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Biomedical subjects

M Millodot

Publications and source records attributed to M Millodot.

At least 19 recordsLinked to original sources
PubMed1989

Presbyopia correction and the accommodation in reserve.

One method of determining the additional correction for presbyopia suggests leaving a percentage of the amplitude of accommodation in reserve. The rationale for this assumption seems logical because using all of the available accommodation is not sustainable without discomfort. However there is no empirical evidence indicating what percentage of the amplitude of accommodation should actually be left in reserve. Common figures adopted have been one-half and one-third. In this investigation the percentage of accommodation used is deduced mathematically after having determined the following: 1. The 'add' by the direct subjective clinical method. 2. Measured the amplitude of accommodation. 3. Measured the reading distance in 305 presbyopes ranging from 40 to 83 years of age. The results showed a small decline in the amplitude of accommodation up to the age of 52, after which age the measurements were scattered about a steady level. This finding suggests that after the age of 52 the results are based on the depth-of-focus of the eye. Females had slightly greater accommodation than males of the same age. The power of the add was significantly correlated to the age of the subject. The mean percentage of accommodation used for the 305 subjects was found to be 50.7%, thus confirming the rule of leaving half of the accommodation in reserve, although there were large variations: there were differences between males and females and with age the percentage of measured accommodation used, after having determined the correct add, diminished. Similarly the percentage of accommodation also decreased for shorter reading distances.

Accommodation, Ocular↗
PubMed1988

Differences in adaptation of tonic accommodation with refractive state.

It has recently been demonstrated that intersubject variations in tonic (dark focus) levels of accommodation are related to corrected refractive state (McBrien and Millodot, 1987). The aim of the present study was to investigate the effect of sustained visual tasks on the tonic level of accommodation in different refractive groups. Eleven hyperopes, 16 emmetropes, ten early onset myopes and ten late onset myopes had their tonic accommodation measured with the objective infrared optometer Canon Autoref R-1 before and after a 15 min sustained visual counting task. The post-task tonic accommodation level was monitored for 15 min to assess the decay rate of any observed task-induced changes in tonic accommodation. Subjects repeated the experimental procedure for four task locations (6 m, pre-task tonic position, 37 cm and 20 cm). Late onset myopes showed significant positive (myopic) changes in their tonic level of accommodation at both near viewing distances, which showed no evidence of decay during the 15 min post-task monitoring period. Hyperopes, however, underwent transient "counteradaptive" decreases in their tonic level of accommodation after sustained near viewing. Emmetropes and early onset myopes showed little change in tonic levels at the two near distances. Differences between groups were also obtained at tonic and far viewing distances. Post-task changes in tonic accommodation demonstrated only a weak negative correlation with pre-task tonic accommodation levels at each task distance. It is proposed that the observed differences in adaptation of tonic accommodation among refractive groups may be related to variations in autonomic innervation of the ciliary muscle.

Accommodation, Ocular↗
PubMed1987

A biometric investigation of late onset myopic eyes.

There is a paucity of biometric information on late onset myopia (onset after 15-years-old). The present investigation compared the ocular component dimensions of 30 late onset myopes and 30 emmetropes, who were both age and sex matched. Measurement techniques included A-scan ultrasonography and keratometry. The findings revealed that the ultimate cause of late onset myopia is vitreous chamber elongation. Late onset myopes were also found to have significantly deeper anterior chambers and thinner crystalline lenses, no differences being found in corneal curvature measures. Possible mechanisms leading to these observed biometric differences are discussed in the light of recent findings.

Adolescent↗
PubMed1987

The relationship between tonic accommodation and refractive error.

It is now well established that accommodation adopts an intermediate resting position in the absence of visual stimulation. It also has been shown that this tonic position of accommodation exhibits considerable variation among individuals (ie, 0-4 D). Previous studies have been somewhat inconclusive as to whether these individual differences in tonic accommodation are related to the refractive properties of the eye. This study investigates the relationship between tonic accommodation and refractive error with particular reference to both the value and the time course of tonic accommodation. Sixty-two subjects were used for the study, with an age range of 19-25 yr. Subjects were placed into one of four refractive groups, after considering factors pertaining to the theories of refractive development. Darkroom measures of tonic accommodation were determined using the infrared objective autorefractor, Canon Autoref R-1. Significant differences were found among the four refractive groups, with corrected hyperopes having the highest dioptric value of tonic accommodation and corrected late-onset myopes having the lowest dioptric value. The significant difference found between early- and late-onset myopes is suggested as a possible reason for the inconclusive results of some previous studies. It also was found that the time taken to reach a stable tonic position of accommodation was much slower for the hyperopic group than for the other refractive groups. The results are discussed in the light of recent findings on accommodative hysteresis and with respect to a dual innervation to the ciliary muscle.

Accommodation, Ocular↗
PubMed1986

Amplitude of accommodation and refractive error.

This paper investigates the amplitude of accommodation of early onset myopes, late onset myopes, emmetropes, and hyperopes. Eighty subjects were used, 41 women and 39 men, with an age range of 18-22 yr (mean = 19.82). All subjects wore an optimum refractive correction, giving visual acuities of 20/20 or better. Significant differences were found between the four refractive groups, with late onset myopes having the largest amplitude of accommodation, followed by early onset myopes, emmetropes, and hyperopes. The results are discussed in the light of previous findings on tonic accommodation and the accommodative response gradient and interpretated in the context of a dual innervation to the ciliary muscle.

Accommodation, Ocular↗
PubMed1986

The effect of refractive error on the accommodative response gradient.

The accommodative responses of early-onset myopes, late-onset myopes, emmetropes and hyperopes were measured over a range of 5 dioptres using an objective infra-red autorefractor. Differences were found between the four refractive groups, with hyperopes accommodating more for near targets than emmetropes, followed by early-onset myopes then late-onset myopes. Moreover a strong correlation between the accommodative response gradient and refractive error was found, suggesting that hyperopes accommodate more to a particular target than do emmetropes or myopes. The results are interpreted in the context of a dual innervation to the ciliary muscle.

Accommodation, Ocular↗
PubMed1985

Clinical evaluation of the Canon Autoref R-1.

A clinical evaluation of the Canon Autoref R-1 was carried out on 93 subjects (186 eyes). Results were compared to clinical refractive data. A measure of the R-1 reliability was obtained both for successive measurements and measurements repeated on separate occasions. The validity of the instrument was found to be similar to that found in other investigations.

Adolescent↗
PubMed1985

Variation of astigmatism with accommodation and its relationship with dark focus.

The refraction of 122 eyes was measured for five distances of fixation with an objective Canon Auto Refractor. The cylindrical component was found to vary, but by a small amount. We found no evidence of a deliberate reduction in astigmatism with accommodation, in accord with other studies. The axis was also found to vary by at least 5 degrees in about half of the subjects tested, which is also in good accord with other investigations. The dark focus of 31 eyes was not found to correlate with a minimum astigmatic distance, since this only occurred in 11 out of the 31 eyes. For these 11 eyes, the correlation of dark focus and the dioptric distance where astigmatism was minimum was equal to + 0.54. It implied that for those eyes (about one-third of all types) there was a tendency to yield the best optical quality at the dark focus.

Accommodation, Ocular↗
PubMed1979

Eyelid closure causes myopia in humans.

Humans with unilateral ptosis are more myopic in the closed eye than in the normal eye. The effect seems unrelated to visual loss. In pairs of humans ranked for sleep and myopia, the more myopic tends to sleep the more. We conclude that eyelid closure causes myopia.

Adolescent↗
PubMed1979

Measurement of corneal sensitivity and thickness with PMMA and gas-permeable contact lenses.

Corneal sensitivity and thickness were measured in nine subjects. Two different types of hard contact lenses were fitted to each subject; a traditional polymethylmethacrylate (PMMA) lens on one eye and a gas-permeable cellulose acetate butyrate (CAB) lens on the other eye. Corneal sensitivity did not differ significantly between the two eyes after 6 and 10 hr of wear, but PMMA lenses tended to induce slightly more corneal edema than CAB lenses. As oxygen tension behind such contact lenses is different, whereas mechanical stimulation is nearly the same, this would appear to support the view that corneal sensitivity changes are related to mechanical stimulation. However, careful scrutiny of the data does not support this view unequivocally, and other suggestions are made.

Acetates↗
PubMed1978

Effect of long-term wear of hard contact lenses on corneal sensitivity.

The effect of years of wear of hard contact lenses on corneal sensitivity has been evaluated. By testing 91 subjects who had worn lenses for up to 22 years and a control group of 42 people, it was found that corneal sensitivity diminished exponetially. However, in five subjects who had abandoned use of their lenses, recovery occurred within four months, indicating that the process is reversible. The loss of corneal sensitivity with hard contact lens wear ought to be heeded, since the eye is thus at a greater risk of infection.

Adult↗
PubMed1978

The discrepancy between retinoscopic and subjective measurements: Effect of age.

Comparison of the retinoscopic and subjective measurements of 1078 eyes of all ages showed that the discrepancy (retinoscopy--subjective) was positive and equal to 0.3--0.4 D in young eyes. The discrepancy diminished with age, eventually becoming negative in the 6th decade of life. The results are explained by postulating that retinoscopic light is reflected from the internal limiting membrane in young eyes and from a layer posterior to the photoreceptors in older eyes.

Accommodation, Ocular↗
PubMed1978

The discrepancy between retinoscopic and subjective refraction: effect of light polarization.

To identify the surface from which the retinoscopic reflex originates, retinoscopy was performed on 305 eyes of various ages with polarized and depolarized light. The discrepancy between retinoscopic and subjective refraction measures in the two conditions shows that two fundus reflective layers contribute to retinoscopy: a specularly reflecting layer presumably at the retina/vitreous interface and a diffusely reflecting layer near the pigment epithelium. In young eyes the retina/vitreous interface predominates; the other predominates in older eyes.

Adolescent↗