The Orbit - A Complete MBBS Ophthalmology Guide

1. Introduction

• Volume: approximately 30 mL
• Shape: pyramid with base facing anteriorly and apex directed posteromedially
• The orbital axis diverges about 23° from the visual axis

2. Bony Orbit - The Seven Bones

The Orbital Rim (Base of the Pyramid)

3. The Four Orbital Walls

Roof (Superior Wall)

• Formed by the orbital part of the frontal bone (mainly) + small contribution from the lesser wing of sphenoid posteriorly
• Thin plate of bone separating the orbit from the anterior cranial fossa and the brain
• Special features:
  • Anteromedially: trochlear fovea (pulley for superior oblique muscle) and possible intrusion of the frontal sinus
  • Anterolaterally: lacrimal fossa (contains orbital part of lacrimal gland)
• Clinical note: roof fractures are common in children from minor brow trauma

Medial Wall

• Four bones (medial to lateral): Maxilla + Lacrimal + Ethmoid + Sphenoid (mnemonic: "MALES")
• Largest contributor: orbital plate of ethmoid (contains ethmoidal air cells - very thin, "lamina papyracea")
• Junction of roof and medial wall: anterior and posterior ethmoidal foramina (for anterior/posterior ethmoidal nerves and vessels)
• Anterior portion: lacrimal groove formed by lacrimal bone (posterior lacrimal crest) and frontal process of maxilla (anterior lacrimal crest) - contains the lacrimal sac
• Both medial walls are parallel to each other
• Clinical: medial wall is the thinnest wall - fractures allow air from ethmoid sinuses into orbit ("orbital emphysema" - patient feels fullness on blowing nose)

Floor (Inferior Wall)

• Also the roof of the maxillary sinus
• Bones: orbital surface of maxilla (mainly) + contributions from zygomatic and palatine bones
• The inferior orbital fissure begins posteriorly and runs along the lateral boundary of the floor
• Clinical: most commonly fractured wall in blow-out fractures - muscle entrapment causes diplopia

Lateral Wall

• Two bones: zygomatic bone (anterior) + greater wing of sphenoid (posterior)
• The superior orbital fissure lies between the greater and lesser wings of the sphenoid
• This is the thickest, strongest wall - fractures require major force
• Clinical: lateral wall separates the orbit from the middle cranial fossa and temporal lobe

4. Important Orbital Openings (Foramina and Fissures)

5. Orbital Septum - The Key Dividing Structure

• Pre-septal (preseptal) space - anterior to the septum (eyelid proper)
• Post-septal (orbital) space - posterior to the septum (true orbit)

6. Contents of the Orbit

• Eyeball (globe)
• Optic nerve - CN II (from globe to optic canal)
• Extra-ocular muscles (EOMs): 4 rectus muscles (superior, inferior, medial, lateral) + 2 oblique muscles (superior, inferior) + levator palpebrae superioris
• Lacrimal gland - in lacrimal fossa of the frontal bone (anterolateral)
• Cranial nerves: CN III (oculomotor), IV (trochlear), V1 (ophthalmic), VI (abducens)
• Ophthalmic artery and its branches
• Ophthalmic veins (superior and inferior) - drain to cavernous sinus
• Ciliary ganglion - parasympathetic ganglion in posterior orbit (lateral to optic nerve)
• Orbital fat - fills the space and cushions structures

7. Clinical Conditions of the Orbit

7A. Orbital Cellulitis (Bacterial)

• Rapid onset pain (worse on eye movement)
• Proptosis (may be non-axial/dystopia if abscess present)
• Painful ophthalmoplegia
• Reduced VA, impaired colour vision
• RAPD (afferent pupillary defect) - suggests optic nerve compression
• Tender, erythematous, warm eyelids + chemosis
• Pyrexia - often marked
• Choroidal folds, optic disc swelling on fundoscopy

• Optic neuropathy, exposure keratopathy, raised IOP
• Subperiosteal abscess (most common along medial wall)
• Intracranial: meningitis, brain abscess, cavernous sinus thrombosis

• WBC, blood cultures, nasal swab cultures
• High-resolution CT orbit + sinuses + brain (modality of choice - confirms diagnosis, excludes abscess)
• MRI also used

• Mandatory hospital admission + urgent ENT review
• IV antibiotics: ceftazidime + oral metronidazole (cover anaerobes)
• Continue IV until apyrexial 4 days, then oral 1-3 weeks
• Optic nerve function monitored every 4 hours (VA, color vision, pupil)
• Surgery: drain subperiosteal abscess; drain infected sinuses if not responding

7B. Preseptal (Periorbital) Cellulitis

• No proptosis, no chemosis
• Normal VA, normal pupillary reactions, normal ocular motility
• Swollen, firm, tender, red eyelid

7C. Chandler-Hubert Classification of Orbital Complications of Sinusitis

7D. Blow-out Fractures of the Orbit

Orbital Floor Fracture

• Periocular ecchymosis and oedema
• Subcutaneous emphysema (crackling on palpation - air from maxillary sinus)
• Infraorbital nerve anaesthesia: numbness of lower lid, cheek, side of nose, upper lip, upper teeth and gums (infraorbital canal frequently involved)
• Diplopia - mechanisms:
  • Haemorrhage/oedema restricting inferior rectus/oblique movement (temporary)
  • Mechanical entrapment of inferior rectus or inferior oblique within the fracture - diplopia in upgaze and downgaze
  • Contusion of extraocular muscles
  • Orbital haemorrhage
• Enophthalmos (sunken eye) - due to increased orbital volume; may be masked initially by oedema

• Most resolve with observation if no entrapment/significant diplopia
• Surgery (repair of orbital floor) indicated for:
  • Persistent diplopia with motility restriction
  • Enophthalmos >2 mm
  • Large fracture with significant tissue prolapse
  • Infraorbital anaesthesia (relative indication)

Medial Wall Fracture

• Usually associated with floor fractures; rarely isolated
• Signs: periorbital ecchymosis, subcutaneous emphysema (nose blowing)
• Medial rectus entrapment: defective abduction and adduction
• Treatment: release entrapped tissue, repair bony defect

Roof Fracture

• Rare; more common in children
• Haematoma/ecchymosis of upper eyelid
• Large fractures: globe pulsation (CSF pressure transmission)
• Exclude CSF leak (meningitis risk)

Orbital Haemorrhage (Retrobulbar)

• Can cause acute orbital compartment syndrome with compressive optic neuropathy - can cause irreversible blindness
• Causes: trauma, iatrogenic (peribulbar/retrobulbar block), vascular anomalies
• Emergency management: lateral canthotomy and cantholysis to decompress the orbit

7E. Idiopathic Orbital Inflammatory Disease (Orbital Pseudotumour)

• Usually unilateral in adults; bilateral in children
• Acute/subacute onset of ocular and periocular redness, swelling, and pain
• Congestive proptosis
• Mild to severe ophthalmoplegia
• Features of optic nerve dysfunction (esp. posterior orbit involvement)
• Choroidal folds possible

• Spontaneous remission after a few weeks
• Intermittent episodes with eventual remission
• Severe prolonged inflammation → progressive fibrosis → "frozen orbit" (ophthalmoplegia + ptosis + visual impairment)

• CT: ill-defined orbital opacification and loss of definition of orbital contents
• Biopsy: often required to confirm diagnosis and exclude lymphoma, neoplasia, sarcoidosis, granulomatosis with polyangiitis

7F. Thyroid Eye Disease (Graves' Orbitopathy / Dysthyroid Orbitopathy)

• One point each for: pain on eye movement, eye pain at rest, eyelid erythema, conjunctival injection, chemosis, eyelid swelling, caruncle/plica inflammation
• Score ≥3/7 = active disease requiring immunosuppression

• Oral prednisolone 60-80 mg/day initially, tapered by response
• IV methylprednisolone preferred (better tolerability, superior outcomes):
  • Lower-intensity regimen: 0.5 g once weekly x 6 weeks, then 0.25 g once weekly x 6 weeks
  • Acute compressive optic neuropathy: pulsed IV methylprednisolone 0.5-1 g (3 successive days or alternate days)
• Contraindications to IV: significant hepatic dysfunction, cardiovascular disease, uncontrolled hypertension/diabetes

• Compressive optic neuropathy unresponsive to steroids
• Intractable corneal exposure
• Cosmetically disabling proptosis

7G. Rhino-Orbital Mucormycosis

• Rapid spread from nasal/sinus mucosa → orbit → intracranially
• Black eschar on nasal mucosa
• Orbital cellulitis signs + severe proptosis
• Very high mortality if untreated

• Correct underlying metabolic defect (control DKA)
• IV antifungal (amphotericin B)
• Daily packing/irrigation with antifungal agent
• Wide surgical excision of devitalized/necrotic tissue; exenteration in unresponsive cases
• Adjunctive hyperbaric oxygen

8. Differential Diagnosis of an Acutely Inflamed Orbit

9. Key Mnemonics and Memory Aids

• Maxilla, Zygomatic, Frontal, Ethmoid, Lacrimal, Sphenoid, Palatine

• Lacrimal nerve, Frontal nerve, Trochlear nerve (CN IV), Superior ophthalmic vein, Oculomotor nerve (CN III), Ciliary nerve (nasociliary), Abducens (CN VI)

10. Summary Table - Quick Revision