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PubMed2005

Comparison of the accommodation theories of Coleman and of Helmholtz by finite element simulations.

PURPOSE: The accommodation process of the human eye is still a controversial subject. Coleman assumes that the lens, together with the zonula fibers, forms a diaphragm which is held in a catenary shape due to the pressure difference between the aqueous and vitreous body of the lens. The aim of the paper is to compare the results of two simulations (according to the Helmholtz and to the Coleman theories) with ultrasonographic data. METHODS: An axisymmetric static finite element model of the lens was generated using the literature data for geometry, material properties and loads. The refractive power of the lens was calculated for two different ages (29 and 45 years). RESULTS: The application of a pressure to the posterior lens surface did not yield an increase in refractive power change during accommodation. Rather a decrease in accommodation related refractive power was found. CONCLUSIONS: Physiologically relevant refractive power changes are obtained by a simulation in accordance with the Helmholtz theory. A simulation in accordance with the Coleman theory does not yield physiological values of refractive power change.

Accommodation, Ocular↗
PubMed2005

Age related changes in the characteristics of the near pupil response.

Static and dynamic aspects of the near pupil response were studied in human subjects in the age range when accommodative amplitude steadily declines. Dynamic accommodative and pupillary responses to step stimuli were recorded in 66 subjects (ages: 14-45 years). Exponential fits to data provided amplitude, peak velocity and time constants. Accommodative amplitude decreased linearly with age (p < 0.05). Pupil constriction per diopter of accommodative response increased exponentially with age (p < 0.05). The amplitude of pupil constriction for a 2D stimulus decreased linearly with age (p < 0.05) and for a 5D stimulus did not change with age (p = 0.90). The latency of pupil constriction did not change with age (p = 0.65), while the mean peak velocity decreased linearly with age (p < 0.05). An increase in the amount of pupil constriction per diopter of accommodative response, but not per diopter of stimulus amplitude, suggests that the near effort per se does not increase with age. There is a slight reduction in the speed of near pupil response with age.

Accommodation, Ocular↗
PubMed2005

"Bothersome blur": a functional unit of blur perception.

Knowledge regarding the amount of blur perceived to be "bothersome" to an individual, namely that which is assumed to be annoying and to adversely affect task performance, remains limited. A Badal optical system was used to measure the blur detection, bothersome blur, and non-resolvable blur dioptric thresholds monocularly either to an isolated 20/50 or 20/200 Snellen E, or to three 20/50 lines of text. Subjects were comprised of 13 visually normal young adults and 3 absolute presbyopes. Cycloplegia was used to paralyze accommodation in the young adults. Within each target type for the young adults, the mean bothersome blur threshold was always significantly larger than that found for blur detection and significantly smaller than that found for non-resolvable blur. Across target types and blur criteria, the bothersome blur thresholds for the isolated 20/50 E (1.02 D) and the 20/50 text (1.34 D) were not significantly different, although in 12 of the 13 subjects the latter were larger (p<0.002, sign test). However, both were significantly smaller than for the isolated 20/200 E (1.80 D). In a subset of young adult subjects, bothersome blur was found to be repeatable over time. The results were similar in the absolute presbyopes. The bothersome blur threshold was primarily influenced by target detail and secondarily by target extent. These findings have important implications with respect to tolerances for optical lens design and refractive surgery outcomes, as well as provide insight into basic aspects of human blur perception.

Accommodation, Ocular↗
PubMed2006

In vitro dimensions and curvatures of human lenses.

The purpose of this study was to determine dimensions and curvatures of excised human lenses using the technique of shadowphotogrammetry. A modified optical comparator and digital camera were used to photograph magnified sagittal and coronal lens profiles. Equatorial diameter, anterior and posterior sagittal thickness, anterior and posterior curvatures, and shape factors were obtained from these images. The data were used to calculate lens volumes, which were compared with the lens weights. Measurements were made on 37 human lenses ranging in age from 20 to 99 years. These showed that lens dimensions and the anterior radius of curvature increase linearly throughout adult life while posterior curvature remains constant. The relative shape (or aspect ratio) of the posterior lens is unchanged through adult life since both equatorial diameter and posterior thickness increase at the same rate. The ratio of anterior thickness to posterior thickness is constant at 0.70. It is suggested that in vivo forces alter the apparent location of the lens equator, that the in vitro lens shape corresponds to the maximally accommodated shape in vivo and that the shapes of the accommodated and unaccommodated lens progressively converge toward each other due to lens growth with age, with a convergence point located near the age of total loss of accommodation (55-60 years). Together, these observations provide additional support for the Helmholtz theory of accommodation.

Accommodation, Ocular↗
PubMed1995

Age-related response of human lenses to stretching forces.

Five human lenses of varying ages were subjected to radial stretching forces which mimic the action of the ciliary muscle in vivo. Although the number of lenses was small, it was found that the human lens becomes more resistant, with age, to radial stretching forces and that lenses over the age of 50 years showed very little response to stretch. In prepresbyopic lenses, stretching has a greater effect on the shape of the anterior surface than it does on the shape of the posterior surface. The five lenses studied showed individual variations in lens size and shape and a trend to increased curvature with age, particularly marked for the anterior surface.

Accommodation, Ocular↗
PubMed2005

[Persistent visual problems thirty years after severe head trauma].

INTRODUCTION: Visual disturbances are common after severe head trauma. Some authors report 50 percent of these patients having damage to the visual system, ocular motor nerve palsies, or central eye movement disturbances. Visual disturbances influence prognosis and outcome of neurological rehabilitation, and, in most cases, diagnosis and treatment require interdisciplinary care. CASE REPORT: We report here a 50-year-old patient who suffered from severe head trauma at age 20. After coming out of a 4-week coma, he suffered visual disturbances such as blurred vision and diplopia. Visual acuity was 1.0 and the visual fields were intact. The symptoms remained unexplained for a long time. One year after the trauma, right trochlear palsy was diagnosed and surgical treatment was performed. However, the symptoms persisted, and one year later, a second operation was performed without changing the symptoms. During many years, prisms or refractive corrections were used to improve the visual disturbances, and finally, a psychiatric treatment was started. At age 40, the patient became presbyopic and his visual problems increased, especially for reading. When he was examined for the first time in our consultation, corrected visual acuity was 1.0 with a small hyperphoria of the right eye. During careful examination of the eye downgaze, a conjugate pure torsional nystagmus was observed, which disappeared in primary gaze position. CONCLUSION: Pure torsional nystagmus is difficult to diagnose, especially when it is provoked only by one gaze direction such as in our patient. As a sequel of the severe head trauma with brainstem contusion, this type of nystagmus is a rare form of central vestibular nystagmus, and may by modified by head rotation or suppressed by convergence. The "treatment" of the patient's blurred vision when reading was easy by ordering goggles with a near part placed near the primary gaze position and a 6 base down prismatic correction. Reading was then possible without provoking torsional nystagmus.

Coma↗
PubMedDate not supplied

Compensation procedures for the anisometropic presbyope.

When changing to bifocal lenses from distance anisometropic lenses, presbyopic patients may experience diplopia when attempting to do close work. Prescription failure can be dramatically reduced by utilizing the procedures outlined in this article. Why would a patient experience diplopia when attempting to read with bifocals? The diplopia may simply be a result of the optical difference in power between the two lenses. Anisometropia creates a disparity in the size of the retinal image as well as a prismatic imbalance when looking away from the optical center. A practical approach to coping with these problems is recommended.

Accommodation, Ocular↗
PubMedDate not supplied

Visual problems of the aging musician.

The presbyopic musician presents the ophthalmologist with an often demanding set of visual needs. Many modalities, some necessarily creative, are available to aid older musicians in their vision-dependent art.

Aging↗
PubMed2000

Why we need reading-glasses before a zimmer-frame.

Accommodative loss represents the fastest human biological decrement. Why? To examine this, the shape of the eye-lens during accommodation or its relaxation is analysed with special reference to age-related changes. The capsule is viewed as a force transmitter: the distribution of forces along centripetal capsular arcs and the resulting stresses and strains are calculated. The effect of zonular 'shifts' is considered. Age-related accommodative loss can be modelled successfully if changes in mechanical properties are linked to those in lenticular and capsular shape.

Accommodation, Ocular↗
PubMed2003

Changes in the internal structure of the human crystalline lens with age and accommodation.

Scheimpflug images were made of the unaccommodated and accommodated right eye of 102 subjects ranging in age between 16 and 65 years. In contrast with earlier Scheimpflug studies, the images were corrected for distortion due to the geometry of the Scheimpflug camera and the refraction of the cornea and the lens itself. The different nuclear and cortical layers of the human crystalline lens were determined using densitometry and it was investigated how the thickness of these layers change with age and accommodation. The results show that, with age, the increase in thickness of the cortex is approximately 7 times greater than that of the nucleus. The increase in thickness of the anterior cortex was found to be 1.5 times greater than that of the posterior cortex. It was also found that specific parts of the cortex, known as C1 and C3, showed no significant change in thickness with age, and that the thickening of the cortex is entirely due to the increase in thickness of the C2 zone. With age, the distance between the sulcus (centre of the nucleus) and the cornea does not change. With accommodation, the nucleus becomes thicker, but the thickness of the cortex remains constant.

Accommodation, Ocular↗
PubMed2001

Monovision outcomes in presbyopic individuals after refractive surgery.

PURPOSE: To characterize monovision outcomes and patient satisfaction with conventional monovision (dominant eye corrected for distance) and crossed monovision (dominant eye corrected for near) in presbyopic individuals after excimer laser photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK). DESIGN: Retrospective observational case series. PARTICIPANTS: One hundred forty-four consecutive patients, 45 years or older, who were treated with excimer laser refractive surgery between December 1995 and June 1998. METHODS: Patients in whom the surgical outcome was monovision (MV) (distance vision spherical equivalent [SE] -0.50 to +0.50 diopter (D), near vision SE -3.75 to -1.00 D and anisometropia 1.00 D or greater), crossed MV (dominant eye corrected for near vision and the nondominant eye for distance vision) and full correction (bilateral SE -0.50 to +0.50) were identified. Data were abstracted and analyzed statistically. MAIN OUTCOME MEASURES: Preoperative and postoperative visual acuity and refraction. Patient satisfaction with monovision RESULTS: Forty-two patients had surgical outcome of MV. In MV patients, the average distance vision SE, near vision SE, and anisometropia were -0.04 +/- 0.27 D, -1.95 +/- 0.70 D, and 1.92 +/- 0.74 D, respectively. Patient satisfaction was 88% with MV. Twelve patients attained crossed MV. All patients with crossed MV were satisfied with their vision. Patient satisfaction with MV showed no relationship to gender, age at initial surgery, preoperative trial of monovision, laterality of treatment, type of monovision, or predictability of outcomes. CONCLUSIONS: Monovision may be a valuable option for presbyopic individuals considering refractive surgery. Crossed monovision can result in satisfactory visual outcomes.

Cornea↗
PubMed2002

Multifocal intraocular lens implantation in prepresbyopic patients with unilateral cataract.

OBJECTIVE: To evaluate the benefits of implantation of a zonal-progressive multifocal intraocular lens (IOL) in prepresbyopic patients with unilateral cataract. STUDY DESIGN: Prospective, nonrandomized, comparative trial. PARTICIPANTS: Ninety-five eyes of 95 prepresbyopic patients aged between 14 and 40 years with either multifocal or monofocal IOL implantation at two institutions and with more than 6 months follow-up. INTERVENTIONS: Temporal clear corneal phacoemulsification and foldable IOL implantation was performed in all eyes. In 54 patients, a zonal-progressive optic multifocal IOL (Array SA40-N, Allergan, Irvine, CA) was implanted, whereas 41 patients received a monofocal IOL and served as controls. RESULTS: Preoperative patient demographics, mean postoperative spherical equivalent, astigmatism, and uncorrected and best-corrected distance visual acuities were similar in the two groups. Patients with a multifocal IOL achieved a significantly better uncorrected near visual acuity than patients with monofocal IOL (0.18 versus 0.37; P = 0.0001). With distance correction only, mean near visual acuity was 0.17 versus 0.43 (P = 0.0001). Best-corrected near visual acuity was 0.11 for both groups (P = 0.91), with +1.43 diopters (D) for the multifocal group and +2.35 D for the monofocal group (P = 0.0001). Spectacle dependency differed significantly between the two groups, with 21 patients (51%) of the monofocal group commonly requiring an additional plus add for near tasks compared with 5 patients (9%) in the multifocal group (P = 0.001). Stereopsis was superior in the multifocal group (P < 0.001), with 42 (77%) and 33 (61%) patients with a multifocal IOL responding positively to the Lang and Titmus tests, respectively. In the monofocal group; only 20 (48%) and 9 (22%) patients gave correct answers. CONCLUSIONS: Multifocal IOL implantation is a viable alternative to monofocal pseudophakia in prepresbyopic patients with unilateral cataract.

Adolescent↗
PubMed1979

Prescribing glasses for myopia.

Myopia is a one-symptom refraction problem: blurring of distance vision. Correction of the myopia should eliminate this symptom, but a prescription for glasses may, in fact, produce other symptoms equally--or more--disturbing. A few of the more common pitfalls in prescribing for the myopic patient are: (1) failure to recognize accommodative stress masquerading as low myopia; (2) confusion as to when, or whether, to prescribe for the unilateral myopic patient; (3) overcorrecting the myopic refractive error and failure to appreciate the symptoms that result from overcorrection; (4) difficulties in gaining acceptance of bifocals by the myopic presbyopic patient; (5) appropriate selection of bifocal segments; and (6) acquired myopic anisometropia caused by sclerosis of the crystalline lens. Although myopia would appear to be the least troublesome of all refractive problems, the maximizing of visual potential with comfort and safety needs refracting skills, experience, and clinical judgement.

Aniseikonia↗
PubMed1987

Contact lenses.

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Astigmatism↗