What is optic nerve hypoplasia

Figure 1. Various features of optic nerve hypoplasia

Sometimes, the patients with optic nerve hypoplasia were referred to me as optic neuritis.

Optic nerve hypoplasia (ONH) is defined as small disc size. Then, How small is the ONH?
The optic disc size can be measured by distance from optic nerve head to macula.

Figure 2. Definition of ONH.

Normal optic disc (DD) was placed 2 and half or 3 discs between macula and optic nerve head (DM).
If the 4 or more discs can be placed between macula and optic nerve head, we can say it is ONH. In other words, ONH was diagnosed when DM/DD is over than 4.