Distraction Sickle

Distraction Sickle, a crescent-shaped white field visible at the optic nerve disc during ophthalmoscopy. The optic nerve passes into the eyeball through an opening in the sclera and choroid, which form a canal (canalis sclerotico-chorioidalis). Depending on the shape of this canal, the appearance of the optic nerve disc circumference changes. In cases where one wall of the canal goes straight forward while the other (temporal wall) takes an oblique direction, the optic nerve disc is pulled inward toward the canal. During ophthalmoscopy, due to the transparency of the optic nerve disc tissue, the temporal wall of the canal adjacent to the circumference of the disc can be seen in the

Distraction sickle: 1 - sclera; 2 - chorio-idea; 3 - pigment epithelium; 4 - retina; 5 - sickle. (From Ahepfeld.)

form of a narrow light sickle - D. s. (according to Jager - cone). Such a sickle (or cone) is also observed in cases where the choroid with the pigment epithelium of the retina (due to their insufficient development) does not end at the very edge of the optic nerve. Then between the edge of the optic nerve and the pigment ring, the exposed sclera is visible in the shape of a sickle. D. s. is most often observed in myopic eyes, where traction of the optic nerve occurs more frequently and is more pronounced. The sickle is also often found in emmetropic and even hypermetropic eyes. The development of the sickle is explained mainly by the everting action of the superior oblique muscle (Stilling).