HZO occurs when latent varicella zoster virus (VZV), residing in the ganglion of the trigeminal nerve, reactivates and affects the ophthalmic nerve (V1). Primary infection with VZV commonly occurs in childhood as chickenpox. Anterior uveitis occurs in 40-50% of all cases of HZO (but only when the tip of the nose is involved).
Here is a good article from the British Medical Journal: Download H Zoster O
Image from BMJ article