Periorbital (Preseptal) & Orbital Cellulitis

The critical distinction is anatomical: preseptal (periorbital) cellulitis is anterior to the orbital septum and does not threaten vision; orbital cellulitis is posterior to the septum and can cause permanent vision loss. The single most useful bedside differentiator is whether vision and eye movements are affected — if they are, treat as orbital cellulitis until proven otherwise.

Children under 4 years have an incomplete orbital septum and so carry a higher risk of infection spreading from preseptal to orbital.

Periorbital (preseptal) cellulitis

Soft tissue infection of the eyelids anterior to the orbital septum, usually from spread of infection from sinusitis, a stye (hordeolum), dacryocystitis, or eyelid trauma.

• Organisms: Staph and Strep most common; H. influenzae type b in the unimmunised; HSV/HZV also possible
• Features: erythema and swelling of periorbital soft tissue; vision and eye movements not affected; no systemic symptoms

Management (oral antibiotics if no systemic features):

Scenario	Regimen
Simple infection	Flucloxacillin 500 mg QID for 7 days (12.5 mg/kg QID in children)
Concurrent sinusitis or H. influenzae b risk (child <5, incompletely vaccinated)	Amoxicillin+clavulanate DF 12-hourly for 7 days (22.5+3.2 mg/kg)
High MRSA risk	Trimethoprim/sulfamethoxazole 160/800 mg BD for 7 days

If systemic features or no improvement after 48 hours of oral therapy → IV flucloxacillin 2 g 6-hourly for 7 days.

Orbital cellulitis

Usually arises from paranasal sinus infection; occasionally endogenous spread in immunocompromised patients. Organisms: Staph, Strep, H. influenzae b, and fungal causes in immunocompromised/diabetic patients. Children under 4 years are at increased risk (same incomplete-septum mechanism).

Features: reduced vision, limited and painful eye movements, diplopia, proptosis, chemosis, ± systemic symptoms (fever, vomiting).

Investigations: blood cultures, CT orbits.

Management: refer to hospital — may need surgical drainage of collections; IV broad- spectrum antibiotics (e.g. ceftriaxone + flucloxacillin); de-escalate to amoxicillin+ clavulanate DF once improving.

Differential for a swollen/tight orbit

Not every tight or proptotic orbit is infective. Consider:

• Orbital compartment syndrome — acute, usually post-traumatic, raised intraorbital pressure with markedly raised IOP and RAPD; a time-critical surgical emergency (lateral canthotomy) — see penetrating-eye-injury.md
• Thyroid eye disease — proptosis with lid retraction/lag, usually without the fever and severe pain-on-eye-movement of orbital cellulitis — see thyroid-eye-disease.md
• Orbital vascular/neoplastic lesions (varices, vascular malformations, tumours) — proptosis that may fluctuate with Valsalva manoeuvres, without the acute infective picture