28 Optical Coherence Tomography in Neurophthalmology
Optical coherence tomography (OCT) is a rapid noncontact method that allows in vivo imaging of the optic nerve head (ONH) and retinal nerve fiber layer (RNFL). Since its introduction in early 1990s, the technology has deeply disseminated into clinical practice. 1 , 2 , 3 , 4 High acquisition speed and high resolution have made it better than any available imaging methods. Although OCT had been used for diagnosis and prognosis in varied ophthalmologic conditions, to start with it, was used for ONH and retinal evaluation. 2 , 3 , 4 In ONH, it was initially used for the determination of thickness of the retinal nerve fiber layer (RNFL), which is an important parameter for glaucoma assessment. 5 , 6 In 1995, Schuman and associates showed that the RNFL thickness, as measured by OCT, demonstrated a high degree of correlation with functional status of the optic nerve, as measured by visual-field examination. 5
Although glaucoma screening has been the most common indication for ONH examination by OCT, imaging of the ONH and RNFL can be useful in other rare and varied neurologic conditions. In this chapter, we go through the common nonglaucomatous conditions in which OCT can be used for imaging.
28.1 Optic Nerve Head
The ONH is vertically oval, with an average diameter of 1.85 to 1.9 mm. The horizontal diameter ranges from 1.70 to1.80 mm. The central excavation in the ONH is the optic cup, and it is horizontally oval. The border between the ONH and the optic cup is the neuroretinal rim. The peripapillary region is divided into the alpha and beta zones (adjacent to disc).
28.1.1 Optic Nerve Head in OCT
The ONH is scanned by the optic disc cube mode in the OCT. It is a 200 × 200 scan. Fig. 28.1 shows the OCT map of ONH taken in spectral-domain OCT. Scan quality should be checked for every OCT image, with particular attention to the segmentation of RNFL and the signal strength. The value of signal strength ranges between 1 and 10, with 10 representing the best and 1 the worst image signal. Poor signal strength is often related to incorrect scan focus or media opacity. Acquiring a scan with maximal possible signal strength is recommended for RNFL measurement. The scan circle should be centered on the optic nerve center. The central zone shows the average thickness of the nerve fiber layer; next, it shows the quadrant variation in RNFL thickness, followed by the clock hour’s variation. The color coding for the distribution of RNFL thickness is given and compared with the normal distribution in the population. The normative display provides a useful reference to determine whether the RNFL measurements are within or outside the normal ranges. Accordingly, green indicates 95% of the distribution of normal values, yellow indicates 5% of the distribution of normal values, and red represents 1% of the distribution of normal values. The lowermost part of the scan map shows the graph comparing patients’ RNFL thickness in microns with the normative data distribution. The other modules available in neurologic imaging are advanced visualization, RNFL thickness analysis, and global progression analysis (GPA), which can be used in glaucomatous progression and analysis. OCT GPA is a trend-based analysis with progression analyzed and reported as change over time using serial RNFL measurements. At least four visits are required to generate the GPA report. The GPA overlays serial RNFL thickness profiles and performs linear regression analysis of average RNFL thickness against the duration of follow-up.
28.2 Specific Conditions
28.2.1 Optic Neuritis
Optic neuritis (ON) is an inflammation in the optic nerve (Fig. 28.2), which causes subacute onset of vision loss in children and young adults. The Optic Neuritis Treatment Trial gives the overall scenario of the clinical presentation and management of optic neuritis. OCT provides a noninvasive means to quantify the structural effects of an inflammatory insult to the optic nerve. OCT has been used in patients with multiple sclerosis (MS), who are predisposed to develop ON; it has been shown that thickness of the RNFL was 46% lower in MS eyes relative to control eyes. 7 Atrophy of the RNFL was associated with lower visual evoked potential (VEP) amplitudes, worse log minimum angle of resolution (logMAR) visual acuity scores, reduced visual-field mean deviation, and decreased color vision in ON patients. 8 Trip et al noted that macular volumes were 11% lower in ON eyes compared with control eyes. 8 In addition, a strong correlation was found between OCT measurement of axonal loss and multifocal VEP latency. 9 Recurrent optic neuritis leads to RNFL atrophy; especially in unilateral ON, the changes can be well appreciated. Noval and associates noted an initial increase in RNFL thickness (Fig. 28.3) that resolved by 1.5 months. In ON eyes, they reported a 25% reduction in RNFL thickness at 6 months. 10 Syc et al used OCT to quantify neurodegeneration as an outcome in MS clinical trials. 11 Excellent reproducibility of average and quadrantic RNFL thickness values (Fig. 28.4), average macular thickness, and total macular volume was found using spectral-domain OCT. 11