Phacoemulsification and foldable posterior chamber intraocular lenses in aircrew: Clinical behavior and aero medical implications

Material and Methods

A total of 14 experienced pilots (both civil and military) with a mean age of 52.9 ± 8.1 years who had undergone phacoemulsification and implantation of foldable PC-IOLs participated in this study.

The study was conducted as per the defined protocol, in following stages:-

1. A detailed history focused on personal particulars, flying experience and cataract and IOL surgery was recorded.

2. An extensive questionnaire proforma was completed by the subject to gather information from the aircrew regarding problems encountered while flying with IOL.

3. General external examination of the eyes and adnexae.

4. Aided and unaided distant visual acuity was recorded separately for each eye by the standard Snellen's chart from a distance of 6 meters. Near vision and intermediary vision was assessed by using near vision test type chart at a distance of 30 cm and 100 cm respectively. The optical state and postoperative refractory error of the eyes was assessed by radioscopy.

5. Visual field testing was done by confrontation test. Color perception was assessed by Martin Lantern test (MLT) for military aircrew and Ishihara book for civil aircrew. Ocular muscle balance was tested by cover test and quantitatively measured by Maddox Rod test. Objective convergence was measured by using Livingston Binocular Gauge. Intraocular Pressure in each eye was measured by indentation tonometry method using Schiotz tonometer. Fundoscopy was done using a self-illuminating ophthalmoscope to exclude any abnormality in the fundi and optical media. Finally a detailed examination of cornea, operative wound, anterior chamber, iris, posterior capsule, position and status of the lens implant and retrolental space was carried out by slit-lamp biomicroscopy.

Results

The aircrew who participated in this study was highly experienced with average preoperative flight time of 10935 ± 2450 hours and an average postoperative flight time of 1850 ± 1450 hours. Out of 14 pseudophakic aircrew, 3 had unilateral and 11 had bilateral implantation. All the eyes had undergone phacoemulsification with foldable PCIOL implantation. The surgery had been performed between 1998 tg 2004. Table 1 gives the data of individual aircrew about their surgery and IOL history.

Two aircrew reported to have a problem with vision due to cataract preoperatively and the disability was severe enough to interfere with flying duty in only one aircrew. The subjective complaints as reported by the aircrew (n=3) included haloes around lights, problem with fixed focal length, discomfort, problem with dust and fumes and distortion of image. Out of 25 pseudophakic eyes reviewed, postoperative complications developed in only two eyes. Subluxation of the lens implant occurred in one eye, where the lens was explanted and a new HEMA acrylic lens was reinserted. Posterior capsule opacification developed in another eye, requiring YAG laser capsulotomy.

Visual performance in all the cases after the lens implant surgery was essentially normal. All the aircrew attained 6/6 best-corrected visual acuity for distant vision, N5 for near vision and N14 for intermediary vision. Postoperatively, there were 15 emmetropic, 6 hyperopic and 4 myopic eyes. The residual postoperative refractive errors for the 25 eyes were within the usual values expected. The mean absolute spherical refractive error was 0.22 dioptres, with a range of 0 to 1.75 dioptres. The mean absolute astigmatic error was 0.64 dioptres, with a range of 0 to 1.5 dioptres. The mean bifocal add was + 1.63 dioptres, with a range of + 0.5 to + 3.5 dioptres. Colour perception was assessed to be CP-I for military aircrew andCP-ll for civil aircrew. The status of ocular muscle balance and objective convergence was noted to be well within the acceptable standards for aviation duty. The intraocular pressure in the pseudophakic eyes was recorded to be 15.3±2.08 mm Hg. Ophthalmoscopic and slit-lamp examination did not show significant clinical abnormality in any of the eyes.

Discussion

Since the earliest days of aviation, possession of a high standard of vision has been an essential part, as most of the flight information is visually acquired [10]. In the present study even though 13 out of 14 aircrew interviewed, reported to have a problem with vision due to cataract preoperatively, the disability was severe enough to interfere with flying duty in only one aircrew. In general, cataracts are not particularly serious unless they affect normal vision. Therefore the presence of cataract does not necessarily require restriction from flying duty as long as adequate visual acuity is maintained [11]. However, in aircrew population, surgery may be required earlier to maintain an optimal visual performance because of the peculiarity of the environment where they operate [1]. In the present study, more than 90% aircrew agreed that even though they did not experience much problem preoperatively, they had undergone cataract extraction and lens implantation surgery as advised by the consulting ophthalmologist and felt that they had benefitted from the surgery.

Replacement in situ of the natural lens by an equally powered IOL restores the physiological parameters of vision much superiorly as compared to other methods. As it occupies a position very similar to the crystalline lens, it eliminates peripheral distortion, ring scotoma and restores the full extent of visual field. Also the image magnification is reduced to minimal. Further it abolishes all the difficulties of inserting and removing contact lenses encountered by many people [2, 5, 12].

Aircrews represent a specific sub-set of general population. The type of cataract surgery and IOLs, if needed to be used in this group of people must be highly efficacious and compatible with the operational requirement. In the present study all aircrew had undergone phacoemulsification procedures with foldable PCIOL implantation. In recent years phacoemulsification has been recognized as the procedure of choice for cataract surgery and has evolved into an effective surgical technique for small incision cataract surgery, permitting emulsification of cataractous lens through a tiny incision. It is possible to remove lens material and implant IOL through an incision as small as 3.0 to 3.5 mm in diameter as opposed to incisions up to 11 - 12 mm in the early days of conventional ECCE. This helps in eliminating many of the complications of wound healing related to large incision. The benefits of this procedure also include safer healing with lesser risk of complications such as inflammation or endophthalmitis, more rapid healing and rapid recovery of visual rehabilitation with less postoperative astigmatism [4, 5, 12].

The subjective appreciation in restoring vision and adequacy of the resulting vision in enabling flying duty was evaluated by a questionnaire study. All the aircrew rated the change in vision after surgery to be highly adequate. IOL surgery as a modality of treatment for correction of aphakia was graded as 'excellent' by 93% and the subjective acceptability of IOL in aviation environment was felt as 'completely acceptable' by all the aircrew.

'Haloes around lights' was observed by one aircrew, which lasted for only one year and then completely disappeared. None of the aircrew complained of increased glare sensitivity. Similar findings have been reported earlier [3,8]. Study carried out by Prasad et al [8], had documented a 20-30% incidence of increased glare sensitivity and halos around lights among aircrew that had undergone conventional ECCE with rigid PC-IOL implantation. Similar problems have been reported in another study [3] compared to much high 40 - 60 % with conventional ECCE and rigid PC-lOLs.

When pupil dilates sufficiently in dim illumination, it exposes the round edge of the optic or the positioning holes of the intraocular lens. This causes light to defract after striking and results in glare and haloes. This can be overcome by modifying the IOL design, thus inserting IOL of an appropriate size without positioning holes. This has been clearly observed in the present study, where none of the aircrew with new generation foldable IOLs, complained of increased glare sensitivity and only one pilot reported seeing haloes around lights, which lasted only for a short duration of time. This could be due to the edge of the optic interfering with transmission of light.

Even though the problem with fixed focal length was observed by only one aircrew, all of them reported to have difficulties in reading without glasses. This problem is inherent to IOL design [4, 12]. There is a requirement of bifocal spectacle lenses for distance and near vision in a pseudophakic. However, this problem does not appear to be significant in the types of aircraft flown by this group of aircrew. On the other hand multifocal lOLs enhance near vision performance without spectacles. However the use of multifocal lOLs have the disadvantages of minor degradation of visual performance, large loss of contrast sensitivity and problems with color mixing, which makes them questionable for use in aircrew [13, 14].

Discomfort in the form of grittiness in the eye was observed-by one aircrew as a rare occasional finding, occurring mostly on exposure to bright light. Similarly one aircrew observed problems with dust and fumes in the early postoperative days. None of these problems interfered with their flying duty at any point of time. Distortion of images was reported by one aircrew who was an old case of astigmatism for the past 20 years for a postoperative period of 30-45 days but disappeared completely afterwards. The above mentioned problems are non-specific and might be expected in any type of intraocular surgery.

Erythropsia following cataract surgery is a known problem [15]. This is due to loss of ultraviolet blocking property when crystalline lens is replaced by an artificial implant. Previous studies have reported the incidence of erythropsia between 20-25% [3, 8]. However none of the aircrew in the present study complained of this problem. This may be credited to the use of modern generation high quality foldable lOLs with the U V blocking additive.

Previous history of complications was given by two aircrew. Subluxation of lens occurred in one aircrew, who developed distortion of image and poor vision between 30"' - 45"' post-operative days. The implant was explanted and a new HEMA acrylic lens was reinserted. The PC-IOL is known to give rise to four specific types of malposition postoperatively: pupil capture, deceleration, windshield wiper syndrome and sunset syndrome [12,16,17]. However, technological advancement in the methods of insertion and retention of IOL as well as development of newer microsurgical techniques has eliminated most of these problems.

YAG laser capsulotomy was done in one eye as it developed Posterior Capsule Opacification (PCO) postoperatively. Earlier studies [8, 13] have reported the occurrence of opacification of posterior capsule between 40 to 60% after conventional ECCE with implantation of rigid PC-IOLs. The lesser incidence of such opacification in the present study can be attributed to the highly biocompatible implant material, as compared to previous generation IOLs.

Posterior capsule opacification is a common squeal of extra capsular extraction [18, 19]. Among many factors, IOL material is one of the important factor attributed to opacification of the pos-terior capsule. The incidence of PCO was reported to be significantly high for silicon IOLs (27.9%) as compared to PMMA (7%) and is least for acrylic foldable IOLs. The acrylic foldable IOL adheres to the lens capsule more firmly than PMMA and silicon IOL. These differences seem to play a significant role in causing PCO [20, 21]. An adequate YAG laser capsulotomy improves the vision, where it is impaired and refraction has not helped. The lesser incidence of PCO in the present study can be attributed to the highly biocompatible implant material, as compared to previous generation IOLs.

Conclusion

The evolution of cataract surgery with the introduction of intraocular lens implantation has been one of the major achievements in restoring vision. Intraocular lenses provide a precise pseudophakic optical rehabilitation with minimal magnification and excellent optical properties. The advent of small incision surgery made possible by phacoemulsification and foldable PC-IOLs represents another milestone in this field, reducing most of the postoperative complications thus enabling the aircrew an early return to flying. Most of the post surgery problems, such as increased glare sensitivity, haloes around lights and erythropsia, can be eliminated to a significant level by modifying the IOL design.

Thus the preferred recommended cataract procedure in an aircrew is phacoemulsification with implantation of foldable IOL into the posterior chamber. The ideal IOL should be highly biocompatible, possess a unifocal design and contain an ultraviolet blocking and blue-light filtering additive. Multifocal lenses should not be used in an aircrew and preferably IOL should not have positioning holes.