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Spectral Domain Optical Coherence Tomography and Glaucoma

Glaucoma may be the second leading cause of blindness in the world, affecting about 60 million people globally.1,2 The true prevalence may be underestimated as up to half of patients with glaucoma are undiagnosed, and a large proportion of glaucoma patients who are eligible for registration as legally blind remain unregistered.3–7 
 
Glaucoma is a characteristic optic neuropathy characterized by retinal ganglion cell death, which then leads to retinal nerve fiber layer (RNFL) thinning and optic nerve head (ONH) cupping. The most important risk factor is elevated intraocular pressure. Since these structural changes in the RNFL and ONH may result in irreversible visual field (VF) loss,8,9 the early diagnosis of glaucoma is vital for the early initiation of treatment that may stop or slow down further permanent vision loss. One reason why early diagnosis often does not occur is that patients are usually asymptomatic early in their disease. Another problem preventing early diagnosis is that the standard clinical methods for diagnosing glaucoma (ie, VF testing) can only diagnose glaucomatous vision loss after up to 40% of the nerve tissue is lost irreversibly.10–13 VF testing is also not an ideal method to diagnose glaucoma because these tests rely on not only the subjective test response of the patient but also the subjective interpretation of the physician. 
 
Because of the need to diagnose glaucoma not only earlier but also in a more objective manner, various imaging technologies have been developed over the past decade in an attempt to quantitatively analyze progressive glaucomatous structural changes, such as RNFL thinning and/or ONH cupping, both of which are known to precede clinically detectable VF defects. So far, there are 3 main commercially available imaging technologies that are used to image glaucomatous structural changes: confocal scanning laser ophthalmoscopy (CSLO) (Heidelberg Retina Tomograph—HRTII and III, Heidelberg Engineering, Germany), scanning laser polarimetry (SLP) (GDxVCC, Carl Zeiss Meditec Inc, Dublin, CA), and optical coherence tomography (OCT). We feel that spectral domain optical coherence tomography (SDOCT), also called Fourier domain OCT, has the greatest potential for imaging glaucomatous structural changes. 
 
Before describing SDOCT in more detail, we will describe the science as to why CSLO, SLP, and traditional time domain optical coherence tomography (TDOCT) have not been proven to be better than a complete eye examination (which includes VF testing and stereo disc photography).14,15