Keep Your Eye on the Individual's Visual Function, Part 2
Refractive correction lets 75-80 percent get better than 20/40 vision without glasses. Jobs in which glasses are a hindrance might benefit from these procedures.
- By Bernard R. Blais, M.D., FAAO, FACOEM, FACS, Thomas Tredici, Sr., M.D.
- Dec 01, 2003
THE present status of ocular engineering now allows for many new concepts that change or modify the optical system of the eye. Some of these are corrections for existing ocular impairments, but they require completely new approaches to the fitness-for-duty status and qualifying of incumbents or existing employees. Let's consider a few of these changes.
For the aphakic individual, one who has had a cataract removed, treatment modalities have gone through the following changes:
1. In the past, the individual was required to wear thick, aphakic convex lenses.
2. Aphakic contact lenses replaced this need in most cases.
3. An intraocular lens was introduced into the capsule of the lens or the anterior chamber that provide a fixed focal distant.
4. A multifocal intraocular lens was introduced that functions similarly to external multifocal lenses.
5. Nearing FDA approval is the accommodating intraocular lens, which allows for the measurement of far, intermediate, and near vision in an equivalent method as the original lens. The lens is designed to provide patients with a full range of vision without glasses. The contrast sensitivity is the same with the accommodative intraocular lens and the standard intraocular lens.
Orthokeratology is contact lens fitting technique in which the corneal apex is flattened (molded) with progressively flatter contact lenses to attempt to reduce myopia. Orthokeratology can only be done using hard contact lenses. It works best with the old PMMA lenses and was in vogue especially in the mid 1970s.
The cornea did not mold sufficiently, in many cases. The alteration is temporary--after three weeks without lenses, corneas revert to their original shape. Results are unpredictable. Ocular complication is possible (astigmatism).
Orthokeratology is not an approved form of contact lens fitting for U.S. Air Force aircrews, but for a small, select number of individuals, this can improve their distance vision as long as the contacts are utilized.
Radial Keratotomy (RK)
In the mid-1970s, Russian eye surgeon Fyodorov rediscovered the procedure. Eight to 16 radial cuts are made into the cornea from the limbus toward the pupil. The depth of the incision may approach Decemet's membrane. A procedure such as radial keratotomy may be looked at favorably by some as a method to correct the refractive error of ametropic aviators. Radial keratotomy is performed under topical anesthesia. A formula is utilized depending on the refractive error, age, corneal curvature, etc. Calculations are made as to the length, depth, and number of incisions to correct the refractive error. On healing, the center of the cornea flattens, thus reducing the myopia.
RK is more permanent than orthokeratology. For myopia of 1 to 4 diopters, this technique has a 75-80 percent chance of obtaining 20/40 uncorrected acuity. In some cases, glasses or contacts will still be needed to obtain 20/20 visual acuity.
Photorefractive Keratectomy (PRK)
Photorefractive keratectomy (PRK) had its beginning in the 1980s. PRK uses computer and laser techniques to remove a microscopic amount of corneal stroma and reshape it into the curve necessary to correct the refractive error (usually myopia).
Excimer laser in the UVC band at 193 nanometers is utilized. Usually only about 40-50 microns of stroma (one-tenth) is removed. Corneal stroma is never made thinner than 250 microns. Thermal effects are insignificant. Numerous studies show very good results: About 70 percent of the cases achieve 20/20 uncorrected vision, but some of these may regress. About 95 percent can achieve 20/40 uncorrected visual acuity. There may be a prolonged recovery period with recurrent erosions, glare, ghost, and night driving problems, but usually vision is quite good.
Laser In Situ Keratomilleusis is useful over the entire range of myopia. Flap incision with hinge is the method, with PRK done in exposed bed. Final visual results of PRK and LASIK are equal. Post-op morbidity is more marked with PRK. For the patient, it may take four to eight weeks before returning to full-time duties.
Contraindications to PRK/LASIK include:
- Very high myopia/astigmatism
- Active ophthalmic disease
- Neovascularization of the cornea
- Evidence of keratoconus
- Concurrent topical or systemic Rx:
Corticosteroids, antimetabolites, Accutane, Cordarone, or Imitrex
- Medical conditions that may impair healing
- Collagen vascular disease, autoimmune disease, immunodeficiency disease, ocular herpes
Intrastromal Corneal Ring
Two small, semicircular plastic ring segments made of PMMA are inserted into the mid-stroma. The rings place tension on the cornea, altering the curvature. This method is now in the research phase, but with surprisingly good results.
Refractive Surgery in Occupational Medicine
How might this method of refractive correction fit in an occupational medicine scenario? Seventy-five to 80 percent should get better than 20/40 vision without glasses. Twenty to 25 percent still may need glasses to achieve 20/20. Jobs in which glasses are a hindrance might benefit from these procedures (such as firefighters, security personnel, goggle and scope users, painters, etc.).
For all jobs following surgical correction, visual function should be tested initially and repeated to determine stability of the procedure. If the surgery was RK (radial keratotomy), the worker should be protected from physical force to the eye because RK wounds take a long time to heal, if ever. A finger may rupture the eyeball even after months of healing. For PRK or LASIK, follow routine eye safety rules. PRK or LASIK do not appear to weaken the cornea as does RK, but the individual might have difficulty with glare and night driving.
If LASIK was done, the corneal flap could possibly be displaced even years after the surgery. One would notice an instant change/decrease of vision until the corneal flap was repositioned--it would not be lost because it is hinged with corneal tissue. PRK and LASIK are safe procedures when done by competent, experienced surgeons, yet a small number of cases will develop complications. There are more complications with LASIK than with PRK.
In myopic (nearsighted) workers, if refractive surgery is done on mature adults (age 40+), they soon will discover that they cannot see to read, see the computer screen, or do close or fine work unless they use reading glasses. They have traded one pair of glasses for another. All workers older than 45 doing near work of any kind would be affected by any new advances in the correction of presbyopia because PRK, LASIK, or intrastromal rings do not correct presbyopia. These procedures only affect distance vision, whether it be myopia, hyperopia, and/or astigmatism.
To date, several surgical procedures to improve presbyopia have been attempted without success. The use of a contact lens to create monovision works for a small number of individuals but has some inherent drawbacks:
- It is difficult for some to adjust to.
- It degrades depth perception (stereopsis).
Shortfalls of Refractive Surgery
- Refractive error not fully corrected or over-corrected.
- Tissue removal not centered.
- Prolonged corneal haze.
- Increased intraocular pressure (temporary while on Rx).
- All of the PRK complications are possible.
- Corneal flap cut off.
- Corneal flap wrong depth.
- Microtome cut into AC.
- Epithelial cell down growth.
- Day eye syndrome.
The Impact of Future Techniques
Just evolving now are refined laser/optical techniques, such as "wavefront analysis" having the potential for improving visual results even beyond 20/20 to 20/15--20/12---20/10--?? Wavefront-guided LASIK and PRK technology is proposed for the correction of both lower and higher order optical aberrations. For residual errors, a second wavefront-guided procedure would be performed.
Fitness-for-duty evaluations require more detail because the individual cannot by law be allowed to choose and pass with an additional reading segment and still fulfill the legal requirements. The key elements of fitness-for-duty evaluation still remain:
1. A detailed evaluation of essential functions of the job.
2. Very careful evaluations of the visual ergonomics of the job.
3. Visual screening of the employee or incumbent.
4. Comparison of the visual evaluations to the visual needs of the position.
5. Provide needed devices to fulfill the Americans With Disabilities Act (ADA) of 1990 and OSHA's 29 CFR 1910.132 and 1910.133, 134.
As La Haye and Sustello mentioned in their article "Safety Eyewear and The Older Worker" (Occupational Health & Safety, October 2001, page 38, Vol. 70, No. 10), paste-on bifocals may be better than "cheaters," but they do not fulfill the visual ergonomics requirements of the employee except for the fixed focal distance chosen. Nor does using them resolve the fitness for duty of these individuals who have had or are using means to modify the visual power of the eye, temporarily or permanently.
The occupational medicine team must perform a job analysis and ocular (visual) screening, issuing the proper prescription or non-prescription glasses as mandated by law, as well as by the need for a productive, safe employee. The team must become more involved than ever in the past with the changes that are taking place in the refractive power of the eye. These changes may have a definitive effect on individuals' ability to perform in their workplaces.
Part 1 of this article was published in the October 2003 issue of Occupational Health & Safety.
This article originally appeared in the December 2003 issue of Occupational Health & Safety.