Stereoscopic photographs are a useful tool in the detection of glaucoma, however, the difficulty in obtaining them has seen them underused in favour of optical coherence tomography (OCT). A new device is attempting to redress this balance, enabling stereo-photography to be used alongside OCT for optimal practice.
Glaucoma is major global health problem:1 the World Health Organization reports that it is the second largest cause of permanent blindness worldwide; various authors estimate that the disease causes 2.1 to 4.4 million such cases worldwide,2,3 and a retrospective series found that blindness occurred in about 20% of glaucoma patients.4
Glaucoma is a degenerative disease characterised by typical morphological changes in the optic nerve head (ONH) and the retinal nerve fibre layer (RNFL), followed by a corresponding loss of the visual field (VF), having a major impact on the quality of life of patients5,6. The complex nature of glaucoma requires a comprehensive assessment of each patient, including measurement of IOP, and ONH and VF evaluation, which are critical steps in the diagnosis and monitoring of the disease
Optic nerve head assessment
The detection of structural damage to the ONH is central to the diagnosis of glaucoma and is extremely important for monitoring both patients at risk and those with established disease. Glaucoma, by definition, is an optic neuropathy and, therefore, particular attention must be given to examination of the optic nerve
The ONH is the site at which the dropout of retinal ganglion cells is identified most easily using current clinical techniques and is postulated as the primary site for damage.7,8 Traditionally, the assessment of ONH health is performed by stereoscopic photographs; a technique that improves the detection of RNFL and optic disc changes, including optic disc haemorrhages (ODHs) that would not be evident on ophthalmoscopy or monoscopic photographs.
The Ocular Hypertension Treatment Study compared stereoscopic photographs with clinical examination in 1,618 participants (3,236 eyes) who were followed for 96.3 months. The photographs enabled the detection of ODHs in 128 eyes of 123 participants. Twenty one cases (16%) were detected by both clinical examination and review of photographs, and 107 cases (84%) were detected only by review of photographs (P<0.0001).10
Regarding the monoscopic photograph of the retina, the lack of stereopsis could lead the operator to assess the clinical situation on the basis of the colour of the area rather than the contour of the neuroretinal rim;9,11 but the stereoscopic photography provided higher levels of interobserver agreement compared with monoscopic assessments. Observers reading photographs in the context of major clinical trials are generally reported to have low interobserver variability, while others have reported much greater variability.12
Despite its advantages, traditional stereoscopic photography has some limitations. Firstly, the process of capturing the image is not easy, since the cameras are complex and the process is time-consuming. In addition, the reliability of the image is highly influenced by such factors as the camera angle, photographic technique, lighting and magnification;13 while the experience of the operator evaluating the photographs also has an effect.14,15
The above-mentioned constraints of stereoscopic photography have favoured the diffusion of high-tech imaging technologies, such as optical coherence tomography (OCT), which allows fast and reproducible high-resolution quantitative evaluation of ONH and RNFL with good diagnostic accuracy.16 Imaging with OCT, however, also has some inherent drawbacks, such as the lack of qualitative information about the structures being evaluated and the fact that it is a fast-evolving technology affected by early obsolescence, thus limiting the ability to monitor patients over the long term.
A new technology to increase the scope of stereoscopic photograph analysis
The stereoscopic photograph-for its reliability, the qualitative nature of the image, and possibility of monitoring patients over the long term-provides relevant information for the assessment of the ONH that warrants being integrated with the data obtained with OCT. However, its aforementioned technical limitations have caused a decline of stereoscopic photography, thus reducing the availability of valuable data for the diagnosis and monitoring of glaucoma.
The new automated perimeter combined with a white light scanning ophthalmoscope, Compass (Centervue), provides confocal images of the retina. The stereo photograph feature of the device overcomes some of the limitations of traditional stereo photographs, due to the specific techniques used.
The most significant advancement is represented by its process of image acquisition: the first photograph is automatically captured with a focus on the rim; while for the second photograph, the device automatically focuses on the lamina cribrosa. This double focus enhances its three-dimensional effect and, together with the confocal system and the white light source, enables the capture of high-quality images.
In our experience, we obtained excellent images without the need for pupil dilation – the assessment of ONH was possible even in the presence of media opacities, such as early and mild cataract. The characteristics of the device define a new role for stereoscopic photography as a method to be used alongside OCT, to improve the capabilities of diagnosis and monitoring the course of glaucoma.
Dr Francesco Oddone, MD, PhD
Dr Oddone is head of the Glaucoma Research Unit at Bietti Foundation, Rome, Italy.
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