Fig.1-2. Folding of the retina as a result of epiretinal membranes causes a shortening of the retina in both anteroposterior and circumferential directions (Thompson, 2006).
Circular or equatorial folds are generally in the region of the equator and roughly parallel to it, forming a ridge in the fundus. They may extend around the entire circumference of the globe. A circular fold is often displaced posteriorly over part of its course, sometimes close to the posterior pole of the globe. Occasionally, it runs a spiral course, which starts at the equator and ends near the disc. A circular fold is more prominent and more frequently visible above the horizontal meridian than below it (Fig.1-2). (Thompson, 2006).
A circular fixed fold at the equator is often accompanied by a posterior vitreous detachment or a large lacuna, the anterior limit of which corresponds to the fold. The vitreous gel anterior to the fold shows increased density. Its posterior aspect may become covered by a newly formed equatorial membrane (Thompson, 2006).
Surface maembranes and tractional RD
Surface membranes can be difficult to diagnose when the retina is attached. Many successfully repaired retinal detachments without redetachment show surface membrane formation if examined histopathologically. Occasionally, surface membranes can result in localized traction retinal detachments, often seen just posterior to a scleral buckle. These localized traction detachments tend to be stable and should be differentiated form recurrent rhegmatogenous retinal detachments associated with open retinal breaks (Ayelward, 1999).
Traction retinal detachments have a concave surface generated by retinal pigment epithelium pump pulling against traction, in contradistinction to the convex profile of rhegmatogenous detachments. Should a new retinal break acutely develops or a pre-existing break reopens, already present but unsuspected membranes may result in dramatic contraction of the newly redetached retina to give the clinical impression of sudden development of severe PVR (Ayelward, 1999).
Two kinds of star-shaped folds exist: limited and generalized. Limited star-shaped folds are more frequent in the lower than in the upper half of the fundus (Fig.1-3). A generalized star-shaped fold has its center at the disc. It is not unusual to see a star-shaped fold at or near the macula, where it may form a large macular pucker, although a pucker is seldom star-shaped. Several star-shaped folds may be present in the same eye. A star-shaped fold is generally the result of shrinkage of a preretinal membrane. The preretinal membrane may exist before a retinal detachment develops. In this case, the star forms gradually as the detachment progresses. More rarely, the star-shaped fold results from shrinkage of a subretinal membrane. The size of star shaped fold may vary from 1 to 2 disc diameters (DD) to entire fundus. The latter is formed by shrinkage of a preretinal membrane that grew from the disc area and caused multiple, roughly triangular folds whose apices were near the disc (Schepens et al., 2000).