The Horizontal Raphe of the Human Retina and its Watershed Zones

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.

Paracentral acute middle maculopathy following coronary angiography

“…The PAMM lesion for our patient was also noted to occur in the area between the superior and inferior retinal vascular arcade, along the horizonal raphe (HR), which has also been described as a watershed zone. 13 Given the etiology of PAMM, our patient may represent a unique demonstration of a dual watershed effect due to both the vertical intraretinal location of the affected DCP, and the horizontal positioning within the HR. 6 , 13 …”

Multimodal Imaging, OCT En Face, and OCT Angiography of an Anomalous Retinal Artery: Case Report and Review of the Literature

“…The retina presents a temporal horizontal raphe that divides the retina into 2 hemispheres: superior and inferior [ 4 ]. These hemispheres contain 2 different vascular systems that communicate only at the level of the 3 capillary plexuses, but in the parafovea only at the level of the deep capillary plexus [ 2 , 3 ]. However, at the temporal raphe, there is a distinct separation between the nerve fiber layer of the superior and inferior temporal retina, and this has been attributed to the earlier development of the central retina and to the existence of the 2 watershed zones [ 4 ].…”

“…However, at the temporal raphe, there is a distinct separation between the nerve fiber layer of the superior and inferior temporal retina, and this has been attributed to the earlier development of the central retina and to the existence of the 2 watershed zones [4]. The separate circulation of the superior and inferior hemispheres of the temporal retina might be a consequence of development and, in particular, of the organization of the long posterior ciliary arteries, and it has been suggested that it may also play a protective role in the case of vascular occlusive diseases [3]. Retinal vessel development initiates in the optic nerve and proceeds centripetally, respecting the temporal raphe and reaching the retinal periphery before birth [1].…”

“…The large retinal vessels run at the level and parallel to the nerve fiber layer, and the superior and inferior distribution of the large retinal vessels in the temporal retina allows the temporal nerve fibers of the optic nerve to run horizontally in the papillomacular bundle and to surround the macula to form a temporal raphe that is the basis of the horizontal meridian of the visual field [ 3 , 4 ]. It is believed that the division of the human retina in 2 hemispheres with different vascularization and nerve fiber layer may have protective visual effects in the case of vascular occlusive or optic nerve diseases [ 3 ].…”

“…The formation of the nerve fiber temporal raphe has been attributed to the earlier development of the central retina and also to the existence of the watershed zones [ 3 , 4 ]. However, anatomical variations of the retinal vasculature have been described in which large retinal vessels arteries or veins cross the horizontal retinal raphe.…”