The Choroid in Retinal Vascular Diseases

Choroidal and Retinal Vasculature—Why Should We Evaluate the Choroid in Retinal Vascular Diseases?

The retina has two sources of oxygenation. The inner retina is supplied with oxygen by the retinal vasculature, whereas the outer retina, especially the photoreceptor layers, is supplied by the choroidal vasculature. Therefore, a structurally and functionally normal choroid, in addition to normal retinal vessels, is essential for proper retinal function. Retinal vasculature is supplied by the central retinal artery. It supplies a capillary network that is drained by venules. In the choroid, the short and long ciliary arteries supply blood, which is then drained by the vortex veins.

Retinal vasculature forms the inner blood–retinal barrier (BRB), which limits the passage of molecules to those with specific receptors or transporters on the capillary endothelial cells. In contrast, the capillaries in the choroidal vasculature have a larger lumen with more fenestrations, allowing for the passive transport of small molecules. Choroidal arteries are typically short and have few branches, which supply the choriocapillaris at right angles. Systemic conditions such as high blood pressure and diabetes mellitus (DM) can directly affect the choriocapillaris.

Retinal and choroidal oxygenation is crucial for maintaining a normal physiology; thus, retinal disease occurs when the retina and choroid are deprived of adequate amounts of oxygen. Various experiments that have used oxygen electrodes to assess oxygen levels in the choroid and retina have shown that oxygen consumption in the outer retina is highest in the parafoveal region, whereas inner retinal oxygen tension in the fovea is very low (approximately 5 mmHg). This indicates that there is no retinal vasculature supplying the fovea, and that the choroid is the predominant source of oxygen.

In most tissues of the body, including the retina, blood flow is autoregulated so that fluctuations in perfusion pressure do not cause proportional changes in blood flow. This is achieved by compensatory dilation and constriction of arterioles and capillaries that allow blood flow to return to normal shortly after a change in pressure. Inefficient autoregulation can lead to hypoxia and neovascularization, as seen in diabetic retinopathy (DR). However, in contrast to its retinal counterpart, choroidal vasculature does not exhibit autoregulation, although this is still the subject of debate.

Retinal vasculature contains no circulatory anastomoses. Thus, if retinal vessels are obstructed, the retina easily becomes ischemic, as there are few collaterals to perform compensation. Depending on the retinal area, some retinal vascular diseases may show a different degree of anatomic involvement and functional loss. For example, in eyes with retinal ischemia due to either retinal artery or vein occlusion, oxygen supply from the choroidal vasculature to the thinner peripheral retina allows for greater viability of the retinal tissue and maintenance of peripheral visual fields.

Therefore, evaluation of the choroid and choroidal vasculature may be important in retinal vascular diseases, as the vasculature may partly modulate their severity. In particular, altered choroidal blood flow may be associated with decreased photoreceptor function. Furthermore, retinal and choroidal vascular abnormalities may occur simultaneously, especially in patients with systemic disease. In addition, retinal vascular diseases may induce the expression of molecules like vascular endothelial growth factor (VEGF) that can mediate changes in choroidal vasculature. Therefore, choroidal evaluation is important in retinal vascular diseases.

Choroidal Imaging in Retinal Vascular Diseases

Retinal vascular diseases are caused by abnormalities in retinal vasculature or by damage to retinal vessels that leads to closure. This disease category can be separated into (1) retinal vascular diseases caused by systemic disorders such as DR and hypertensive retinopathy and (2) those not directly caused by systemic disorders such as retinal artery occlusion (RAO), retinal vein occlusion (RVO), macular telangiectasia (MacTel), Coats’ disease, ocular ischemic syndrome (OIS), and pediatric retinal vascular diseases like retinopathy of prematurity (ROP). Among them, DR is the most prevalent retinal vascular disease and is the most common cause of vision loss in working-aged adults.

Choroidal evaluation and quantitative analysis of choroidal blood flow in retinal vascular disease using traditional imaging modalities like ultrasonography and indocyanine green angiography (ICGA) has been limited. With the recent advent of enhanced depth imaging OCT (EDI-OCT) and swept-source OCT (SS-OCT), choroidal change in retinal vascular diseases has been studied, and a number of reports, mostly those on DR and RVO, have improved our understanding of choroidal change in retinal vascular diseases.

Most studies have assessed choroidal thickness (CT) by manually measuring the perpendicular distance from the outer margin of the retinal pigment epithelium (RPE) to the inner sclera on the limited subfoveal or perifoveal area (rarely the peripapillary area). As the choroid is a highly vascularized tissue composed of an anastomosed network of choriocapillaries, measuring the CT at a few sample points in the subfoveal areas cannot accurately determine the CT or volume of the entire macula; thus, this method provides limited information about changes in the entire choroid. In particular, a wider posterior pole or peripheral area may be relevant in choroidal change, whereas the subfoveal area may be of specific interest for macular diseases such as age-related macular degeneration. CT or volume mapping are recommended for a comprehensive assessment of the choroid. Choroidal mapping has been used in very few studies on retinal vascular disease; however, it is particularly beneficial for assessing retinal and choroidal changes, as analysis of the entire peripheral area is more relevant than analyzing one or several points of the choroid. CT or volume mapping can be generated by a six-radial scan protocol with a commercial spectral domain OCT (SD-OCT) device ( Fig. 16.1 ) using an EDI technique or by a high-density multiple raster scanning protocol using SS-OCT. However, EDI-OCT and SS-OCT can include interpolation and segmentation errors, respectively, which necessitate careful interpretation on the results.

In this chapter, we summarize choroidal changes, in particular, CT in several retinal vascular diseases, and the thickness changes following treatment.

Retinal Vascular Diseases Associated With Systemic Disorders

Diabetic Retinopathy

It seems natural that a retinopathy-inducing disease like DM would also affect the choroid. Indeed, histopathologic studies have revealed various choroidal changes secondary to DM, which are collectively referred to as “diabetic choroidopathy.” Relevant changes related to diabetic choroidopathy are known to predominantly affect the choriocapillaris. A histopathologic examination showed microaneurysms, dilation or narrowing of the vascular lumen, and dropout of the choriocapillaris similar to retinal capillary dropout in retinopathy. Fluorescein angiography (FA) or ICGA images of eyes with diabetic choroidopathy show delayed choroidal vascular filling, appearing as multiple, lobular choroidal hypofluorescence, due to choriocapillaris degeneration, which may correlate with the severity of DR. Although the concept of diabetic choroidopathy has long been established, it has not received due attention, as traditional imaging modalities such as FA, ICG, and ultrasonography have a limited ability to identify choroidal changes caused by DM. The advent of advanced OCT technologies such as EDI-OCT and SS-OCT has revived the concept of diabetic choroidopathy, and several reports have shown choroidal changes in DM and their complications.

Choroidal changes in diabetes mellitus without diabetic retinopathy

There are conflicting reports regarding total CT in patients with DM ; however, most studies have shown choroidal thinning in diabetic eyes. One study examined both macular and peripapillary CT in DM, and both thicknesses showed significant thinning with increasing severity of DR. Patients with DM in one previous study showed choroidal thinning regardless of the presence of DR or macular edema (ME). A histopathological study using scanning electron microscopy showed obstruction of the choriocapillaris and vascular degeneration of the choroid, supporting observed OCT findings.

However, the large population–based Beijing Eye Study showed slightly thicker subfoveal choroid in patients with DM and indicated that the presence and stage of DR were not associated with change in subfoveal choroidal thickness (SFCT). Thus, choroidal change in patients with DM can occur as microvascular alterations independent of retinal change or retinopathy.

Choroidal change in diabetic retinopathy and macular edema

A recent OCT study on morphologic changes in the choroid of eyes with Dr showed irregular or “S”-shaped choroid-scleral borders, a displaced thickest point, and focal thinning of the choroid ( Fig. 16.2 ).

Several reports showed significant thinning of the choroid in DR. Adhi et al . showed choroidal thinning in eyes with DR, which was more remarkable in those with proliferative DR (PDR) and diabetic ME (DME). Interestingly, there was no significant difference in the thickness of the large choroidal vessel layer between DR and healthy patients, whereas significant thinning was noted in the choriocapillaris and medium choroidal vessel layers, depending on the severity of DR.

However, Kim et al . reported that the CT significantly increased with increasing severity of DR. Furthermore, Kase et al . showed significant choroidal thickening in eyes with severe nonproliferative DR (NPDR) and PDR; however, eyes with mild to moderate NPDR demonstrated remarkable choroidal thinning.

One report showed that eyes with DME had subfoveal choroidal thickening compared to eyes without DME, and this subfoveal choroidal thickening was most remarkable in eyes with serous retinal detachment (SRD)–type DME. It has been suggested that increased production of VEGF mediates vasodilation, and the subsequent increase in choroidal blood flow may lead to choroidal thickening in patients with severe stages of DR and DME. Another study by Gerendas et al ., however, showed that macular CT is significantly reduced in DME compared to healthy eyes. This study is noteworthy in that it evaluated the correlation between retinal thickness and CT by generating thickness maps. The study showed no significant correlation between the two thickness parameters; however, it did show that a thinner choroid was likely correlated with areas of large retinal leakage.

Although it is known that patients with DM or DR exhibit diverse choroidal changes, the mechanisms of choroidal thinning and thickening have yet to be determined. As the aforementioned cross-sectional studies did not assess longitudinal choroidal or retinal changes, they may be limited in ability to elucidate an association or cause–effect relationship between DM or DR and choroidal change. Furthermore, these studies are limited in that subfoveal or perifoveal points, which may not represent the entire diabetic choroid, are only measured for CT and thus, point-by-point variations in the choroid may lead to inconsistent results. Therefore, thickness or volume mapping of the choroid in eyes with Dr should be used to more accurately assess choroidal change.

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