Fig. 4.1
Heidelberg SPECTRALIS® SD-OCT EDI from a normal 10-year-old boy. ELM (OLM) external (outer) limiting membrane, NFL nerve fiber layer, IS/OS (EZ) inner segment/outer segment junction or ellipsoid zone, GCL ganglion cell layer, IDZ interdigitation zone, IPL inner plexiform layer, RPE retinal pigment epithelium, INL inner nuclear layer, OPL outer plexiform layer, FoH fibers of Henle, ONL outer nuclear layer
OCT technology has developed at a rapid pace, and new advances have been incorporated into practice even before their clinical significance is understood. Spectral domain OCT (SD-OCT) was one of the first advances from the original time domain OCT. Spectral domain , also known as Fourier domain , offers scan rates up to 100 times faster than time domain OCT. All of the wavelengths of returning light are analyzed simultaneously allowing faster data collection and less motion artifact. The resolution is also deeper. Enhanced depth imaging displaces the SD-OCT machine closer to the eye and delivers the most tightly focused portion of illumination to the choroid or inner sclera. This displacement enhances the sensitivity for imaging deeper structures including the choriocapillaris and choroid. The resultant image is inverted, but software programs in the OCT machine rotate the image to the orientation that we are accustomed to using to examine the retina, e.g., inner retina at the top of the image and retinal pigment epithelium and choroid at the bottom (Fig. 4.2).
Fig. 4.2
Heidelberg SPECTRALIS® SD-OCT and corresponding EDI OCT in a normal eye
The newest advances in OCT include en face OCT, swept source OCT (SS-OCT) , and OCT angiography. En face OCT combines SD-OCT with transverse confocal analysis to produce transverse (en face) images of retinal and choroidal layers at a specified depth. Analyses of the retinal and choroidal structures in this plane may allow better quantification of abnormalities and correlations with outcomes such as visual acuity [1]. As of November 2016, only one platform for SS-OCT and OCT angiography (Carl Zeiss Meditec, Inc.) was FDA-approved for research use in the USA. Through the use of a different type of laser and camera, as well as two fast parallel photodiode detectors, SS-OCT is able to achieve a high imaging speed allowing the acquisition of high-resolution images and reducing the negative effect of eye movement on scan quality. This technology also improves the ability to image deep ocular structures while providing uniform sensitivity for more superficial structures such as the vitreous within the same scan. OCT angiography utilizes a processing algorithm to produce images of capillary-level blood flow with en face visualization of separate layers in the retina and choroid. In uveitic eyes, OCT angiography may be mainly useful in the diagnosis of choroidal neovascular membranes, although it can also be used to detect small areas of capillary loss in retinal vasculitis cases that appear normal on conventional fluorescein angiography imaging. To date, there are few published studies that analyze uveitic retinal pathology with SS-OCT or OCT angiography, but it is probably only a matter of time before these OCT technologies are utilized more widely in research as well as in clinical practice.
Clinical Utility of OCT
In uveitis, OCT can provide a wealth of information. In some cases, OCT imaging may provide the key information necessary to narrow the differential diagnosis. OCT can be used to quantify active inflammatory disease or demonstrate resolution of inflammation. OCT may also explain vision loss. The following case examples illustrate the utility of OCT in various manifestations of uveitis.
The utility of OCT in diagnosing and managing macular edema of any etiology, including uveitic, is well known. Interestingly, multiple studies have documented discrepancies between fluorescein angiography (FA) and OCT: both FA+/OCT− and FA−/OCT+ cases of uveitic macular edema may occur, highlighting the importance of utilizing multimodal imaging when diagnosing and managing uveitis patients [2, 3]. OCT is particularly well suited to illustrate the outer retinal and choroidal inflammation in the white dot syndromes including serpiginous choroiditis, multiple evanescent white dot syndrome, punctate inner choroidopathy, and multifocal choroiditis. Enhanced depth imaging OCT illustrates the massive and diffuse choroidal thickening associated with active Vogt-Koyanagi-Harada syndrome as well as birdshot chorioretinopathy and posterior scleritis . Several recent reviews provide excellent summaries of the OCT findings in uveitis [4, 5].
Case 1
39-year-old woman with non-Hodgkin’s lymphoma was undergoing chemotherapy. One week before her ophthalmic examination, she had a bone marrow biopsy under sedation. The next day, she noticed an “egg-shaped” scotoma in her left eye. Visual acuity was 20/20 OU. There were no obvious abnormalities on color fundus (Fig. 4.3a). Infrared/red-free photos show a wedge-shaped lesion superonasal to the fovea.
Fig. 4.3
SD-OCT through the lesion shows a corresponding area of irregularity of the outer nuclear layer and outer retina (Fig. 4.3b, c).
She was diagnosed with acute macular neuroretinopathy (AMN) also known as acute macular outer retinopathy which is probably a microvascular occlusive event, not truly an inflammatory condition.
Role of OCT: Crucial for diagnosis in a case with very subtle clinical findings.
Case 2
76-year-old woman with HLA-A29 positive birdshot chorioretinopathy was receiving treatment with systemic immunosuppression which was initiated late in the course of disease due to delays in diagnosis. The best corrected visual acuity in her right eye was 20/40. Color montage of the fundus shows typical chorioretinal lesions. Humphrey visual field 24-2 testing shows superior field loss (Fig. 4.4a).
Fig. 4.4
SD-OCT shows central macular atrophy, poor definition of retinal layers, loss of outer retinal structures, and chronic-appearing intraretinal edema (Fig. 4.4b).
Role of OCT: Demonstrates structural abnormalities which may not have been suspected based on clinical exam.
Case 3
A 16-year-old boy presented with sudden vision loss in his left eye. His VA was 20/20 OD, count fingers OS, and he had relative afferent pupillary defect in the left eye. The granular appearance of the central macula and the presence of irregular white spots in the posterior pole and periphery are consistent with a diagnosis of multiple evanescent white dot syndrome (MEWDS) (Fig. 4.5a).
