Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effects of optical distortions. It has been developed in astronomical telescopes to remove the effect of atmospheric turbulence from the satellites or stellar images and only in recent years its use has been extended to Ophthalmology.

What are the optical aberrations of the human eye?
The human eye is an optical system having several optical elements, such as the cornea and the crystalline lens, for focusing light rays representing images onto the retina. Imperfections in the components and materials within the eye may cause light rays to deviate from the desired path. These deviations are referred to as optical aberrations. Optical aberrations from the eye include low-order and high-order aberrations (HOA): low-order aberrations, such as defocus and astigmatism, are the dominant optical aberrations in the human eye optics: they account for approximately 90% to the overall wavefront aberration (WA) of the eye. Although HOA make a small contribution (on average ≤10%) to the total variance of the eye WA, they have a deleterious effect on image quality. Optical berrations in the eye optics not only blur images formed on the retina (thus impairing vision), but also blur images taken of the retina of the human eye.

How to detect the early signs of retinal diseases?
Diagnosis of retinal diseases at an early stage is crucial for the treatment and avoidance of serious visual loss. Across the developed world, the major causes of vision loss can be attributed to age-related macular degeneration (AMD), diabetic retinopathy (DR) and glaucoma. Diagnosis usually occurs once damage has already happened with some extent, because of the relatively poor resolution of current retinal imaging techniques to detect abnormalities of the retinal microstructures, including photoreceptors cells, microvessels and nerve fiber bundles. Current retinal imaging modalities (eg, SLO, colour retinography etc) rely on the use of the eye’s optical system as that objective lens and can have a maximum lateral resolution of 20-25 µm.

When applied to the optical system of the eye, adaptive optics can correct for the total wavefront aberration of the ocular system and provide substantial improvements in the sharpness of retinal images that are normally degraded from the said aberrations. The introduction of AO ophthalmoscopes has improved the transverse resolution to approximately 2 µm so that single photoreceptor cells, especially cones, and retinal micro-vessels or nerve fiber bundles can be resolved. AO retinal imaging can therefore provide information about the retinal micro-structures that cannot be obtained with current retinal imaging techniques.

Dr. Marco Lombardo is the principal investigator of the first research protocol study on the application of adaptive optics in Ophthalmology in Italy. He is currently working on noninvasive detection of pathological changes in patients with diabetes.

Interview to Dr. Lombardo about his experience with an AO retinal camera prototype (rtx1 retinal camera, Imagine EyesOrsay, France).

With regards to recent data, achieved in studies, could you please highlight and elaborate on the benefits this instrument offers surgeons and patients that other imaging techniques do not?
Adaptive optics (AO) can provide very early stage diagnosis and prognosis information that could not be obtained from other retinal imaging techniques. AO is a technology used to improve the performance of optical systems by reducing the effects of optical aberrations. When applied to the optical system of the eye, AO can correct for the total wavefront aberration of the system and provide substantial improvements in the sharpness of retinal images that are normally degraded from the said aberrations, with the ultimate definition of single photoreceptors.
With the advent of a compact reliable AO retinal camera into the clinical environment there is now the opportunity to image the human retina at microscopic spatial resolution in real time. The AO retinal camera introduces a powerful instrument for the early diagnosis of diseases affecting the macular area. The ability to image the photoreceptor layer in vivo provides the opportunity to better understand the pathological processes leading to visual impairment and to non-invasively monitor the normal retinal function, the progression of retinal diseases and the efficacy of therapies at a cellular level.

Which clinical applications have you found most promising in your research with this instrument?
Several clinical examples can be discussed to demonstrate the beneficial use of AO fundus imaging. Macula oedema from diabetic retinopathy and essudative macular degeneration represent the most significant cause of visual impairment in Europe and United States. The opportunity to detect and monitor pathological variations in the photoreceptor layer at a very early stage of the disease can represent the basis for designing new diagnostic and treatment protocols to preserve the normal integrity and function of the retina. For example, in ARMD, AO images reveals granular contents during drusen early stage of formation and allows to check the aspect of the cone mosaic between drusen areas. In diabetic retinopathy, AO enables to verify the integrity of the cone mosaic with faint intraretinal edema which could not be seen in SLO or OCT images. Diagnosis and treatment of macular diseases at a pre-clinical, theoretically reversible, stage can be of great benefit to the community. 

AO image focused at the photoreceptor layer showing the cone mosaic of the parafoveal region. Patches of cones with higher brightness across a 4° field size of the photoreceptor layer. This is likely due to variations in reflectance of cones.