In the age of premium IOLs, cataract patients are offered technologically advanced IOLs that correct either presbyopia (ie, multifocal, accommodating) or astigmatism (ie, toric) or provide sharper vision, especially under low-light or nighttime conditions (ie, aspheric). The patient's expectations after cataract surgery are the highest, and ophthalmologists feel pressured to consistently deliver excellent postoperative visual outcomes. In this environment, the ability to evaluate retinal visual acuity and predict postoperative vision has become an essential tool for setting patient expectations and identifying what patients may benefit from these technologies. The following series of articles summarize several tools that may help ophthalmologists predict visual acuity after cataract surgery.

The Guyton-Minkowski Potential Acuity Meter (PAM; Haag-Streit, Köniz, Switzerland) (Figure 1) measures retinal visual acuity behind a cataract or other media opacity.¹ 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.

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. 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. 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.

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. Nonocular conditions that make PAM testing difficult- to-impossible to conduct, include poor patient posture or mental status, literacy, nystagmus, and patient fatigue.

APPLICATIONS
In their original report, Minkowski et al1 found that among cataractous eyes having best preoperative visual acuity of 20/200+, the postoperative visual acuity was correctly predicted to within three lines in 100% of patients. In 91% of patients, the prediction was within two lines. PAM correctly predicted postoperative visual outcomes of 20/40+ in 95% of patients. Most studies reported that PAM correctly predicted visual acuity to within two lines in approximately 80% to 90% of patients.^2,3

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. In our practice, PAM results are a basis for IOL selection. Patients with poor results will not receive a multifocal IOL, because good retinal acuity is a requisite for obtaining good results with these expensive IOLs.

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.⁴ When both eyes have cataracts and similar preoperative visual acuities, PAM may be used to select what eye will undergo cataract surgery first.

PAM testing is used to identify patients with coexisting ocular diseases (eg, retinal or nerve pathologies) who may benefit from cataract surgery. A few studies have reported that PAM is a potentially useful predictor of postsurgical results in patients with cataracts and coexisting posterior segment disease (eg, macular degeneration, macular hole).⁵ 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.

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.

Harvey Siy Uy, MD, practices at the Asian Eye Institute, in Makati, Philippines. Dr. Uy states that he has no financial interest in the products or companies mentioned. He may be reached at tel: +632 8982020; fax: +632 8982002; or harveyuy@yahoo.com.

1. Minkowski JS, Palese M, Guyton DL. Potential acuity meter using a minute aerial pinhole aperture. Ophthalmology. 1983;90:1360-1368.
2. Uy HS, Munoz M. A comparison of the potential acuity meter and pinhole tests in predicting postoperative visual acuity after cataract surgery. J Cataract Ref Surg. 2005;31:548-552.
3. Chang MA, Airiani S, Miele D, Braunstein RE. A comparison of the potential acuity meter (PAM) and the illuminated near card (INC) in patients undergoing phacoemulsification. Eye. 2006;20:1345-1351.
4. Klein TB, Slomovic AR, Parrish RK II, Knighton RW. Visual acuity prediction before neodymium-YAG laser posterior capsulotomy. Ophthalmology. 1986;93:808-810.
5. Alio JL, Artola A, Ruiz-Moreno JM, et al. Accuracy of the potential acuity meter in predicting the visual outcome in cases of cataract associated with macular degeneration. Eur J Ophthalmol. 1993;3:189-192.