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PubMed1995

Animal models of emmetropization: matching axial length to the focal plane.

BACKGROUND: It has long been recognized that more people are emmetropic than would be expected from a random combination of the refractive and axial components of the eye. However, it has been difficult to determine whether this is the result of an active emmetropization mechanism. METHODS: This paper reviews some of the studies in animals that have been conducted during the past 20 years. Four basic paradigms have been used to determine whether the visual environment helps guide eyes to emmetropia: 1) observing the normal pattern of ocular development, 2) shifting the location of the focal plane with minus- (and plus-) power lenses, 3) removing focused images by visual form deprivation and, 4) restoring form vision after a period of visual deprivation. RESULTS: Data from many studies suggest that an active emmetropization mechanism guides the postnatal development of the eye, matching the axial length to the focal plane. In normal development, the axial length initially is generally short so that the photoreceptors are in front of the focal plane of the unaccommodated eye. The subsequent axial elongation eventually moves the photoreceptors to, but not past, the focal plane. When animals are raised with the focal plane shifted posteriorly with minus-power lenses, the eyes elongate to approximately match the displaced focal plane. When information about the location of the focal plane is removed by visual deprivation, the eyes elongate past the point of emmetropia and become myopic. When developing eyes that have become myopic from a brief period of form deprivation are re-exposed to patterned images, they can slow their axial elongation, gradually eliminating the myopia. Data from several species suggest that the axial length is regulated within the eye itself, involving direct, spatially local communication from the retina to the sclera. It also appears that the regulation of axial elongation involves active control of the scleral extracellular matrix. CONCLUSIONS: If humans have a similar mechanism, then successful emmetropization in children may involve two components. One is to inherit a fully functional emmetropization mechanism. Equally important is exposure to a "normal" visual environment. Deficiencies in either, or an interaction between a compromised mechanism and a non-optimal visual environment might also prevent emmetropization.

Accommodation, Ocular↗
PubMed2003

Neural pathways subserving negative lens-induced emmetropization in chicks--insights from selective lesions of the optic nerve and ciliary nerve.

PURPOSE: Active emmetropization describes the process by which young eyes regulate their growth to eliminate refractive errors. The purpose of this study was to re-investigate the role of the brain in compensation to imposed hyperopic defocus (negative lenses), specifically, to assess whether a retina-brain link and/or an intact ciliary nerve are required for this emmetropizing response. Data from previous related studies are equivocal. METHODS: Unilateral lesion surgery involving either or both optic nerve section (ONS) and ciliary nerve section (CNS), was performed on 2-3 day old White-Leghorn chicks to interrupt communication between the eye (retina in the case of ONS) and brain. After a recovery period of 4 days, lesioned eyes were fitted with either -5 or -15 D lenses or diffusers (6-9 per group). An additional lesion group underwent unilateral CNS and was fitted with -5 D lenses bilaterally. Finally 3 groups that underwent the same unilateral optical treatments but no surgery were included as controls for analyzing lesion-induced changes. Complete sets of measurements, involving retinoscopy for refractive errors, and high frequency A-scan ultrasonography for axial ocular dimensions, were made at the beginning (baseline), and end of a 4 day treatment period. Additional ultrasonography data were collected after 1 and 2 days of treatment. Optical treatment effects were expressed as changes in interocular differences from baseline values. RESULTS: All three lesions produced hyperopic shifts in refraction (evident in baseline values), although this effect was minimal for the ONS+CNS group. Choroidal thickening as well as increased anterior chamber depth and lens thinning were observed in all cases but vitreous chamber depth was reduced in only the ONS group. In response to the -5 D lens, the control (nonlesioned) group showed nearly complete compensation, while full compensation was not achieved to the -15 D lens over this short treatment period. The diffuser group showed the largest change, which was also in the direction of myopia. Both the ONS and CNS groups showed near normal compensation, as indexed by the changes in refractive errors relative to their respective baseline values. In contrast, the ONS+CNS lens groups overcompensated, by 130% and 54% for the -5 D and the -15 D lens groups respectively. Form deprivation responses were slightly exaggerated in both ONS and ONS+CNS groups, the latter group again showing the largest response. Enhanced vitreous chamber growth was evident under all conditions and correlated well with the refractive changes across the groups. DISCUSSION: The data imply that an intact retina-brain link is not required for compensation to hyperopic defocus and thus emmetropization. However, the data also imply interactions between higher centers and the eye. The emmetropization set-point appears to be recalibrated after ONS surgery. The data also indicate a role of the ciliary nerve as an important conduit for signals that exercise a restraining influence on eye growth.

Accommodation, Ocular↗
PubMed1984

Myopia induction in animals following alteration of the visual input during development: a review.

In this review the effects of changes in the quality of the visual environment on the development of myopia during eye growth in various mammalian and avian species are described. The effect of changes in the light/dark cycle on myopia development has been studied only in the avian eye, mainly that of the domestic fowl. In the eyes of chicks reared from hatching to maturity under continuous illumination, the following findings were reported: myopia, astigmatism, increases in axial length and equatorial width, shallow anterior chamber, increase in corneal diameter, reduction of corneal curvature, increased intra-ocular pressure (IOP), low outflow facility, reduction in aqueous space, buphthalmos, macrophthalmos and glaucoma. The above mentioned changes were consistent in the majority of the studies. In a few experiments where a change in one of the above mentioned parameters was not found, no tendency for the opposite condition was reported. When the illumination level of the visual environment was changed in the avian eye there was an increase in the total size of the eye as well as exophthalmos. Other parameters were not examined. It is quite possible that myopia and eye enlargement in avians are caused by entirely different processes than myopia and eye enlargement in mammals since they can be induced either by changes in the diurnal rhythm or by low intensity light. The involvement of the pineal gland in the control of eye growth in avians is therefore possible. The effect of continuous dark rearing was studied in the avian and in the mammalian eye. In developing chicks reared in continuous darkness some enlargement of the eye took place, but a condition of hyperopia was found as opposed to the expected myopia. This result is in agreement with the results of experiments performed on monkeys and cats reared from infancy to adulthood in complete darkness. The effect of near vision conditions during growth was studied in monkeys, cats and chicks confined to small chambers, cages or rooms. A slight myopia was usually obtained in all of the above species but the incidence of myopia increased consistently in the experimental animals compared to the normal controls. The effect of optically restricting the visual field during growth was studied in chicks. Using special occluders, the eyes of the chicks were exposed only to the frontal fields of vision. This manipulation induced a considerably high myopia and an increase in the axial length of the eye and in the depth of the anterior chamber. Removal of the occluders resulted in a reversal of the induced myopia.(ABSTRACT TRUNCATED AT 400 WORDS)

Accommodation, Ocular↗
PubMed2002

Baseline refractive and ocular component measures of children enrolled in the correction of myopia evaluation trial (COMET).

PURPOSE: To describe baseline refractive and ocular component measures in children with myopia enrolled in the Correction of Myopia Evaluation Trial (COMET). COMET is a multicenter, randomized clinical trial to evaluate whether progressive-addition lenses slow the progression of juvenile-onset myopia compared with single-vision lenses. METHODS: Four hundred sixty-nine children with myopia between -1.25 and -4.50 D spherical equivalent and without eye or systemic conditions known to affect refractive development were recruited from four geographically and ethnically diverse communities in the United States. Their ages were 6 to 11 years inclusive, and 52% were girls. The main outcome measure for the overall trial is progression of myopia determined by cycloplegic autorefraction after inducement of cycloplegia with 2 drops of 1% tropicamide. Axial length, the secondary outcome measure, was assessed by ultrasonography. The distance correction was determined by subjective methods before cycloplegia, with noncycloplegic autorefraction values as the starting point. RESULTS: Because data were similar in both eyes, they are reported for the right eye only. The mean spherical equivalent measured by cycloplegic autorefraction was -2.38 +/- 0.81 D. Young children had significantly less myopia than older children (P = 0.03), but the amount of myopia did not differ by gender or ethnicity. Mean axial dimensions were 4.0 +/- 0.2 mm (anterior chamber), 3.4 +/- 0.2 mm (lens), 16.8 +/- 0.7 mm (vitreous chamber), and 24.1 +/- 0.7 mm (axial length). Girls' eyes had significantly shorter axial length than boys' (P < 0.0001). Mean corneal radii were 7.73 +/- 0.25 mm (horizontal) and 7.59 +/- 0.24 mm (vertical). Ninety-five percent of the eyes had a ratio of axial length to corneal radius higher than 3.0. CONCLUSIONS: These baseline measures provide cross-sectional data on a large group of ethnically diverse children with myopia. Refractive and axial component dimensions are consistent with data in other studies showing that myopic eyes have longer vitreous chambers than emmetropic eyes. The measures reported herein will serve as a basis for examining changes that occur over a minimum of 3 years of follow-up of children enrolled in COMET.

Accommodation, Ocular↗
PubMed1980

Craniopharyngioma simulating bilateral internal ophthalmoplegia.

A 15-year-old girl had rapid onset of an apparent bilateral internal ophthalmoplegia. Subsequent evaluation revealed a large craniopharyngioma. It is uncommon for a mass to cause such eye findings and unique for a craniopharyngioma to manifest in this fashion.

Accommodation, Ocular↗
PubMed1998

Accommodation amplitude after lens refilling with injectable silicone by sealing the capsule with a plug in primates.

OBJECTIVE: To restore accommodation in primate eyes by refilling the lens capsule with injectable silicone compounds. MATERIALS AND METHODS: Eight eyes of 8 monkeys (Macaca fascicularis) were treated by the lens refilling procedure. To prevent leakage of the injected liquid silicone before it polymerized in the capsule in vivo, a silicone plug for sealing the capsular opening was developed. After endocapsular phacoemulsification following an upper minicircular capsulorhexis, the plug was introduced into the capsulorhexis opening. A silicone mixture was injected into the capsular bag through the delivery tube of the plug. Automated refractometry was performed 1 week and 3 months after surgery. Accommodation amplitude was determined as the difference between the refractions before and 1 hour after topical application of 4% pilocarpine chloride. RESULTS: Five of 8 eyes could be refilled. In 4 of 5 eyes, refraction could be measured. Accommodation amplitude ranged from 1.0 to 4.5 diopters, with a mean of 2.3 +/- 1.3 diopters (8.0 +/- 2.0 preoperative values). At 3-month examination, thick posterior capsule opacification precluded refractometry in all eyes. CONCLUSIONS: The lens refilling procedure with the use of a silicone plug for sealing the capsular opening was feasible in primate eyes. The accommodation amplitude attained was a small fraction of the value before surgery. This may result from the loss of so-called intracapsular accommodation, ie, active participation of lens fiber cells in accommodation. However, since the obtained accommodation may be sufficient for near vision after cataract surgery, this lens refilling procedure warrants further study. Elucidation of the mechanism of intracapsular accommodation may also be necessary.

Accommodation, Ocular↗
PubMed1998

The distance angle to target in surgery for intermittent exotropia.

BACKGROUND: Patients with intermittent exotropia may have an increase in their angle of strabismus in the distance when the angle is measured either after 1 hour of monocular occlusion or while the patients fixate on a distant target outdoors. The hypothesis that surgery should be performed for this larger deviation has been suggested but not tested. OBJECTIVES: To test the hypothesis that surgery should be performed for the increased angle of strabismus in the distance in patients with intermittent exotropia and to investigate the factors that influence the angle of misalignment. METHODS: A prospective, clinical trial was conducted of patients with intermittent exotropia in whom the angle of misalignment in the distance increased after 1 hour of monocular occlusion or while the patients fixated on an outdoor target. The study group underwent surgery for the largest deviation measured; the control group underwent surgery for the initial angle measured at 6 m. All patients in whom the angle of misalignment increased while the patients were looking at an outdoor target were additionally measured in indoor illumination at 24 m and also at 6 m under floodlights that simulated outdoor illumination. Ninety patients undergoing surgery were randomized. RESULTS: Forty-three (86.0%) of the 50 patients undergoing surgery for the largest angle measured had a satisfactory outcome vs 25 (62.5%) of the 40 patients in the control group (P<.001). The mechanism for the increase in exotropia while fixating on an outdoor target was studied in 76 patients, and the results were variable. CONCLUSIONS: The angle of strabismus in patients with intermittent exotropia undergoing surgery should be measured while the patients fixate on an outdoor target and after 1 hour of monocular occlusion. Surgery should be performed for the largest angle measured.

Accommodation, Ocular↗
PubMed1998

Distance/near differences in intermittent exotropia.

BACKGROUND: Burian's classification of exotropia based on the difference between the distance deviation and near deviation (distance/near differences) leaves some questions unanswered. Controversy exists concerning whether the divergence excess pattern is caused by an excess of divergence or by excessive accommodative convergence. Much of the literature on this subject has been confusing because investigators did not eliminate tenacious proximal fusion as an artifact in calculating the ratio of accommodative convergence to accommodation (AC/A ratio). Previously, one of us (B.J.K.) proposed a classification system that respected this artifact and subdivided the classification system proposed by Burian. METHODS: A total of 202 consecutive patients with an exotropia underwent a series of measurements to determine the respective role of accommodative convergence and tenacious proximal fusion as a cause for their distance/near differences. In addition, the value obtained by a rapid prism adaptation test as a possible substitute for 1 hour of monocular occlusion was studied. RESULTS: In 98 patients, the initial distance deviation exceeded the near deviation. In 10 patients, the distance/near differences were caused by a high AC/A ratio, which would have been mislabeled by Burian's classification system. Brown's recommendation of using +3.00-diopter lenses at near to diagnose simulated divergence excess would have led to the misdiagnosis of a high AC/A ratio in 61 of these patients. In 26 patients, the near deviation exceeded the distance deviation. Burian's classification would have incorrectly labeled 2 patients as having convergence insufficiency when, in fact, they had pseudoconvergence insufficiency. The new proposed classification system proved 100% sensitive and 100% specific (6 of 6 patients for both parameters) for identifying preoperatively exotropic patients who postoperatively developed an esotropia at near with a high AC/A ratio. Rapid prism adaptation tests at near proved useful for identifying the presence of tenacious proximal fusion, but were not accurate in its quantification. CONCLUSIONS: The validity and utility of the new classification system was confirmed. Identification of exotropic patients with a high AC/A ratio and consideration of nonsurgical treatment is important. The rapid prism adaptation test is qualitatively, but not quantitatively, the same as 1 hour of monocular occlusion.

Accommodation, Ocular↗
PubMed1998

Effects of H-7 on the iris and ciliary muscle in monkeys.

OBJECTIVES: To determine the effects of H-7 on (1) iris and ciliary muscles (CMs) in living monkeys; (2) isolated monkey CM strips; (3) actomyosin contractility in cultured Swiss 3T3 cells. METHODS: (1) Pupillary diameter (calipers) and accommodation (refractometer) in living monkeys were measured after topical, intracameral, or intravitreal administration of H-7 followed by systemic pilocarpine hydrochloride. (2) Pilocarpine-induced contraction of isolated monkey CM strips following administration of H-7 was measured in a perfusion chamber. (3) Actomyosin contractility in Swiss 3T3 cells cultured on thin silicone rubber film was determined by measuring cell-induced film wrinkles before and after administration of H-7. RESULTS: Topical H-7 prevented anesthesia-induced miosis but did not affect resting refraction. Intracameral or intravitreal H-7 dilated the pupil and inhibited miotic but not accommodative responses to pilocarpine. H-7 inhibited pilocarpine-induced contraction of isolated monkey CM strips and reduced Swiss 3T3 cell contraction. CONCLUSIONS: H-7 inhibits actin-based contractility in non-muscle cells and in monkey iris sphincter and CM. Under our in vivo experimental conditions, the effect on the iris predominates over that on the CM.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗
PubMed1999

Diagnosis and treatment of exotropia with a high accommodation convergence-accommodation ratio.

BACKGROUND: Patients with exotropia often have a slow-to-dissipate fusional mechanism at near, which masks the true near deviation. Consequently, determination of the accommodation convergence-accommodation (AC/A) ratio in patients with exotropia must be based on near measurements obtained after prolonged monocular occlusion (typically 1 hour). When determined in that manner, the presence of a high AC/A ratio before surgery in an exotropic patient has been reported to be predictive of an esotropia at near after surgery. OBJECTIVE: To investigate the diagnosis and management of exotropia with a high AC/A ratio. METHODS: Three hundred four consecutive patients with exotropia were studied. In addition to the usual measurements, measurements were obtained at near after 1 hour of monocular occlusion, with and without additional +3.00-diopter lenses. Also, a gradient AC/A ratio was obtained by using additional minus lenses at distance fixation. RESULTS: One hundred fifty-four (50.7%) of 304 patients would have been thought to have a high AC/A ratio if that diagnosis was based on measurements obtained before prolonged monocular occlusion. In fact, only 22 patients (7.2%) actually had a high AC/A ratio; 132 patients (43.4%) had a pseudo-high AC/A ratio. Six of 22 patients with a high AC/A ratio underwent surgery to correct the exotropia. The presence of a high AC/A ratio before surgery had sensitivity, specificity, and positive and negative predictive values of 100% for predicting a postoperative esotropia at near associated with a high AC/A ratio. The remaining 16 patients with high AC/A ratios were treated with overcorrecting minus lens therapy (including a bifocal). Ten of them have been followed up to at least 18 years of age, by which time 9 have shown normalization of the AC/A ratio. CONCLUSIONS: Near measurements used to calculate the AC/A ratio in exotropic patients must be made after prolonged monocular occlusion. Otherwise, many patients with a pseudo-high AC/A ratio will be thought to have a true high AC/A ratio. The presence of a high AC/A ratio is infrequent in patients with esotropia, but it is highly predictive of a postoperative esotropia at near fixation.

Accommodation, Ocular↗
PubMed1999

Does overcorrecting minus lens therapy for intermittent exotropia cause myopia?

BACKGROUND: Overcorrecting minus lens therapy has been used as a treatment for intermittent exotropia. It is based on the principle that an exotropic deviation will be decreased by stimulating accommodative convergence with additional minus power in spectacles. Because excessive accommodation has been implicated as a cause of myopia, there is theoretical concern that overcorrecting minus lens therapy for exotropia may cause myopia. OBJECTIVE: To investigate the effect of overcorrecting minus lens therapy for exotropia on the progression of myopia. DESIGN: A retrospective chart review. SUBJECTS AND METHODS: Seventy-four patients with intermittent exotropia were treated with overcorrecting minus lens therapy for at least 6 months (6-month treatment group), and a 34-patient subset of them received overcorrecting minus lens therapy for 5 years (5-year treatment group). The mean change in refractive error (spherical equivalent of the fixing eye) of these 2 groups 5 years after initial examination was compared with the mean change in refractive error of a control group of 45 patients with intermittent exotropia who did not receive overcorrecting minus lens therapy. RESULTS: At the time of initial examination, the mean (+/-SD) refractive error was 0.00 +/- 1.40 diopters (D) in the control group, 0.00 +/- 1.50 D in the study group, and -0.10 +/- 1.50 D in the 5-year study group, all of which were essentially identical. Five years after initial examination, the mean change in refractive error was -1.40 +/- 2.80 D in the control group, -1.52 +/- 1.80 D in the 6-month treatment group, and -1.54 +/- 1.80 D in the 5-year treatment group. These differences in the change in refractive error (myopic shift) were not statistically significant (t test), and the differences are clinically unimportant. CONCLUSION: Overcorrecting minus lens therapy for intermittent exotropia does not appear to cause myopia.

Accommodation, Ocular↗
PubMed2001

Fifteen-year outcome of surgery for the near angle in patients with accommodative esotropia and a high accommodative convergence to accommodation ratio.

OBJECTIVE: To determine the 15-year outcome of patients with partly accommodative esotropia with a high accommodative convergence to accommodation (AC/A) ratio who underwent surgery based on the angle of esotropia at one-third meter while wearing full hyperopic correction. METHODS: A retrospective chart review to determine the 15-year outcome of 25 patients whose 6-month outcome had been previously reported as part of a prospective, randomized, masked clinical trial. All patients had partly accommodative esotropia with a high AC/A ratio and underwent surgery based on their esotropia at one-third meter while wearing full-distance optical correction. RESULTS: Fifteen years after surgery, 19 of the 22 patients for whom follow-up data are available had between 0 and less than 10 prism diopters of esotropia. Only 6 of the 19 needed to continue to wear optical correction to maintain satisfactory alignment; however, 8 more needed spectacles for visual purposes. Only 1 patient needed to use a bifocal add to have satisfactory alignment at one-third meter. All patients showed some degree of sensory fusion, with 4 obtaining 40 seconds of stereopsis and another 8 obtaining between 60 and 200 seconds of stereopsis. CONCLUSION: Surgery for the near angle obtained with patients wearing their full hyperopic distance correction provides excellent motor and sensory results in patients with partly accommodative esotropia with a high AC/A ratio.

Accommodation, Ocular↗
PubMed2003

Theoretical and measured pseudophakic accommodation after implantation of a new accommodative posterior chamber intraocular lens.

OBJECTIVE: To analyze different techniques of measuring accommodation after implantation of a new accommodative posterior chamber intraocular lens (PCIOL). METHODS: In this comparative, nonrandomized interventional study, we analyzed 15 eyes of 15 patients (aged 44-84 years) at 6 months after cataract surgery and PCIOL implantation (Akkommodative 1CU; HumanOptics AG, Erlangen, Germany) and compared these results with those of an age-matched control group (n = 15). We used the following methods to measure accommodation: dynamic measurement with objective (videorefractometry [PowerRefractor; PlusOptix, Erlangen] and streak retinoscopy) and subjective (subjective near point [push-up test and accommodometer] and defocusing) techniques, as well as static measurement of the change in anterior chamber depth (ACD) using the IOLMaster (Zeiss, Jena, Germany) after pharmacological stimulation using 2% pilocarpine eye drops. MAIN OUTCOME MEASURES: Theoretical accommodation calculated from the forward shift of the lens optics (decrease of ACD) using paraxial geometrical optics and measured accommodation amplitude. RESULTS: Accommodation amplitude (mean +/- SD; range; median) results after 6 months in the study and control groups were as follows: 1.00 +/- 0.44; 0.75-2.13; 1 diopter (D); and 0.35 +/- 0.26; 0.10-0.65; 0.25 D, respectively, using the PowerRefractor; 0.99 +/- 0.48; 0.13-2.00; 0.88 D; and 0.24 +/- 0.21; -0.13-0.75; 0.25 D, respectively, using retinoscopy; 1.6 +/- 0.55; 0.50-2.56; 1.7 D; and 0.42 +/- 0.25; 0.00-0.75; 0.50 D, respectively, using subjective near point; and 1.46 +/- 0.53; 1.00-2.50; 1.75 D; and 0.55 +/- 0.33; 0.25-0.87; 0.50 D, respectively, using defocusing. Anterior chamber depth decreased in the study and control groups as follows: 0.78 +/- 0.12; 0.49-1.91; 0.65 mm; and 0.16 +/- 0.09; 0.00-0.34; 0.18 mm, respectively, after applying 2% pilocarpine eyedrops, indicating an accommodation of 1.16 +/- 0.22; 0.72-1.88; 1.05 D vs 0.22 +/- 0.13; 0.00-0.47; 0.23 D (P =.001). CONCLUSIONS: Accommodation after implantation of a presumably accommodative PCIOL can be measured with clinical methods or derived from the biometric data of the eye and the measured ACD decrease using geometrical optics. For clinical purposes, pseudophakic accommodation should be assessed with a variety of different techniques, including subjective and objective measurements. The theoretical approach using geometrical optics may be an additional indicator for the accommodative response in patients with pseudophakic eyes and may allow a subdivision of the measured accommodation into true pseudophakic accommodation and pseudoaccommodation.

Accommodation, Ocular↗