A 19 year old young girl with previous history of repeated pain over medial canthus and chronic use of nasal decongestants, presented with abrupt onset of fever with chills & rigor, diplopia on lateral gaze, moderate proptosis & chemosis. On examination optic disc is congested. Most likely diagnosis is -

Repeated pain over medial canthus and chronic use of nasal decongestants suggest chronic ethmoidal sinusitis. Patient's other symptoms suggest that she has landed up in complicated sinusitis Now let’s see the symptoms one by one - There is sudden onset of - Chills & rigor → Systemic symptoms Diplopia on lateral gaze → Suggestive of VI C.N. (abducent) involvement leading to lateral rectus palsy Proptosis & chemosis Optic disc congestion So, among the given options - Orbital apex syndrome (OAS) can be ruled out as visual loss is often the initial manifestations of an OAS (not seen in this patient). The remaining two options; cavernous sinus thrombosis & orbital cellulitis can impose a diagnostic difficulty. Both of these have almost similar presentation with some differences. Abrupt onset of chills &rigor, moderate proptosis and lateral gaze palsy favour cavernous sinus thrombosis (CST). In orbital cellulitis onset is slow & systemic features are mild & there is restricted ocular movement in all directions from the beginning. Optic disc congestion & vision loss in late stages are found both in CST & orbital cellulitis.