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PubMed2002

Numerical modelling of the accommodating lens.

Data on geometric and material properties of the human lens derived from various published sources are used to construct axisymmetric, large displacement, finite element models of the accommodating lens of subjects aged 11, 29 and 45 years. The nucleus, cortex, capsule and zonule are modelled as linearly elastic materials. The numerical model of the 45-year lens is found to be significantly less effective in accommodating than the 29-year lens, suggesting that the modelling procedure is capable of capturing at least some of the features of presbyopia. The model of the 11-year lens shows some anomalous behaviour, and reasons for this are explored.

Accommodation, Ocular↗
PubMed2001

Experimental increase in accommodative potential after neodymium: yttrium-aluminum-garnet laser photodisruption of paired cadaver lenses.

PURPOSE: Loss of lens elasticity is one of several proposed mechanisms responsible for the decline in accommodation with age and is the most accepted explanation for presbyopia. We wish to confirm the lens elasticity premise and attempt to experimentally reverse the age-dependent loss of accommodative potential as measured by polar strain. DESIGN: Experimental human autopsy eye study. PARTICIPANTS AND CONTROLS: Thirty-six cadaver lenses were tested to determine the age-dependent polar strain. Eleven lens pairs were then tested with one lens treated with neodymium:yttrium-aluminum-garnet (Nd:YAG) laser and the other left untreated before rotation as an age control. TESTING: Using a custom-made rotational apparatus (described by Fisher, 1971), freshly excised cadaver lenses (<48 hours postmortem) were rotated at 1000 rpm on a 9-mm diameter pedestal to simulate the physiologic pull of the zonules. Lenses were initially tested to determine the age-dependent polar strain. One lens in a pair was then treated with an Nd:YAG laser and the other left untreated before testing. Treatment consisted of 100 suprathreshold pulse placed in a central annular pattern of 2- to 4-mm diameter. Treatment energies varied from 2.5 to 7.0 mJ/pulse, depending on the relative clarity of the lenses. Polar strain was both microscopically measured and calculated from projected photographs before and after rotation of both lased and unlased lenses. Statistically significant differences were determined by paired t test. MAIN OUTCOME MEASURES: Polar strain (decrease in axial thickness with rotation) of the lens. RESULTS: An age-dependent decrease in polar strain was observed that paralleled the findings of Fisher. Both measured and projected polar strain were greater in the lased than unlased lens, and this difference was highly significant by paired t test (P = 0.001 and P = 0.004, respectively). CONCLUSIONS: Age-dependent loss of lens elasticity (polar strain) can be experimentally reversed (increased) by selective intralenticular photodisruption.

Accommodation, Ocular↗
PubMed2001

Laser in situ keratomileusis monovision.

PURPOSE: To evaluate the outcome of monovision after laser in situ keratomileusis (LASIK). SETTING: Private practice, Little Silver, New Jersey, USA. METHODS: In this retrospective study, 432 consecutive patients (864 eyes) who had LASIK between January 1998 and October 1999 with a follow-up of 6 to 28 months were examined. All patients older than 40 years were asked to complete a questionnaire, and study results were based on analysis of data from the patient charts correlated with the questionnaire responses. RESULTS: Women chose monovision over full distance correction more than twice as often as men (P =.00). Although the patients who had full distance correction reported somewhat higher levels of postoperative satisfaction than the monovision patients, the differences were not statistically significant (P =.41). Overall, 96% of patients were satisfied with monovision after LASIK, and 4% chose enhancement to full distance correction. CONCLUSION: Monovision after LASIK was a valuable and viable option for the management of presbyopia.

Adolescent↗
PubMed1994

Refractive changes induced by electrocautery of the rabbit anterior lens capsule.

The pathologic basis for presbyopia is classically attributed to lenticular sclerosis or atrophy of the ciliary muscle, but recent work suggests that thickening and loss of elasticity of the anterior lens capsule play an important role. As no practical means for attenuating or reversing the aging process of lens protein has been identified, alteration of the lens capsule eventually might prove to be a desirable alternative to spectacle correction in presbyopic individuals. This paper describes changes in the refractive properties of the lens resulting from alteration of the anterior lens capsule by application of focal cautery, using both an in vitro, in situ and an in vivo rabbit model. In vitro thermal treatment (electrocautery) of the capsule significantly increased the anterior curvature of the lens by an average of +2.95 diopters. Histologic examination of the treated lenses showed thinning of the capsule in the treated areas, as well as focal vacuolar degeneration in the lens substance beneath the lesions. In vivo thermal treatment of eyes induced a significant shift toward myopia, compared with control eyes. The accommodative range increased post-treatment relative to the controls, but the effect diminished over time, stabilizing near baseline at two to three weeks after treatment. Histologic examination showed localized changes but no signs of diffuse cataract formation. We conclude that the anterior capsule may play a significant role in the refractive power and accommodative changes in the crystalline lens.(ABSTRACT TRUNCATED AT 250 WORDS)

Accommodation, Ocular↗
PubMed1983

Twins living apart test: progress report.

A new approach is proposed in twin research based on the study of twins who, though reared together, have subsequently lived apart for a period of at least five years. With respect to the more powerful study of twins reared apart, the twins living apart test has the advantages of being more realistic and affording easier access to sufficiently large samples. In this pilot study, the test has been applied to a sample of 92 monozygotic pairs now aged 35-45; 15 pairs were still living together and 77 had lived apart for over five years. As a first approach, comparisons have been made, in the cotwins of the two subsamples, with respect to the following traits: height, weight, presbyopia, presbyacusia, alcohol consumption, tobacco consumption, blood pressure (systolic and diastolic), dental caries, and hours of sleep.

Adult↗
PubMed2002

Accommodation dynamics as a function of age.

Experiments are described in which the dynamic accommodation response to a stimulus whose vergence was varying sinusoidally with time between 1.33 and 2.38 D was measured as a function of frequency for 19 subjects, with ages distributed between 18 and 49 years. Response to abrupt stimulus change between the same levels was also measured. The results showed that at any age, for the sinusoidal stimuli, gain fell with frequency and phase lag increased: at fixed frequency, gain fell with age and phase lag increased. Neither reaction nor response times for step stimuli changed with age. Analysis of the sinusoidal data suggests the possible existence of a cut-off frequency of about 2 Hz, which varies little with age and above which the system cannot respond. The phase data is compatible with the existence of a frequency-independent time delay, which increases with age from about 0.17 s at 20 years to 0.48 s at 40 years. The results are discussed in terms of current knowledge of the factors contributing to the development of presbyopia.

Accommodation, Ocular↗
PubMed1999

Causes and prevalence of non-vision impairing ocular conditions among a rural adult population in sw Uganda.

Information is scanty about the extent of ocular morbidity in developing countries, particularly for non-vision impairing conditions (NVIC), although these constitute the majority of consultations in eye clinics. We have conducted a survey to determine the prevalence and causes of NVIC in a Ugandan adult population and compared our findings with the work pattern of the district hospital. Adults were screened using Snellen's illiterate E chart and those found with visual impairment (acuity less than 6/18) in either eye were referred to a low-vision clinic, and those with obvious ocular disease or symptoms, but without visual impairment, to an outreach clinic. A total of 2886 (53%) out of 5479 adults were screened. Of these, 257 (8.9%) were referred to the outreach clinic, of whom 173 (67%) attended. Of those attending the low-vision clinic 83% had visual impairment confirmed, and 92% of those attending the outreach clinic were confirmed not to have visual impairment. The four commonest NVIC observed at the outreach clinic were: presbyopia (48%), allergic conjunctivitis (20%), early cataract (9%) and infective conjunctivitis (8%), the same conditions as those most commonly seen at the district hospital. In this community, over 80% of NVIC are caused by four conditions which are potentially either correctable, operable or treatable. Training health workers to recognise and treat these conditions would serve the great majority of eye patients. Hospital activity data can be used cautiously to assess the relative frequency of NVIC in the community.

Adolescent↗
PubMed1990

Is the quality of vision with contact lenses adequate? Not in all instances.

There are several circumstances in which contact lenses (CLs) provide vision inferior to that possible with spectacles. Over time, both soft and rigid gas-permeable (RGP) lenses accumulate deposits that can cloud vision. RGP lenses are especially prone to scratching; some varieties may even become crazed, giving a spiderweb quality to vision. Certain uncommon corneal complications associated with lens wear--infection, neovascularization, ulcers, and severe epithelial edema--may also impair vision, sometimes permanently. CLs often provide less than optimal vision for patients with presbyopia or severe astigmatism. Difficulties with axis stabilization arise with both soft and rigid toric lenses, and these can take several attempts to resolve. Monovision fitting currently offers the best option for presbyopes, but it has an effect on stereopsis, compromising depth perception. Furthermore, patients often require considerable time before adapting to monovision lens use. Finally, even under normal circumstances, certain inherent features of CLs can diminish visual acuity. The water in soft lenses diffuses light; the loss of visual crispness is particularly notable with high-water-content lenses. On the other hand, the small diameter of RGP lenses often contributes to ghosting--a phenomenon in which the edge of the lenses is visible whenever the pupils are widely dilated.

Astigmatism↗
PubMed1981

Spectacles for the emmetropic presbyopic optometrist.

Presbyopia brings special seeing problems for an emmetropic optometrist. Incorporating the bifocal into a half-glass offers several advantages. The best parameters for this spectacle form are derived from experience, and also by comparing it to single-vision half-glasses, bifocals, and trifocals. Particular facility in both prescribing and fabricating derives from the great number of power and size relations between upper and lower segments available. One can extrapolate similar benefits in this spectacle form for other occupations and hobbies as well as for presbyopic wearers of single-vision contact lenses.

Allied Health Personnel↗
PubMed1985

Bifocal adds in Nigeria.

A total of 447 female and 915 male presbyopic patients were examined in the Benin-City, Nigeria optometry clinic. Comparison of bifocal adds with published data from other areas of the globe suggests that climatic temperature has less effect on the age of onset of presbyopia than other authors have claimed.

Adult↗
PubMed1986

Suppression behavior analyzed as a function of monovision addition power.

Monovision, the use of a monocular addition for near viewing, is a clinical technique sometimes used to correct presbyopia. This technique attempts to maintain binocular function while requiring a degree of central suppression. We examined suppression behavior in spectacle monovision as a function of addition power. Suppression behavior was determined by measuring the length and frequency of periods of suppression for a range of monocular addition powers. Testing was conducted at viewing distances of 6 m and 40 cm. The most striking result is the existence of a near/distance differential: the monocular addition power required to stabilize suppression at near is greater than at distance. It was also found that suppression behavior may be stable even in the presence of a relatively high level of stereoscopic acuity. The relation between the contradictory needs for suppression and binocular functioning under monovision conditions is discussed.

Adult↗
PubMed1986

Bifocal adds and environmental temperature.

Eight hundred and eighty-eight optometrists of North and Central America were asked to provide data on their typical bifocal addition (add) for 45-, 50-, 55-, and 60-year-old males and females. Analysis of the 577 responses shows no significant difference of adds with the mean annual temperatures of the optometrists' communities at any of the four age levels. The mean annual temperatures in the survey ranged from -7 degrees Centigrade (19 degrees Fahrenheit) in Alaska to 26 degrees Centigrade (79 degrees Fahrenheit) in Puerto Rico. Previously reported inverse correlations are explained in terms of a self-selection bias by patients at the earlier presbyopic age levels when age of onset criteria are used instead of more advanced presbyopic age levels when all persons seek ophthalmic services. The mean adds for females at all four age levels were between 0.05 and 0.06 D higher than for males, theoretically attributable to differences of physical stature rather than of age of onset or advancement of presbyopia.

Accommodation, Ocular↗
PubMed1988

Stereopsis in presbyopes wearing monovision and simultaneous vision bifocal contact lenses.

One of the advantages suggested for contact lens bifocals over monovision treatment of presbyopia is that there may be less impairment of binocular function. We evaluated nearpoint stereoposis on the Titmus stereotest for 10 presbyopic patients. Testing was done under five conditions; monovision and binocular correction with each of four marketed simultaneous bifocal contact lenses. Simultaneous vision bifocals tested produced at least as much reduction in stereopsis as monovision compared to baseline spectacle correction. Repeat testing of bifocal stereopsis with best near over-refraction suggested that a substantial portion of the stereo reduction could be attributed to insufficient effective adds with the bifocal contacts.

Contact Lenses↗
PubMed1991

Binocular accommodative facility in prepresbyopic adults and its relation to symptoms.

Previous studies measuring accommodative facility have been confined to young adults and children. Standard test procedures were used to measure interactive facility (i.e., binocular accommodative flip test) in a sample of 45 adults from 30 to 42 years of age. The results showed a marked reduction in the average number of cycles per minute performed for the +/- 2.00 D flip test. Symptoms in the sample were also tabulated and their relation with interactive facility investigated. We found no direct relation between symptoms and accommodative facility. Further studies are needed to establish normative data for accommodative facility for adults approaching presbyopia.

Accommodation, Ocular↗
PubMed1992

Role of visual acuity, stereoacuity, and ocular dominance in monovision patient success.

Monovision (MV) contact lens correction of presbyopia induces substantial reductions in stereoacuity and small reductions in binocular visual acuity (VA). This study examined those effects in a group of successful and a group of unsuccessful MV patients. Compared to performance with a full binocular correction, the unsuccessful group demonstrated significant losses in both functions. These reductions were smaller in the successful group and of marginal statistical significance only for stereoacuity. Laterality of sighting dominance and laterality of distance/near correction had minimal effect on the results.

Adult↗
PubMed1993

Effects of target distance and pupil size on letter contrast sensitivity with simultaneous vision bifocal contact lenses.

Using simulated presbyopia we measured the effects of target vergence and pupil size on letter contrast sensitivity while subjects wore accurately centered simultaneous vision bifocal contact lenses. These experiments examined the increase in depth of focus provided by two bifocal contact lenses, and the effects of pupil size on near and distance vision. Monocular contrast sensitivity was assessed with three different designs of soft contact lenses. First, as a single vision control, we measured contrast sensitivity with a Bausch & Lomb, Optima 38 lens corrected for distance. We then measured contrast sensitivity with an Allergan Optical, Hydron Echelon, which is a diffractive simultaneous bifocal and a CIBA Vision, Spectrum, which is a concentric design, center-near, two-zone simultaneous vision bifocal. In general both bifocal contact lenses improved contrast sensitivity at near but decreased contrast sensitivity at distance (when compared to the single vision lens). With a fixed 3.5-mm artificial pupil, the Echelon lens had two clear peaks of sensitivity, one at distance (0.00 D) and one at near (-2.00 D), but the Spectrum lens had no clear peaks. Performance of the Spectrum lens depended on its center zone size. Near (-2.00 D) and distance (0.00 D) contrast sensitivity was measured with pupil sizes that ranged from 1 to 6 mm. The results showed that visual performance with a diffractive lens was almost independent of pupil diameter up to 3 to 4 mm but, as predicted, performance with the two-zone lens depended critically on pupil size.

Contact Lenses, Hydrophilic↗
PubMed1999

Comparison of suppression, stereoacuity, and interocular differences in visual acuity in monovision and acuvue bifocal contact lenses.

PURPOSE: The relationship between visual acuity and stereoacuity has been well documented: as binocular visual acuity increases, stereoacuity improves. We compared interocular differences in visual acuity and stereoacuity in two presbyopic soft contact lens modalities, monovision and a new soft bifocal contact lens, the Acuvue Bifocal. The Acuvue Bifocal is hypothesized to show a smaller interocular acuity difference, increased stereoacuity, and decreased suppression over monovision at distance and near. METHODS: Monovision patients wearing Acuvue or Surevue soft contact lenses were tested for visual acuity, stereoacuity, and suppression at distance and near. Stereoacuity was tested with the Randot Stereotest (near) and the BVAT (distance). Suppression was evaluated with the Acuity Suppression Vectogram (near) and the BVAT (distance). Patients were then fit with the Acuvue Bifocal in each eye. After wearing the lenses for 1 week, the same tests of visual acuity, stereoacuity, and suppression were performed. RESULTS: The mean interocular acuity difference (IAD) at distance with monovision was 0.712 logarithm of the minimum angle of resolution (logMAR) (SD = 0.275) and 0.188 logMAR (SD = 0.252) (p < 0.001) with the Acuvue Bifocal. At near, the mean IAD with monovision was 0.420 logMAR (SD = 0.183) and 0.137 logMAR (SD = 0.147) (p < 0.001) with the Acuvue Bifocal. Of the monovision subjects, 89% (17 of 19) demonstrated suppression at near while only 26% (5 of 19) did with the bifocal lenses (statistically significant at p < 0.001). Stereoacuity at near improved from a median of 200 sec arc with monovision to 50 sec arc with the bifocal lenses. CONCLUSIONS: In this study, correcting presbyopia with the Acuvue Bifocal versus monovision resulted in a statistically significant decrease in the interocular difference in visual acuity at distance and near. The decreased interocular difference in visual acuity improved certain aspects of binocularity as demonstrated by a decrease in suppression and an increase in stereoacuity.

Contact Lenses, Extended-Wear↗
PubMed2001

Ultrasound biomicroscopy of the aging rhesus monkey ciliary region.

Ultrasound biomicroscopy of the living rhesus monkey ocular ciliary region was undertaken to identify age-dependent changes that might relate to the progression of presbyopia. Monkeys were anesthetized and pharmacologically cyclopleged, the eyelids were held open with a lid speculum, and sutures were placed beneath the medial and lateral rectus muscles. Ultrasound biomicroscopy imaging of the nasal and temporal quadrants of the eye were performed, and the live images were recorded to videotape. Subsequent image analysis was performed to obtain objective morphometric measurements of the ciliary body region. The ciliary body inner radius of curvature, outer radius of curvature, inner arc length, area, thickness, perimeter, zonular fiber length, and circumlental space were measured. Zonular space was calculated. The circumlental space decreased with increasing age in the temporal quadrant. The other morphologic measurements were not significantly correlated with age or body weight. Most morphologic measurements were significantly different comparing temporal vs. nasal quadrants. Bifurcation of the posterior zonular fibers was frequently observed. Although temporal circumlental space was the only measurement found to change with age, ultrasound biomicroscopy of the living rhesus ciliary region did identify distinct nasal vs. temporal asymmetries, which may reflect anatomical requirements for convergence-associated accommodation.

Accommodation, Ocular↗