FIELD OF THE INVENTION

The invention generally relates to methods and devices for treating refractive error in an eye. Particular embodiments may have application to influencing the rate of progression of refractive error, by slowing the progression of refractive error and/or preventing the progression of refractive error. Particular embodiments may have application to the control of progression of myopia. Other embodiments may have application to the control of the progression of hyperopia.

BACKGROUND

Myopia (short-sightedness) is a disorder of the eye in which distant objects cannot be clearly focused, but near objects can be. Images of distant objects are brought to focus in front of the retina; that is, the focusing power of the eye is too strong ‘at distance’. The condition may be corrected by the use of a negatively powered lens, which causes the distant images to focus on or nearer the fovea. Myopia can be a serious and progressive condition that leads to increasing visual impairment despite the use of corrective lenses. It is becoming increasingly common, with some countries in South-East Asia reporting that 80% of children aged 17 years suffer from the condition.

Hyperopia (long-sightedness) is a disorder where distant objects can be focused, but near objects cannot be focused. Hyperopia may be corrected by the use of positive power lenses.

It is generally agreed that the process of normal eye development—emmetropization—is regulated by a feedback mechanism, which regulates the length of the eye to maintain good focus both at distance and at near—or emmetropia. While it is also generally agreed that this feedback mechanism is somehow disturbed in eyes with refractive error, so that the eye grows too long in myopia and not long enough in hyperopia, there has not been consensus about the nature of the feedback mechanism or how the progression of myopia and hyperopia can be controlled. Both biochemical and optical (focal defect) mechanisms have been suggested.

While it is generally assumed that the feedback stimulus is somehow related to focal defects of the eye, the matter cannot be simple because, in progressive myopia, the condition may become worse—i.e., the eye continues to lengthen excessively—even though lenses that correct for distance vision are worn.

It has been proposed that an optical feedback mechanism is somehow upset by deficiencies in the accommodative effort of the eye due to excessive near work. The deficiency is considered to manifest as lag of accommodation (imprecise and insufficient accommodation) in some myopic eyes at near, resulting in defocus, which stimulates further undesirable axial elongation of the eye.

Bifocal lenses and PALs (progressive addition lenses) in spectacles have been proposed as a possible way to relieve the accommodative stress and defocus in the hope that the stimulus for elongation would be removed. U.S. Pat. No. 6,752,499 (Alter) describes prescribing commercially available bifocal contact lenses for myopic eyes that also exhibit near point esophoria to control the progression of myopia. Both concentric distance center and near center contact lenses were employed in myopic eyes with near point esofixation disparity. The zones of the concentric distance and near zones lie within the pupil.

In U.S. Pat. No. 6,045,578 (Collins et al.) it is proposed that emmetropization is regulated by the degree and direction of a spherical aberration present at the fovea. It is proposed that young myopes have higher levels of negative spherical aberration that promotes inappropriate eye growth and that the use of ophthalmic lenses to impart positive spherical aberration will counteract axial growth and thus the progression of myopia.

In international patent publication number WO 200604440A2 (Phillips et al) it is proposed that defocus at the fovea for both distance and near vision inhibits the feedback stimulus for excessive eye growth. This publication proposes the use of a bi-focal contact lens that simultaneously provides the central retina with (a) clear vision for both distance and near and (b) myopic defocus for both distance and near. Again, the visual image will be degraded if this proposal is used.

U.S. Pat. No. 7,025,460 (Smith et al) describes the results of animal trials demonstrating that it is the optical state of the peripheral retina, not the fovea, that dominates the feedback stimulus for emmetropization. Thus, Smith et al, proposes that controlling off-axis focal points of the eye relative to the central on-axis focal points through manipulation of the curvature of field of the visual image provides a method of abating, retarding or controlling the progression of myopia and hypermetropia (impaired near vision caused by insufficient eye length).

FIGS. 1 and 2show how Smith et al describe an eye1that has a positive curvature of field. Since the central on-axis image point2is located in front of (i.e. opposite to the direction of light) the retina3, this eye is considered myopic when measured using standard techniques such as auto-refractors, refractor-heads or trial frames, in the manner that eye-care practitioners are familiar. In this representative eye, the off-axis peripheral image points4for large field angles are located behind (i.e. in the direction of light) the retina3. Thus the eye1of this example is relatively hypermetropic for the peripheral visual field.FIG. 2shows a relative field curvature graph for the eye1, which shows that the central to mid-peripheral field5is myopic (focus in front of retina3) but the mid-peripheral to far-peripheral field6is hypermetropic (focus behind retina3).

FIGS. 3 and 4show how the eye1ofFIGS. 1 and 2is prescribed an optical device7for the control of the progression of myopia, as described by Smith et al. This optical device7is designed so that it would generate a negative relative curvature of field8on the eye1. The central, on-axis image9is focused sharply to the fovea11enabling good visual acuity. The optical device7introduces sufficient negative relative curvature of field8to focus the peripheral image points13more anteriorly, or in front (i.e. in the direction against the direction of light in the eye) of the retina3.

U.S. Pat. No. 7,665,842 (Ho et al) describes providing one or more vision priority zones in which peripheral defocus or another aberration is corrected, with the lens also controlling the relative curvature of field for the peripheral retina in another region outside of the vision priority zone(s).

There remains a need for methods and optical devices that address the suggested optical (focal defect) mechanisms of progression of refractive error, particularly in relation to the progression of myopia.

SUMMARY

The present invention relates to the utilization of the finding that refraction in central and peripheral areas of the retina of human eyes, or at least the eyes of a substantial or significant number of people, is such that the natural curvature of field of the visual image is asymmetric around the visual axis of the eye. In more detail, the invention relates to the control of off-axis focal points of the eye relative to the on-axis focal points in an asymmetric manner through the use of an optical device that controls refraction of light for the eye.

For a patient with myopia, a method of treatment may therefore include measuring the refractive characteristics of the eye at the fovea (in other words on-axis or at the central retina) and outside the fovea (in other words off-axis or at the peripheral retina). The myopic patient may then be treated by modifying the refractive properties of the ocular system to provide clear or acceptable vision on-axis and to control the position of the image at the peripheral retina, for example to move the image onto or in front of the retina. For an eye with asymmetry in the peripheral retina, a device to move the image at the peripheral retina will require different refractive powers to treat the asymmetry observed in different parts of the peripheral retina. For a patient with hyperopia, the peripheral images may be moved onto or behind the retina, again treating for observed asymmetry in the eye.

One way of modifying the refractive properties is through the use of a lens. Embodiments of a device for an eye include contact lenses, spectacle lenses, corneal implants or other lenses that have different power profiles for images received by different parts of the peripheral retina. The class of contact lenses includes lenses that operate by having a refractive power and orthokeratology lenses that achieve a change in the refractive properties of the eye by reshaping the cornea.

In some embodiments a lens is provided for affecting the progression of myopia or hyperopia in an eye. The lens comprises a nasal optic zone and a temporal optic zone for refracting light to be received by the peripheral retina. The nasal optic zone and temporal optic zone have different refractive powers. The lens may further include a central optic zone with a refractive power to correct for the refractive power of the eye on-axis and to provide clear or acceptable distance vision at all distances.

The lens may therefore comprise regions for affecting light destined to various sectors of the retina in a rotationally asymmetric manner relative to the central on-axis focal points. The lens may affect the peripheral light rays received by the retina of the patient's eye in any given sector so that their focal points are on or anterior to the surface of the retina for myopes, or on or posterior to the surface of the retina for hyperopes. In this way, for at least some patients, the stimulus for aberrant eye growth may be removed in substantially all sectors or over an increased amount of the retina. The removal of stimulus may be performed especially in those regions deemed to be of most importance for inhibiting such eye growth.

In some embodiments a method of prescribing an optical device for affecting images received by the peripheral retina includes observing whether the recipient of the optical device has a symmetrical or asymmetrical curvature of field. The prescribed optical device may then either treat the peripheral image symmetrically or asymmetrically dependent on the observation.

