AV, or arteriovenous nicking (also known as arteriovenous nipping in the UK) is the phenomenon where, on ophthalmologic examination, an arteriole

Arteriole

An arteriole is a small diameter blood vessel in the microcirculation that extends and branches out from an artery and leads to capillaries.Arterioles have muscular walls and are the primary site of vascular resistance...

 is seen crossing a venule

Venule

A venule is a very small blood vessel in the microcirculation that allows deoxygenated blood to return from the capillary beds to the larger blood vessels called veins. Venules range from 8 to 100μm in diameter and are formed when capillaries unite .Venules are blood vessels that drain blood...

 resulting in impaction of the vein with bulging on either side of the crossing. This is most commonly seen in hypertensive retinopathy

Hypertensive retinopathy

Hypertensive retinopathy is damage to the retina due to high blood pressure .-Pathophysiology:The retina is one of the "target organs" that are damaged by sustained hypertension....

.

It is thought that, since the arteriole and venule share a sheath, the arteriole's thicker walls push against those of the venule forcing the venule to collapse. This makes the venule form an hourglass shape around the arteriole. Other theories suggest that this results not from compression from the arteriole but from sclerotic

Sclerosis (medicine)

In medicine, sclerosis refers to the stiffening of a structure, usually caused by a replacement of the normal organ-specific tissue with connective tissue.Types include:...

 thickening or glial cell

Glial cell

Glial cells, sometimes called neuroglia or simply glia , are non-neuronal cells that maintain homeostasis, form myelin, and provide support and protection for neurons in the brain, and for neurons in other parts of the nervous system such as in the autonomous nervous system...

proliferation at the site where the two vessels cross.