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Skull Base & Orbit

Orbit Anatomy: Septum and Cone First

Fellowship-level neuroradiology teaching by E. Brooke Schrickel, MD. Open this topic in the interactive reading room →

Watch · concise explainerVisual summary

Core clinical idea

Localize every orbit case on two anatomic axes before naming anything: the septum (pre- vs post-septal = the infection/triage line) and the cone (intra- vs conal vs extraconal = the differential narrower). Then walk optic-pathway disease by named segment.

Bottom line

Localize every orbit case on two axes — the septum and the cone — then walk the optic nerve by segment.

Core workstation questions

  • Pre-septal or post-septal? (Does this become an admission?)
  • Intraconal, conal, or extraconal?
  • If the optic nerve is involved, which segment, and does the field deficit match?
  • Is the superior ophthalmic vein distended? (Cavernous sinus / fistula?)
  • Is the lamina papyracea intact, or is sinus disease crossing into the orbit?

What changes reporting / management

  • State the pre- vs post-septal compartment on every orbital infection; post-septal threatens vision and brain and drives admission, IV antibiotics, and possible drainage.
  • Name the compartment (intra-/conal/extraconal) to narrow the mass differential before naming a lesion.
  • Localize optic-nerve disease by segment (intraocular/intraorbital/intracanalicular/intracranial) so the deficit and surgical plan line up.
  • Flag a distended superior ophthalmic vein and look at the cavernous sinus for thrombosis or carotid-cavernous fistula.

Practical traps

  • Blurring the optic canal (CN II + ophthalmic artery; a vision problem) with the superior orbital fissure (CN III/IV/VI + V1 + ophthalmic veins; an ophthalmoplegia problem).
  • Treating choroid/uveal enhancement as pathology when it is normal physiology.
  • Memorizing the orbital wall map while failing to make the septum and cone calls that actually triage the case.

Teaching pearls

  • Decide pre- vs post-septal before you name a single muscle — that's the triage line.
  • The cone is the optic nerve's neighborhood; intra- vs extraconal narrows the differential before you have a diagnosis.
  • Name the optic nerve segment, not just 'the optic nerve.'
  • A fat superior ophthalmic vein points you at the cavernous sinus — go look.

Teaching visuals

Anatomy of the Orbit — Compartments & ApexLocalize the lesion: intraconal vs conal vs extraconal — it drives the differential.Axial — both orbitsanterior at top of section · realistic correlateanterior (globes)posterior (apex) ↓Globeanterior chamber + vitreousLensanterior pole of the globeOptic nerve (CN II)runs posteriorly to the apexMedial rectuscone wall, against laminaLateral rectuscone wall, lateralLamina papyraceapaper-thin medial wallEthmoid air cellsbetween the orbitsOrbital apexcone converges; canal + SOF hereCompartments — localize the lesionintraconal vs conal vs extraconalIIintraconalconalextraconalCoronal — 3 compartmentsIntraconal — inside the muscle coneoptic nerve glioma / meningioma,cavernous hemangiomaConal — the extraocular musclesthyroid eye disease(muscle-belly enlargement)Extraconal — outside the conelacrimal lesion, dermoid,subperiosteal collectionAt the apexoptic canal: CN II + ophthalmic arterysuperior orbital fissure: CN III, IV, V1, VI + sup. ophthalmic vein
Anatomy of the Orbit — Compartments & Apex

Source lectures

  • Anatomy of the Orbit with Imaging Correlation

Educational material for radiology residents and neuroradiology fellows. Nothing here drives individual patient care, and it contains no patient data.