Abstract
The horizontal raphe (HR) as a demarcation line dividing the retina and choroid into separate vascular hemispheres is well established, but its development has never been discussed in the context of new findings of the last decades. Although factors for axon guidance are established (e.g., slit-robo pathway, ephrin-protein-receptor pathway) they do not explain HR formation. Early morphological organization, too, fails to establish a HR. The development of the HR is most likely induced by the long posterior ciliary arteries which form a horizontal line prior to retinal organization. The maintenance might then be supported by several biochemical factors. The circulation separate superior and inferior vascular hemispheres communicates across the HR only through their anastomosing capillary beds resulting in watershed zones on either side of the HR. Visual field changes along the HR could clearly be demonstrated in vascular occlusive diseases affecting the optic nerve head, the retina or the choroid. The watershed zone of the HR is ideally protective for central visual acuity in vascular occlusive diseases but can lead to distinct pathological features.
Highlights
The horizontal raphe (HR) was first described in the early 1800s as a horizontal demarcation line that extends from the macula to the temporal Ora dividing the temporal retinal nerve fiber layer into a superior and inferior half [1]
Vrabec histologically examined the HR in the temporal macular region and found nerve fibers bundles crossing the HR for short distances [6]
Understanding the complex developmental anatomy of the retina and choroid in the fetus and childhood allows one to better understand the HR’s origin, composition, and extent along the horizontal meridian, along with the role it plays in the clinical picture of adult occlusive vascular disease involving the optic nerve head, the retina, or the choroid
Summary
The horizontal raphe (HR) was first described in the early 1800s as a horizontal demarcation line that extends from the macula to the temporal Ora dividing the temporal retinal nerve fiber layer into a superior and inferior half [1]. Advanced optics confocal scanning laser ophthalmoscopy (AOSLO) can measure the various retinal layers temporal to the macula These instruments objectively measure the nerve fiber layer thickness and defects in the nerve fiber layer thickness that exists adjacent to the optic nerve head and at the macula in aging and glaucoma [7,8,9,10,11]. Several studies demonstrated that the temporal HR does not follow exactly the horizontal meridian, but is inclined, on average, 10 degrees above the horizontal meridian [8,14,16] Both the confocal scanning laser ophthalmoscope and the OCT play a clinical role in the diagnosis of diseases affecting the nerve fiber layer and the HR. Understanding the complex developmental anatomy of the retina and choroid in the fetus and childhood allows one to better understand the HR’s origin, composition, and extent along the horizontal meridian, along with the role it plays in the clinical picture of adult occlusive vascular disease involving the optic nerve head, the retina, or the choroid
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