**3. Pathophysiology of factors controlling blood flow in the optic nerve head**

The blood flow in the ONH depends upon: a). resistance to blood flow, b). arterial blood pressure (BP), and c). intraocular pressure (IOP) [1, 2].


The blood flow in the ONH is calculated by using the following formula [1]:

Perfusion pressure = Mean BP minus intraocular pressure (IOP). Mean BP = Diastolic BP + 1/3 (systolic BP- diastolic BP).

AION cases can be broadly classified into two groups [1, 2]:

NA-AION is a multifactorial disease with multiple risk factors that contribute to its develop‐ ment: the nocturnal arterial hypotension is the most important risk factor. Often, NA-AION patients have an anatomical predisposition: small discs, where structural crowding of nerve fibers (crowded disk), and reduction of the vascular supply, which may combine to impair

The ONH consists of, from front to back: a). surface nerve fiber layer, b). prelaminar region,

**a.** The surface nerve fiber layer is mostly supplied by the retinal arterioles. The cilioretinal artery, when present, usually irrigates the corresponding sector of the surface layer [1, 2].

**b.** The prelaminar region is situated in front of the lamina cribrosa. It is supplied by centri‐

**c.** The region of the lamina cribrosa is irrigated by centripetal branches from the PCAs, either directly or by the so-called arterial circle of Zinn and Haller, when that is present [1, 2]. **d.** The retrolaminar region is the part of the ONH that lies immediately behind the lamina cribrosa. It is supplied by two vascular systems: the peripheral centripetal and the axial centrifugal systems. The former represents the major source of irrigation to this part. It is formed by recurrent pial branches arising from the peripapillary choroid and the circle of Zinn and Haller (when present, or the PCAs instead). In addition, pial branches from the central retinal artery (CRA) also supply this part. The latter is not present in all eyes. When present, it is formed by inconstant branches arising from the intraneural part of the CRA. From the account of the arterial irrigation of the ONH given above, it is evident that the

**2. Arterial blood supply of the anterior part of the optic nerve**

Arterial blood supply of the anterior part of the ONH is presented in figure 1.

**Figure 1.** Arterial blood supply of the anterior part of the optic nerve.

c). lamina cribrosa region, and d). retrolaminar region.

petal branches from the peripapillary choroid [1, 2].

PCAs are the main source of blood supply to the ONH [1, 2].

perfusion to a critical degree [1, 2].

112 Updates in the Diagnosis and Treatment of Vasculitis

	- **a.** thrombotic lesions: Occlusion of the PCAs is most commonly caused by GCA (resulting in infarction of the ONH and A-AION) and less commonly by other types of vasculitis.
	- **b.** embolic lesions: Multiple emboli in the vessels of the ONH have been demonstrated histopathologically in NA-AION.
