The anterior ethmoid nerve is a branch of the nasociliary nerve (V1), passing through the anterior ethmoid foramen to enter the anterior cranial fossa and then descending through the cribriform plate into the nasal cavity. It divides into internal nasal branches (supplying the anterior lateral nasal wall and anterior nasal septum), and an external nasal branch (supplying the skin of the nasal dorsum and tip — Hutchinson sign in herpes zoster ophthalmicus). The Keros classification describes the depth of the cribriform plate olfactory fossa relative to the ethmoid roof.
The anterior ethmoid nerve and its cribriform plate passage are critical in functional endoscopic sinus surgery: the Keros classification (Types 1-3, depth 1-7+ mm) grades the olfactory fossa depth and associated skull base perforation risk during anterior ethmoidectomy. Keros type 3 (deep olfactory fossa, >7 mm depth) has the highest risk of intracranial penetration during aggressive middle meatal antrostomy. The external nasal branch of the anterior ethmoid nerve explains the Hutchinson sign in herpes zoster ophthalmicus: nasal tip vesicles from external nasal branch involvement predict corneal (nasociliary/long ciliary branch) involvement.
Keros type 3 anatomy with a deep olfactory fossa (greater than 7 mm below the ethmoid roof) creates a tall fragile lateral lamella of the cribriform plate that is easily fractured during aggressive anterior ethmoidectomy, producing a CSF leak and potential intracranial penetration; pre-operative CT Keros classification guides surgical technique and the degree of caution exercised at the skull base.
