The Anatomy of the Choroid

It is thought that the choroid was first described and drawn by Democritus of Abdera around 400 BC. Democritus named the sclera chitoon puknotatos (dense tunic) and its inner layer chitoon malista somphos (more spongy tunic). Later, Hippocrates replaced the terms with leukon (white tunic) and divided the choroid into an outer meninx leptotera (delicate membrane) and inner arachnoeides (cobweb-like tunic). The word chorio/chorion referring to a layer in the eye was used for the first time by Celsus in the first years of our era, after which no significant advances in choroidal anatomical knowledge were documented until 1722, when Frederick Ruysch studied the choroid with wax casts and his son Henric Ruysch named the complex of the retinal pigment epithelium, the Bruch’s membrane and the choriocapillaris, as tunica ruyschiana . The detailed work of both Albrecht Von Haller and Hubert Sattler gave the name to the respective layers of great choroidal vessels (Haller) and middle vessels (Sattler). The choriocapillaris was first described by J. Hovius, although the name choriocapillaris was proposed only later by Eschricht. In 1844 Carl Ludwig Wilhelm Bruch described separately the tunica elastica , now known as Bruch’s membrane. The structure of the vortex veins was described by Fuchs. The description of the hemispheres of the choroid (nasal and temporal) and the posterior ciliary arteries was more recently published by Hayreh in 1973. The use of different histological techniques, such as latex casts, in association with ocular imaging increased gradually our knowledge of the anatomy of the choroid.

Imaging of the Choroid

Since Von Helmholtz described the first in vivo ocular fundus examination in 1851, the development of photographic techniques made also possible the first in vivo fundus image by Jackman and Webster, later improved upon by Starr and Howe. These first images were revolutionary to the study of retinal disease and laid the foundation for future essential developments in choroidal imaging. Novotny and Alvis designed a photographic system for sequential imaging of intravascular fluorescein transit through the ocular fundus in 1959 and fluorescein angiography enabled the study of a wide range of conditions including intraocular tumors, chorioretinal dystrophies, age-related macular degeneration, central serous chorioretinopathy, posterior uveitides, and ischemic pathology. However, limited information of the choroid could be gained by fluorescein angiography alone, due to the masking effect of the retinal pigment epithelium and the permeability of the choriocapillaris.

Indocyanine green angiography was described by Flower and Hochheimer and, used in conjunction with fluorescein angiography, allowed for more distinct analysis of the choroidal and retinal circulations. The development of scanning laser ophthalmoscopes and infrared video cameras has further increased the capabilities and diagnostic range of indocyanine green angiography, greatly increasing our understanding of choroidal circulation and its alteration in conditions such as age-related macular degeneration, polypoidal choroidal vasculopathy, central serous chorioretinopathy, pseudoxanthoma elasticum, intraocular tumors, and inflammatory chorioretinal disease.

Contemporary to the development of fundus imaging, diagnostic ultrasound imaging provided new information on the choroid. The ocular use of ultrasound was first described by Mundt and Hughes, based on A-scans, after whom Baum and Greenwood developed the interpretation of B-scans, paving the way for more precise differential diagnosis and assessment of intraocular tumors, as well as other entities such as choroidal detachments. A significant advantage brought by ultrasound imaging was the ability to measure thickness of choroidal lesions in addition to the en face dimensions determined by funduscopic techniques.

The most recent milestone in choroidal imaging is the development of optical coherence tomography. Optical coherence tomography provides high-definition cross-sectional images of the posterior ocular tissues. Earlier generations of time-domain and spectral-domain devices were not able to visualize the choroidal tissue well due to the blockage of incident light by the retinal pigment epithelium. The introduction of deeper penetration techniques, such as enhanced deep imaging and swept-source optical coherence tomography, enable the full thickness of the choroid to be imaged in many cases and have been revolutionary to our understanding of choroidal disease. In addition, these techniques allow for more accurate measurement of choroidal tissue and choroidal lesions.

Over the last 2–3 years the introduction of commercially available motion contrast and decorrelation algorithms, together with cost reduction in computational power with which to execute them, has led to rapid deployment of optical coherence tomography angiography in both research and clinical settings. Optical coherence tomography angiography provides depth-resolved microvascular flow information, which is spatially aligned with structural optical coherence tomography data and which does not require intravenous dye injection. This technology is still in its infancy and its full potential remains to be discovered and released.

The history of choroidal imaging has featured a steady but accelerating accumulation of data and knowledge about this enigmatic tissue, yet is an exemplary of the paradox between achievement and futility characterized by the realization that the more we know, the more confident we become that we still know very little.