cataract
surgery on the basis of poor PAM results.
PAM is also used to test retinal acuity in eyes with other media problems, for
rapid potential vision screening in patients with vitreoretinal diseases,
microphthalmia, as well as large or irregular refractive errors.
PAM testing is also used to identify patients with posterior capsular opacities
who may benefit from Nd:YAG capsulotomy.
When both eyes have cataracts and similar preoperative visual acuities,
PAM may be used to select what eye will undergo cataract surgery first.
8. APPLICATIONS
PAM testing is used to identify patients with coexisting ocular diseases (eg,
retinal or nerve pathologies) who may benefit from cataract surgery.
There is a tendency for PAM to generate false-positive/overestimated
results in patients with macular degeneration.
The combination of PAM and automated visual field testing was useful in
predicting outcomes following combined cataract and trabeculectomy
surgeries.
The clinical reliability of PAM to predict treatment results for noncataractous
conditions has not been established.
PAM testing is not consistently reliable in predicting visual results after
macular hole surgery.
9. SUMMARY
Since its introduction nearly a quarter of a century ago, PAM testing has
become an established means of predicting visual results after cataract
surgery.
The readily available PAM requires low maintenance.
PAM testing is rapidly performed; it is easy to learn—both for the unskilled
operator and first-time patient.
The long track record of PAM has provided ophthalmologists with
numerous published articles that define its uses, quirks, and limitations.
Current high patient expectations and the increasing use of premium IOLs
have clearly established the need for a predictive screening tool before
cataract surgery.
PAM is an imperfect but reasonably reliable and easily available method
for predictive testing and has a definite place in current clinical practice.
