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QUESTION

WHY WOULD I WANT TO LOOK AT CHOROIDAL THICKNESS ON OPTICAL COHERENCE TOMOGRAPHY?

Glenn Yiu, MD, PhD

The practice of measuring choroidal thickness became prevalent with the advent of enhanced depth imaging optical coherence tomography (EDI-OCT). First described by Spaide et al,¹ EDI-OCT is a specialized mode of OCT that takes advantage of the greater depth attained from the inverted image one gets by placing the OCT device closer to the subject’s eye. Today, many current generation spectral domain OCT systems have a software mode that automates this process and produces an OCT image that provides much higher quality cross-sectional visualization of the choroidal vasculature as well as the choroid-scleral junction. By measuring the distance between Bruch membrane and the choroid-scleral junction at various locations in the macula, it became possible to measure choroidal thickness across a spectrum of retinal and choroidal diseases (Figure 26-1).

Before we discuss which conditions are best distinguished by looking at choroidal thickness, it is important to note that, unlike measuring the thickness of the retina, which consists of neural tissue, the choroid is a vascular structure that can vary significantly between individuals. Choroidal thickness decreases linearly with age, undergoing 20 to 30 mm of thinning with each decade of adult life.² The thickness of the choroid is also strongly associated with axial length and refractive error with highly myopic eyes having the thinnest choroid. Finally, choroidal thickness also varies with time of day, being thicker in the morning and thinnest in the evening, such that measurements taken at different times on consecutive visits may not be easily compared with each other. Hence, while the general rule of thumb is that normal choroidal thickness is roughly equal to the thickness of the retina in the central macula, it is important to take into account the patient’s age, refractive error, and the time of day at which the OCT image was taken. To add to the complexity, the posterior boundary of the choroid (ie, the choroid-scleral junction) is not always clearly delineated, with a portion of normal individuals demonstrating a visible suprachoroidal space between the choroid stroma and sclera.3 Hence, there is still significant controversy on how the choroid should be precisely measured and hardly any commercially available software can reproducibly segment the choroid or automatically generate a choroidal thickness measurement. So, at the current time, most eyecare providers must rely on manual choroidal segmentation or manual measurements of choroidal thickness using calipers integrated into the OCT software.

With these considerations in mind, I think the most useful clinical application for measuring choroidal thickness is in diagnosing retinal conditions that are classified as a pachychoroid spectrum condition, such as central serous chorioretinopathy (CSC). The term pachychoroid, derived from the Greek term pachy meaning thick, refers to a collection of related retinal disorders that are characterized by abnormal thickening and dilation of choroidal vessels and increased choroidal hyperpermeability. Examples include CSC and polypoidal choroidal vasculopathy (PCV), and the nomenclature has expanded to include pachychoroid pigment epitheliopathy, which refers to changes that are considered precursors to CSC, and pachychoroid neovasculopathy, which describes Type I choroidal neovascularization in eyes with pachychoroid pigment epitheliopathy or CSC.⁴ I find that obtaining EDI-OCT is particularly useful in eyes with subretinal fluid and vascular leakage on fluorescein angiography that may resemble exudative age-related macular degeneration (AMD), but may also result from chronic CSC or atypical PCV. Even in an elderly patient with drusen and pigment changes, there is significant overlap between these conditions in clinical and angiographic appearance. Especially in cases where subretinal fluid responds poorly to repeated anti-vascular endothelial growth factor (anti-VEGF) therapy, I would consider obtaining EDI-OCT to evaluate the choroidal vasculature. The presence of a thick choroid (Figure 26-2), particularly in the presence of large choroidal vessel lumen, would support a diagnosis of PCV or CSC and potentially benefit from photodynamic therapy, or in the case of CSC, the use of oral mineralocorticoid antagonists, such as eplerenone. Using additional imaging modalities to characterize the choroid, such as indocyanine green angiography or novel OCT angiography (OCT-A), could further clarify the etiology of the condition. Interestingly, recent studies have also shown that the choroid becomes thinner with successful photodynamic therapy treatment of CSC and PCV, suggesting that tracking choroidal thickness changes may be a means of monitoring treatment response.

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