25 Optical Coherence Tomography and Glaucoma
Glaucoma is characterized by optic-nerve cupping with associated visual-field defects. Diagnosing glaucoma by visualizing the optic nerve is something eye doctors have done for the past century, either with the use of a direct ophthalmoscope or indirectly with the slit lamp. However, measurement of the optic nerve or the polarization of the retinal nerve-fiber layer (RNFL) using various instruments was unable to give physicians an objective and reliable measure of a patient’s glaucoma status. Additionally, early glaucoma was difficult to diagnose because of the overlap between structural measures of normal and early glaucoma. Diagnosing glaucoma early enables treatment at its early stages, thus preventing patients from reaching advanced stages of the disease. Optical coherence tomography (OCT) provides a direct objective measurement of tissues such as the optic nerve and the peripapillary circumferential RNFL. Whereas imaging the RNFL in glaucoma patients and in patients suspected to have glaucoma is currently the most common use for OCT, 1 OCT imaging of the macula and anterior segment have important roles as well. Time-domain OCT has been replaced currently by spectral-domain OCT because it provides an ability to capture measurements over larger areas in a much quicker and reproducible manner. 2 , 3 , 4
25.1 OCT of the Retinal Nerve-Fiber Layer
The hallmark of glaucomatous vision loss is progressive loss of RNFL manifested by loss of the neuroretinal rim and seen as cupping of the optic nerve head. 5 , 6 , 7 However, computerized imaging of the optic nerve head has been limited in its adoption because of the significant variability among both normal and glaucomatous individuals. Imaging the circumferential RNFL, however, is reproducible and reliable 8 , 9 and allows for comparison with normative databases, giving doctors valuable diagnostic information, especially for suspected glaucoma and ocular hypertension. In patients with already diagnosed glaucoma, RNFL measurements provide an objective measure of tissue loss, as well as an objective way to confirm clinical findings such as loss of the neuroretinal rim. 10
Abnormal thinning of the RNFL, particularly in the inferior and superior quadrants, is supportive of the diagnosis of glaucoma. RNFL defects seen on OCT scans have corresponding nerve-fiber defects on clinical examination, often seen best with the red-free filter, and corresponding visual-field defects on automated perimetry. Given the high resolution of OCT imaging, especially using spectral-domain OCT, abnormalities on OCT can precede clinical findings and visual-field abnormalities, making OCT of the RNFL particularly helpful for patients with normal visual field or for patients with poorly reliable visual fields or who are unable to perform them (for example, pediatric patients or patients with significant cognitive impairment).
In addition, OCT of the RNFL can be used to assess cup:disc asymmetry, another hallmark of glaucoma. A patient’s RNFL OCT is compared with the normative database, as well as to the patient’s other eye (Fig. 25.1). This analysis is particularly helpful for patients who are not represented in the normative database, such as those with myopia, and patients with early focal defects that are missed by the quadrant analysis.
Optical coherence tomography of the RNFL is also useful in monitoring for progression (Fig. 25.2). This imaging test takes only a few minutes and requires minimal patient participation; thus, it is a well-tolerated adjunct to automated visual-field testing in monitoring patients. Using eye-tracking capabilities of the high-resolution OCT devices, small changes in the RNFL thickness can be assessed, enabling the doctor to consider advancing treatment or monitoring more closely to prevent vision loss from glaucoma.
Computerized analysis is useful to doctors by highlighting abnormally thin and thick regions compared with the normative database. However, abnormally thin areas (red) and areas of normal or thick RNFL (green) can be misleading. Some common diagnostic masqueraders are listed in Table 25.1.
“Red” disease (false positives). The OCT RNFL analysis shows abnormally thin RNFL, giving the false impression in favor of glaucoma.
Segmentation artifact, where the segmentation software has not properly identified the RNFL borders (Fig. 25.3)
Alignment artifact, where the technician has performed the analysis using an image with a corner or edge cut off
Nonglaucomatous optic neuropathies
Ischemic optic neuropathies can result in segmental superior or inferior thinning.
Inflammatory optic neuropathies, such as optic neuritis, can result in subsequent RNFL thinning, often diffuse or of the temporal quadrant.
“Green” disease (false negatives). The OCT RNFL analysis shows normal or thick RNFL, giving the false impression against the diagnosis of glaucoma
The RNFL may not be abnormally thin despite loss of retinal ganglion cell axons if the tissue is edematous due to inflammation
Epiretinal membrane pulling anteriorly, giving the appearance of normal thickness despite loss of retinal ganglion cell axons
Segmentation artifact example, where the segmentation software has misidentified a prominent inner limiting membrane (Fig. 25.4)
Review the OCT scans themselves, in addition to reviewing the computerized analysis
Use the computerized analyses with caution in patients with ocular comorbidities
Clinical correlation of OCT findings is key 20
Abbreviations: OCT, optical coherence tomography; RNFL, retinal nerve-fiber layer.
Also, OCT can be used for cross-sectional imaging of the optic nerve head. This type of imaging of the optic nerve head can be useful clinically for differentiating other optic nerve head findings from glaucoma, for example, visual-field defect in the setting of buried optic nerve head drusen. 11 In addition, cross-sectional imaging of the optic nerve head can allow for visualization of the lamina cribrosa (Fig. 25.5). Although generally not used clinically at present, imaging the lamina cribrosa is of interest in understanding the pathophysiology of glaucomatous cupping.
Whereas OCT imaging of the RNFL is the most common use for this technology in glaucoma patients, OCT of the macula and of the anterior segment can also be quite useful both for the diagnosis of glaucoma and its management.