Optical Coherence Tomography in Uveitis

Although a complete patient medical history followed by detailed clinical examination remains the cornerstone of the diagnosis and management of patients with uveitis, certain technologic advances have contributed to improvements in patient care. Optical coherence tomography (OCT) is one technology that is being increasingly used in the diagnosis and treatment of uveitis. More recently, the development of optical coherence tomography angiography (OCTA) has given clinicians a less invasive and prompt tool to assess the retinal and choroidal vasculature.

OCT uses low-coherence light to obtain high-resolution images of ocular tissues, including the retina and the optic nerve head. The use of long-wavelength light that penetrates into tissues allows imaging of all layers of the retina and of the choroid by detecting light reflectance in the Fourier domain. OCT of the normal retina can image individual layers of the retina and the overlying vitreous and the underlying choroid. Experts have worked to develop a consensus nomenclature for the classification of the retinal and choroidal layers and bands visible on OCT images of the normal eye. Fig. 6.1 shows an OCT image of a normal retina obtained by using a spectral domain (SD)-OCT machine. OCT is not only used in ophthalmology but also in other areas of medicine (e.g., in cardiology to image the coronary arteries and in dermatology).

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Nomenclature for normal anatomic landmarks seen on spectral domain optical coherence Tomography Panel. Healthy retina imaged using Heidelburg Spectralis. RPE, Retinal pigment epithelium. (From Staurenghi G, Sadda S, Chakravarthy U, Spaide RF, for the International Nomenclature for Optical Coherence Tomography (IN.OCT) Panel. Proposed lexicon for anatomic landmarks in normal posterior segment spectral-domain optical coherence tomography. Ophthalmology. 2014;121:1572–1578.)

OCT images are created from the interference pattern created as scattered light from the tissue is compared with a reference wave. First-generation OCT machines used time-domain technology characterized by low-coherence, near-infrared light from a superluminescent diode light source split to a reference mirror and to the retina. Today, two types of OCT devices are most commonly used in ophthalmology: SD-OCT and swept-source OCT (SS-OCT). SD-OCT machines are more prevalent in clinical practices and use a broadband light source, a spectrometer, and a high-speed line-scan camera to measure the interference patterns. The newer SS-OCT machines use a light source with a wavelength around 1 μm that sweeps across a narrow band of wavelengths and a simpler point photodetector. Current SS-OCT devices allow better imaging of deeper structures, such as the choroid, but are more expensive and have less ideal axial resolution and less normative data compared with SD-OCT. However, the technology continues to improve, and comparisons of state-of-the-art devices will change over time.

OCTA allows assessment of the retina and vasculature. Analysis of successive OCT B-scans at the same cross-sectional area of the retina allows detection of motion caused by flow through the retinal vessels. Although OCTA is less invasive and has a higher resolution compared with fluorescein angiography, it is susceptible to motion and projection artifacts, making some images difficult to interpret. That said, the technology continues to improve, with faster scanning speeds and better software algorithms resulting in better images for analysis.

Macular Edema

The most common use of OCT in diagnosing and managing uveitis centers on the detection, characterization, and quantification of macular edema. Before the introduction of OCT, macular edema was primary diagnosed and monitored clinically with slit-lamp examination or fluorescein angiography. However, these methods were variable and affected by the presence of not only media opacity but also by retinal hemorrhage, pigment, or lipid exudation in the retina. Compared with angiography, OCT is a highly sensitive (96%) and sensitive (100%) test for the presence of macular edema in uveitic eyes. SD-OCT has been shown to be better at detecting macular edema compared with time-domain OCT (TD-OCT) in patients with uveitis; this is not surprising, given the better resolution with SD-OCT. Importantly, as a result of its reproducibility in measuring retinal thickness, OCT can reliably be used to assess therapeutic response for uveitic macular edema.

In addition to quantifying macular edema, OCT can detect two different patterns of macular edema: cystic macular edema and a more diffuse pattern of macular edema. In a study of SD-OCT, in a single eye from 500 consecutive patients with anterior, intermediate, posterior, or panuveitis, cystoid macular edema was seen in 25% of eyes, and diffuse macular edema was seen in 11% of eyes. Cystoid macular edema was most frequent in intermediate uveitis (40%) and panuveitis (36%), and diffuse macular edema was most frequent in panuveitis (18%) and posterior uveitis (17%). Furthermore, the junction between the inner and outer segments of the photoreceptors (the IS/OS junction) and the external limiting membrane can be readily identified with SD-OCT. The integrity of the IS/OS junction may predict improvement in visual acuity after treatment of macular edema or epiretinal membrane removal.

Importantly, OCT is used to more precisely monitor response to therapy and can guide therapy. Resolution of cystoid macular edema after treatment with intravitreal corticosteroid therapy is nicely demonstrated with OCT ( Fig. 6.2 ). Collapse of the cystic spaces and complete resolution of retinal fluid and thickness occurred by 2 months after therapy (see Fig. 6.2B ).