Sinus-related (most common in children) (Streptococcus pneumoniae · Streptococcus pyogenes · Streptococcus anginosus group · Haemophilus influenzae (non-typeable or type b if underimmunized) · oral anaerobes from ethmoid/sphenoid sinusitis (CPS 2026, RCH, AAP Red Book))
Staphylococcus aureus / MRSA (Staphylococcus aureus (including CA-MRSA) — consider when prior MRSA, high local prevalence, severe disease, or subperiosteal abscess (CPS 2026, RCH, IDSA SSTI principles))
Dose is calculated using the ampicillin component.
Max 2000 mg/dose (ampicillin) · Max 8000 mg/day · Duration: 10–14 days · CPS 2026 orbital pathway — 50 mg/kg/dose ampicillin component q6h; covers streptococci, H. influenzae, MSSA, and anaerobes for sinus-associated post-septal infection
• Diagnosis: infection **posterior to orbital septum** (post-septal/orbital cellulitis) — proptosis, painful/restricted extraocular movements, chemosis, decreased visual acuity, or RAPD; distinguish from preseptal (periorbital) cellulitis where globe is not involved (CPS 2026, RCH, AAP).
• **All orbital cellulitis requires hospitalization** with IV antibiotics, urgent ophthalmology AND ENT consultation, and close neuro-ophthalmic monitoring (CPS 2026, RCH, NICE serious illness principles).
• Ampicillin/sulbactam first-line for typical sinus-associated orbital cellulitis — covers streptococci, H. influenzae, MSSA, and anaerobes from ethmoid sinusitis (most common pediatric source) (CPS 2026, RCH, UCSF).
• Obtain blood cultures if febrile/systemically unwell; consider conjunctival swab if drainage present — tailor therapy to culture results (CPS 2026, RCH).
• CT orbits and sinuses: obtain when diagnosis uncertain, suspected subperiosteal/intraconal abscess, no improvement at 24–48 h, or surgical planning needed — contrast-enhanced CT is first-line; MRI if intracranial extension or cavernous sinus thrombosis suspected (CPS 2026, RCH, ophthalmology/ENT guidelines).
Enter weight on Kids to calculate doses
Ceftriaxone • i.v. • 50–75 mg/kg/day • 1×/day
Max 2000 mg/day · Duration: 10–14 days · CPS/RCH orbital pathway — ceftriaxone 50–75 mg/kg/day once daily (max 2 g/day) plus metronidazole for anaerobic coverage from sinus source
• Ceftriaxone + metronidazole: when ampicillin/sulbactam unavailable, penicillin allergy (non-anaphylactic), or preference for once-daily cephalosporin dosing — CPS 2026 orbital alternative (RCH uses ceftriaxone + clindamycin as alternate; metronidazole per user protocol and anaerobic coverage principles).
• IV-to-oral switch criteria: clinically improving after 48–72 h IV therapy, afebrile ≥24 h, decreasing proptosis/swelling, normalizing extraocular movements, stable or improving vision, and tolerating oral intake — switch to amoxicillin/clavulanate to complete 10–14 day total course (CPS 2026, RCH, Lancet ID switch guidelines).
• Do **not** use outpatient oral therapy as initial treatment for confirmed orbital cellulitis — this is distinct from preseptal (periorbital) cellulitis (CPS 2026).
• ENT consultation: mandatory — treat underlying acute bacterial rhinosinusitis; consider functional endoscopic sinus surgery if no improvement at 48 h or subperiosteal abscess present (CPS 2026, RCH, ENT/ophthalmology guidelines).
• Ophthalmology consultation: mandatory — serial exams for vision, pupillary responses, color vision, and extraocular motility; urgent re-evaluation if any deterioration (CPS 2026, RCH, AAP).
Enter weight on Kids to calculate doses
Metronidazole • i.v. • 10–10 mg/kg/dose • 3×/day
Max 500 mg/dose · Max 1500 mg/day · Duration: 10–14 days · 10 mg/kg/dose IV TID (max 500 mg/dose) — anaerobic coverage for sinus-associated orbital cellulitis (CPS 2026, RCH)
Enter weight on Kids to calculate doses
Clinical notes
• Diagnostic distinction: **orbital (post-septal) cellulitis** involves tissues posterior to orbital septum — presents with proptosis, painful/restricted eye movements (ophthalmoplegia), chemosis, and possible decreased visual acuity; **preseptal (periorbital) cellulitis** is anterior to septum with normal globe motility and no proptosis (CPS 2026, RCH, StatPearls).
• Red flags requiring urgent action: ophthalmoplegia, proptosis, reduced vision or color desaturation, pain with eye movements, RAPD, bilateral involvement, altered consciousness, or signs of cavernous sinus thrombosis — ophthalmology AND ENT consultation immediately; do not delay IV antibiotics (CPS 2026, RCH, NICE).
• CT/MRI indications: diagnostic uncertainty, suspected subperiosteal or intraconal abscess, no clinical improvement at 24–48 h on appropriate IV therapy, planning surgical drainage, or concern for intracranial extension — contrast-enhanced CT orbits/sinuses is first-line; MRI for intracranial/cavernous sinus involvement (CPS 2026, RCH, ophthalmology guidelines).
• Surgical drainage indications: subperiosteal abscess (especially medial wall/ethmoid), intraconal abscess, large non-draining collection, vision threatened or deteriorating despite antibiotics, or no improvement at 48 h — ENT and ophthalmology co-manage drainage (CPS 2026, RCH, pediatric ENT literature).
• IV→PO switch: when clinically improving after 48–72 h of IV antibiotics (afebrile, decreasing proptosis, improving motility, stable vision, tolerating oral intake) — switch to amoxicillin/clavulanate 22.5 mg/kg/dose BID to complete 10–14 day total course; return immediately if worsening vision, increasing proptosis, or fever (CPS 2026, RCH).
• CPS — Preseptal and orbital cellulitis in children and youth (practice point) (2026)
• RCH Melbourne — Periorbital and orbital cellulitis clinical guideline (2021)
• AAP Red Book — Periorbital and orbital infections (2024)
