Cortical Opacification Research Articles

Higher-order aberrations of age-related cataract

To analyze higher-order aberrations induced by different types of lenticular opacities. Federal University of São Paulo, São Paulo, Brazil. One hundred thirty-eyes of 65 patients with bilateral cataract, classified according to the Lens Opacities Classification System III (LOCS III), had complete ophthalmic examinations, corneal topography (EyeSys Technologies), and wavefront analysis (LADARWave, Alcon Laboratories). Patients with cataracts that could not be measured by a Hartmann-Shack sensor or those with coexisting ocular disease were excluded. Higher-order aberrations were compared between the nuclear, cortical, and posterior subcapsular groups for statistical significance. One hundred five eyes of 65 patients were assessed. Twenty-eight eyes (23.33%) had predominantly nuclear opacification (NO1-NO6 and C1-2), and 13 (10.83%) had mainly cortical opacification (C1-4). Sixty-four eyes (65.83%) had a mixed pattern of LOCS III classification, which hindered the establishment of a correlation between the aberrometry and cataract type. Eighteen eyes that presented with dense posterior subcapsular cataract (P4-5) and 7 eyes with cortical C5 LOCS III classification were excluded. In eyes with nuclear opacification, the mean spherical aberration with a 6.0 mm pupil was 0.45 microm +/- 0.17 (SD) and the mean coma was 0.29 +/- 0.13 microm. In eyes with predominantly cortical cataract, the mean spherical aberration was 0.12 +/- 0.10 microm and the mean coma was 0.49 +/- 0.23 microm. The cortical cataract group had statistically significantly higher coma than the other groups (P<.001). The nuclear cataract group had statistically significantly higher spherical aberrations than the other groups (P<.001). Different types of early lenticular opacities induced different wavefront aberration profiles. Coma predominated in the cortical cataract group, and spherical aberration predominated in the nuclear cataract group.

Topical Ocular Hypotensive Medication and Lens Opacification: Evidence From the Ocular Hypertension Treatment Study

To determine whether topical ocular hypotensive medication is associated with refractive changes, visual symptoms, decreased visual function, or increased lens opacification. Multi-center clinical trial. We compared the medication and observation groups of the Ocular Hypertension Treatment Study (OHTS) during 6.3 years of follow-up with regard to the rate of cataract and combined cataract/filtering surgery, and change from baseline in visual function, refraction, and visual symptoms. A one-time assessment of lens opacification was done using the Lens Opacities Classification System III (LOCS III) grading system. An increased rate of cataract extraction and cataract/filtering surgery was found in the medication group (7.6%) compared with the observation group (5.6%) (hazard ratio [HR] 1.56; 95% confidence interval [CI] 1.05 to 2.29). The medication and observation groups did not differ with regard to changes from baseline to June 2002 in Humphrey visual field mean deviation, Humphrey visual field foveal sensitivity, Early Treatment of Diabetic Retinopathy Study (ETDRS) visual acuity, refraction, and visual symptoms. For the medication and observation groups, LOCS III readings were similar for nuclear color, nuclear opalescence, and cortical opacification. There was a borderline higher mean grade for posterior subcapsular opacity in the medication group (0.43 +/- 0.6 SD) compared with the observation group (0.36 +/- 0.6 SD) (P = .07). We noted an increased rate of cataract extraction and cataract/filtering surgery in the medication group as well as a borderline higher grade of posterior subcapsular opacification in the medication group on LOCS III readings. We found no evidence for a general effect of topical ocular hypotensive medication on lens opacification or visual function.

Cosmic Radiation Increases the Risk of Nuclear Cataract in Airline Pilots

Aviation involves exposure to ionizing radiation of cosmic origin. The association between lesions of the ocular lens and ionizing radiation is well-known. To investigate whether employment as a commercial airline pilot and the resulting exposure to cosmic radiation is associated with lens opacification. This is a population-based case-control study of 445 men. Lens opacification was classified into 4 types using the World Health Organization simplified grading system. These 4 types, serving as cases, included 71 persons with nuclear cataracts, 102 with cortical lens opacification, 69 with central optical zone involvement, and 32 with posterior subcapsular lens opacification. Control subjects are those with a different type of lens opacification or without lens opacification. Exposure was assessed based on employment time as pilots, annual number of hours flown on each aircraft type, time tables, flight profiles, and individual cumulative radiation doses (in millisieverts) calculated by a software program. Odds ratios were calculated using logistic regression. The odds ratio for nuclear cataract risk among cases and controls was 3.02 (95% confidence interval, 1.44-6.35) for pilots compared with nonpilots, adjusted for age, smoking status, and sunbathing habits. The odds ratio for nuclear cataract associated with estimation of cumulative radiation dose (in millisieverts) to the age of 40 years was 1.06 (95% confidence interval, 1.02-1.10), adjusted for age, smoking status, and sunbathing habits. The association between the cosmic radiation exposure of pilots and the risk of nuclear cataracts, adjusted for age, smoking status, and sunbathing habits, indicates that cosmic radiation may be a causative factor in nuclear cataracts among commercial airline pilots.

ACANTHAMOEBA INFECTION AS A CAUSE OF SEVERE KERATITIS IN A SOFT CONTACT LENS WEARER IN JAMAICA

We report the case of a 29-year-old Jamaican patient who presented with severe pain, redness, and swelling of both eyes. She was a regular soft contact lens wearer who did not maintain standard lens care. She was treated for a possible microbial/viral keratitis using topical ciprofloxacin drops, topical acyclovir ointment, and topical atropine drops. The response was inadequate, and scrapings from her cornea, contact lens cases, and both lenses revealed Acanthamoeba on microscopy, which was shown to be Acanthamoeba polyphaga using polymerase chain reaction. She was treated using chlorhexidine 0.02% hourly, ciprofloxacin every 4 hours, and atropine 1% every 12 hours, along with oral ketoconazole 200 mg twice daily with a dramatic response. However, she subsequently suffered slow corneal epithelial regrowth with severe scarring, vascularization, and cortical lens opacification and was referred for penetrating keratoplasty and cataract surgery. This is the first case of severe keratitis caused by Acanthamoeba to be reported from Jamaica and demonstrates that this emerging pathogen can be a cause of severe keratitis in the tropics.

Ten-year follow-up of a ciliary sulcus-fixated silicone phakic posterior chamber intraocular lens

In 1992, a ciliary sulcus-fixated, silicone, phakic posterior chamber intraocular lens (PPC IOL) (Adatomed, Fyodorov type) was implanted in both eyes of a 42-year-old white woman to correct high myopia (right eye, −17.0 diopters [D]; left eye, −11.5 D). In the right eye, localized cortical opacification was present in the anterior part of the natural lens preoperatively but did not progress during a 10-year follow-up period. With Scheimpflug photography, it was possible to detect a space between the natural lens and the IOL that was not evident on slitlamp examination. Maintenance of space between an IOL and the natural lens appears to be an important factor in preventing cataract formation after PPC IOL implantation.

Higher-order aberrations of lenticular opacities

Purpose: To measure and quantify higher-order aberrations induced by different types of lenticular opacities. Setting: Department of Ophthalmology, University of Auckland, and Department of Ophthalmology, Auckland Public Hospital, Auckland, New Zealand. Methods: Patients with lenticular opacities were recruited from outpatient clinics of a major tertiary referral center for ophthalmology. Patients were included if they had clinically evident, mild to moderate lenticular opacity with no coexisting ocular pathology. Patients were examined using standard preoperative techniques with additional assessment by wavefront aberrometry (Zywave®, Bausch & Lomb) and Scheimpflug photography (EAS-1000, Nidek). For comparison, 20 eyes of 10 subjects with no lenticular opacity (control group) were recruited and assessed in an identical manner. Results: Thirty persons were recruited and 40 eyes assessed, 20 with lenticular opacities. Ten eyes had predominantly cortical opacification, and 10 had mainly nuclear opacification. In eyes with predominantly cortical opacification, the mean logMAR uncorrected visual acuity (UCVA) was 0.5 ± 0.2 (SD) (6/18 Snellen equivalent) and the mean logMAR best spectacle-corrected visual acuity (BSCVA), 0.2 ± 0.2 (6/9). Analysis of aberrometry data for a 6.0 mm pupil in this group revealed an increase in coma of cosine phase (Z 3 1 , P = .06) and tetrafoil of cosine phase (Z 4 4 , P = .07) compared to eyes in the control group. Eyes with predominantly nuclear opacification had a mean logMAR UCVA of 0.7 ± 0.2 (6/30) and a logMAR BSCVA of 0.4 ± 0.2 (6/15). Aberrometry data for this cohort for a 6.0 mm pupil showed a statistically greater amount of spherical aberration (Z 4 0, P = .001) and tetrafoil of cosine phase (Z 4 4 , P = .005; Z 4 –4, P = .004). Conclusions: This pilot study suggests that different types of early lenticular opacities induce different wavefront aberration profiles. Predominantly cortical opacification produced an increase in coma and nuclear opacification induced an increase in spherical aberration compared to eyes without opacities. Both types of lenticular opacities also induced a higher amount of tetrafoil. This could explain the significant visual symptoms in patients with early cataract and relatively good high-contrast Snellen acuity.

Incidence and progression of lens opacities: effect of hormone replacement therapy and reproductive factors.

Hormone replacement therapy (HRT) is associated in cross-sectional studies with a decreased prevalence of lens opacities. We explored the associations of HRT and reproductive factors with the incidence and progression of lens opacities over a 2-year period. Data were derived from 1458 women ages 65 years and older from the Salisbury Eye Evaluation population-based prospective cohort study in Salisbury, MD, 1993-1997. HRT was not associated with incident nuclear, cortical, or posterior subcapsular opacities, or with progression of nuclear or cortical opacification. Women who had an early menopause had a higher risk of nuclear progression, whereas those who had a later menopause had a lower risk (linear trend P = 0.013). Other variables related to reproduction, such as oral contraceptive use, age at menarche, number of births, and history of hysterectomy, were not associated with any of the outcomes. These data suggest no evidence of protection against the incidence or progression of lens opacities with HRT.

Racial Differences of Lens Transparency Properties with Aging and Prevalence of Age-Related Cataract Applying a WHO Classification System

Purpose: To investigate racial differences of lens transparency properties and the prevalence of lens opacification by age. Methods: Lenses of randomly selected Asian (1,038 Japanese and 517 Singaporeans) and Caucasian (1,045 Icelanders) subjects were evaluated for their lens transparency property (LTP). The prevalence of lens opacification was determined with a newly proposed WHO cataract classification system. Results: LTP increased with aging for all nationalities. Light scattering intensity was significantly higher in the Singaporeans followed by the Icelandic subjects. The prevalence of cortical opacification in Singaporeans was significantly higher than those of the other nationalities up to the age of 60, and the prevalence in Asians was significantly higher than that in Caucasians aged 60–69. Cortical opacification was more prevalent than the other types for both Japanese and Icelanders in their 50s, 60s and 70s. Regarding the central optical zone, the prevalence in the Singaporeans was significantly higher than in the other two groups in their 50s and 60s. The prevalence of nuclear opacification in Singaporeans was markedly higher than those of the two other groups for all ages. Subcapsular cataract was the least prevalent type for all age groups and nationalities; however, the highest prevalence was observed in Singaporeans. Conclusions: Lens transparency decreased with age in the Singaporeans more markedly than in the other two groups. The high prevalence of nuclear opacification in Singaporeans is considered to be due to environmental rather than race-specific factors.

Risk factors for nuclear lens opacification: the Reykjavik Eye Study.

The purpose of this study is to examine risk factors for nuclear lens opacification in citizens of Reykjavik. 1,045 persons, 583 females and 462 males aged 50 years and older, were randomly sampled and underwent a detailed eye examination and answered a questionnaire. In all Scheimpflug photography of the anterior eye segment was done including the lens as well as retroilluminated photography of the lens. These photographs were used for the diagnosis of lens opacification. The data was analyzed using a logistic regression model. An increased risk for all grades of nuclear opacifications was found with ageing (OR = 1.228, 95% CI = 1.192-1.264, p = 0.000), cigarette smoking for more than 20 pack/years (OR = 2.521, 95% CI = 1.521-4.125, p = 0.000) and pipe or cigar smoking (OR = 2.478, 95% CI = 1.200-5.116, p = 0.014). Outdoor exposure, cortical lens opacification grade II and III and computer usage were not found to be linked to higher risk of nuclear opacification. No correlation was found between nuclear opacification and the consumption of vitamins, herring, sardines and shrimps, cod-liver oil or plant oil, nor were iris color, hyperopia, systemic steroid use, cardiovascular disease, diabetes, glaucoma and pseudoexfoliation found to have a significant effect. Ageing is a major risk factor for nuclear lens opacification, and smoking is a major modifiable risk factor. Cortical and nuclear lens opacifications do not share the same modifiable risk factors.

Cortical lens opacification in Iceland. Risk factor analysis -- Reykjavik Eye Study.

Cortical lens opacification has been associated with outdoor exposure and UV radiation more than other types of lens opacification. We studied risk factors for cortical lens opacification only, the most common as well as the earliest age related change we observe in the lens. 1,045 persons, 583 females and 462 males, 50 years and older, underwent a detailed eye examination and answered a questionnaire. Participants with cortical lens opacification grade I, totalling 374 persons, were assigned to case-control study I, and to case-control study II those with cortical lens opacification grades II and III, totalling 82 subjects. 378 age and sex matched persons served as controls. Those who spent more than 4 hours/day outside on weekdays, in their 20's--30's and 40's--50's respectively, were found to have increased risk of moderate to severe cortical lens opacification. Thus the relative risk for grades II & III, was 2.80 (95% CI 1.01--7.80) and 2.91 (95% CI 1.13--9.62) respectively. Ageing and systemic cortical steroids use were also found to be risk factors. Outdoor exposure appears to be associated with increased risk of moderate to severe cortical lens opacification. Ageing is, however, the main risk factor.

Author’s reply

The pathogenic mechanism(s) underlying the progressive nature of this unique form of anterior polar cataract remain uncertain. Dr. Christensen’s suggestion that this behavior is the result of a loss of cuboidal epithelium at the base of the pyramidal lesion is an equally valid interpretation of the histologic evidence presented in the article. It is our opinion that there were no differences in histologic findings between pyramidal and flat anterior polar cataracts, although the photomicrographs presented in the article do not fully illustrate this fact. Nevertheless, the concept that lens epithelial cell dysfunction is involved in pyramidal cataract formation and progressive cortical opacification is logically sound and entirely plausible. The authors also agree that the textbook reference cited in our discussion of anterior polar cataract histology may not have been ideally chosen, and we appreciate Dr. Christensen bringing an additional article to our attention and to that of the readers of Ophthalmology.

Pyramidal anterior polar cataracts

Objective To document clinical features and subsequent management of pyramidal anterior polar cataracts in children. Design Retrospective, noncomparative case series and clinicopathologic correlation. Participants Fifteen patients who presented to the pediatric ophthalmology clinic. Intervention All patients underwent measurement of visual acuity, assessment of ocular motility, examination of the anterior and posterior segments, and cycloplegic refraction. Amblyopia treatment was instituted when appropriate. When visual impairment occurred from cataract progression or amblyopia or both, cataract removal with or without lens implantation was performed. After surgery, correction of refractive error and treatment of amblyopia were instituted. Several pyramidal opacities were retrieved during cataract extraction and examined by light and electron microscopy. Main outcome measures Visual acuity at initial presentation, size of lens opacity before surgery, amblyopia status, most recent visual acuity after cataract extraction, and histologic examination of lens opacity. Results Nine children had bilateral and six had unilateral pyramidal cataracts (24 eyes). There was no discernible inheritance pattern. Patients were followed for 27 months on average. Twenty of 24 eyes developed cortical opacification that extended significantly beyond the base of the pyramidal lesion. Nineteen eyes required cataract surgery: 10 eyes underwent lensectomy with anterior vitrectomy and 9 had extracapsular cataract extraction, 8 of which had insertion of a posterior chamber intraocular lens. Amblyopia was present or developed in all six patients with unilateral cataract and in eight of nine patients with bilateral cataract. Visual acuity in many eyes remained poor despite amblyopia therapy. The pyramidal opacities consisted of hyperplastic lens epithelium, which exhibited a loss of polarity and was surrounded by a collagenous matrix. Conclusions Pyramidal anterior polar cataracts are present at birth and may represent a variant of anterior polar lens opacities. They may be unilateral or, if bilateral, they may be either symmetric or asymmetric. They consist of hyperplastic lens epithelium in a collagenous matrix. Patients with pyramidal cataracts are likely to develop amblyopia. This can result from either unilateral occurrence or asymmetry of bilateral opacities and is often worsened by surrounding cortical opacification. Many patients require cataract surgery. All infants and young children with anterior polar opacities showing this configuration should be followed for cataract progression and amblyopia.

Anthropometric status and cataract: the Salisbury Eye Evaluation project

Weight or body mass index (BMI; in kg/m2) is frequently identified as a risk factor for cataract, but the nature of the association is unclear. We aimed to characterize the relation between BMI and stature and risk of different types of cataract. We analyzed data from participants in the Salisbury Eye Evaluation (SEE), a cross-sectional survey of visual status and demographic, nutritional, and environmental factors conducted between 1993 and 1995 in a representative sample of community-dwelling older persons in Salisbury, MD. Multiple logistic regression techniques were used to compare risk factors between individuals with nuclear, cortical, or posterior subcapsular (PSC) opacities and individuals with no cataract. Risk of nuclear opacification was greater in participants with lower BMIs [adjusted odds ratio of 1.13 (95% CI: 1.02, 1.27) with a BMI of 22.5 compared with 28.0] and of taller stature [1.12 (95% CI: 1.01, 1.25) with a stature of 170.5 cm compared with 164]. In contrast, risk of cortical opacification was greater in participants with higher BMIs and of taller stature, but the relation for stature diminished in magnitude and was not significant after adjustment for other risk factors. BMI was not related to risk of PSC opacities, but there was some evidence that taller stature is a risk factor for PSC opacification (P = 0.06) after adjustment for other risk factors. Both BMI and stature are independent risk factors for cataracts in the SEE population, with the nature of the risk dependent on cataract type.

Epidemiological studies on UV-related cataract in climatically different countries.

Cataract epidemiological surveys applying objective judgement through lens images in the climatically different places of Noto and Amami, Japan, Singapore and Reykjavik, Iceland yielded several significant results about the influence of solar UV. 1) The percentage of transparent and of lens opacification was significantly higher in the Reykjavik subjects than in the Singaporeans. 2) The percentages including early changes were higher in Amami and Singapore than in Noto and Reykjavik. 3) Progressed lens opacification was highest in Singapore. While the main type of lens opacification was cortical in Noto and Reykjavik, that of Singapore was nuclear. 4) A significant correlation between cortical opacification and the history of time spent outdoors was noticed. The UV risk for formation and/or progression of cortical opacification should be acceptable from the epidemiological standpoint.

Racial differences in lens opacities: the Salisbury Eye Evaluation (SEE) project.

The purpose of this study was to determine racial differences in the prevalence of different types of lens opacities and cataract surgery. Between 1993 and 1995, the Salisbury Eye Evaluation (SEE) Project enrolled a representative sample of 2,520 community-dwelling persons aged 65-84 years in Salisbury, Maryland, 26.4% of whom were African-American. Participants received a full eye examination, and photographs were taken for documentation of lens status. Photographs were graded using a standardized grading system for the presence of cortical, nuclear, or posterior subcapsular cataract (PSC) opacification in at least one eye. The odds of having cortical opacities were 4.0 times greater among African Americans than among Caucasians (95% confidence interval (CI) 3.3-4.8). Caucasians were significantly more likely to have nuclear opacities (odds ratio = 2.1, 95% CI 1.7-2.6) and PSC opacities (odds ratio = 2.5, 95% CI 1.7-3.6). The odds of cataract surgery were 2.8 times higher among Caucasians, but these differences did not explain the differences in the prevalence of different types of lens opacities by racial group. With lower rates of nuclear and PSC opacities than Caucasians, African Americans may have a lower demand for cataract surgery. However, even with these differences, there is still significant unnecessary loss of vision due to cataract among older African Americans, for whom programs to ensure access to surgical care are indicated.

Calcium Ionophore Induced Proteolysis and Cataract: Inhibition by Cell Permeable Calpain Antagonists

Two dipeptide aldehyde cell permeable calpain inhibitors, cBz-Val-Phe and calpeptin, have been assessed for their ability to prevent cytoskeletal proteolysis and loss of transparency in whole rat lenses. Calcium overload, induced by ionomycin in artificial aqueous humor with 1mM calcium, resulted in lens opacification and degradation of cytoskeletal proteins including spectrin, filensin, and vimentin. No such changes resulted from incubation in ionomycin in the absence of calcium. In calcium overload lenses both inhibitors gave some protection against cytoskeletal protein degradation and loss of transparency. These experiments indicate that calpain has a role in cortical opacification in high calcium lenses and that cell penetrating calpain inhibitors do indeed enter lens cells and reduce both proteolysis and opacification.

Lens hardness in mature cataracts.

A cataract is said to be mature when all the cortical fibres become opaque. Depending on the pathophysiological processes that cause the cortical fibre opacification this phenomenon may be associated with a varying degree of nuclear sclerosis. A relationship between lens hardness and degree of nuclear sclerosis in non-mature cataracts has been demonstrated previously. This study aimed to investigate the relationship between the hardness of mature cataracts and the transmitted nuclear colour, age and rate of progression of the cataract. Thirty-eight patients with mature cataracts were assessed prior to extracapsular cataract surgery. The nuclear colour that was transmitted through the opaque cortex was graded using reference photographs. Age and duration of visual symptoms were recorded and lens hardness was measured by a specially designed lens guillotine. Multivariate analysis of data indicates a relationship between hardness of a mature cataract and the transmitted nuclear colour and age (adjusted R2 = 0.59). There is also a tendency for hardening of the lens as the duration of visual symptoms increases. By considering these clinical markers, the cataract surgeon can estimate the hardness of the lens and therefore its suitability for phacoemulsification.

Variation in Cataract Surgery Practice and Clinical Outcomes

To examine associations between surgical technique, patient and surgeon characteristics, and clinical outcomes of cataract surgery. Seventy-five ophthalmologists were recruited from three cities based on a sampling scheme stratified by surgeon-reported annual volume of cataract surgery. Seven hundred seventy-two patients undergoing first eye cataract surgery were enrolled, with complete preoperative, perioperative, and 4-month postoperative clinical data on 717 patients (93%). Sixty-five percent of surgery was performed by phacoemulsification and 35% by standard extracapsular (ECCE) techniques. Performance of ECCE was associated with the presence of ocular comorbidity and 21 or more years in practice of the surgeon. Performance of phacoemulsification was associated with annual volume of cataract surgery, wherein high-volume (201-399 patients annually) and very high-volume (> 400 patients annually) surgeons had 3.7 and 3.9 times the likelihood of performing phacoemulsification compared with moderate-volume (51-200 cases annually) surgeons. The rates of intraoperative, perioperative, and 4-month postoperative adverse events and the amount of improvement in visual acuity did not differ either by surgical technique or volume stratum. The reported occurrence of posterior capsular opacification within 4 months of surgery was increased in the presence of cortical opacification, one city, and patients operated on by either high- or very high-volume surgeons. In this cohort, no difference in clinical outcomes, as measured by change in visual acuity or occurrence of postoperative adverse events (except for posterior capsular opacification), can be attributed to performance of phacoemulsification versus ECCE or to the reported annual volume of cataract surgery of the surgeon. Self-reported high and very high annual volume of cataract surgery is associated independently with performance of phacoemulsification and surgeon's report of posterior capsular opacification at 4 months after cataract surgery.

Objective Measurement of Cortical and Subcapsular Opacification in Retroillumination Photographs

An image analysis system has been developed for the objective analysis of cortical opacification. Lens photographs are obtained using a Neitz CTR retroillumination camera. The images are digitized using an IBM-PC-based imaging system. We have developed a program, called OPAC, through which the computer analyzes the image using a circular grid consisting of 93 sectors. The program outputs the percent of lens area which is opaque. Ninety-eight photographs, previously classified by the Lens Opacities Classification System (LOCS II), were analyzed using OPAC as well as a manual, square-grid counting system (GRID). The results show a linear correlation between OPAC and GRID with a correlation coefficient of 0.94. The results obtained with OPAC are also linearly correlated with the LOCS II classifications.

Accelerated heavy particles and the lens. V. Theoretical basis of cataract enhancement by dose fractionation.

That accelerated heavy ions are highly cataractogenic is indisputable. The basis of heavy particle effectiveness and the augmentation by fractionation, as recently demonstrated, remains less clear. There is no question, however, that these are tied to the 'track structure' and densely ionizing nature of the radiation. The unique energy deposition characteristics relating to charge and track structure are now being explored to begin to dissect the cellular response of the lens epithelium to radiation exposure. The elucidation of the basis of the cataractogenic effect of accelerated heavy ions is important not only to risk assessment, but also in the consideration of theories of radiation action and the mechanism of cortical opacification arising from a myriad of cataractotoxic agents.