Orbital cellulitis is one of the most dangerous eye emergencies in primary care — the hallmark distinguishing it from pre-septal (periorbital) cellulitis is involvement posterior to the orbital septum, which communicates directly with the intracranial venous sinuses via the superior and inferior ophthalmic veins. The orbital septum is a thin fibrous membrane extending from the bony orbital rim to the eyelid tarsal plates — it is the critical anatomical barrier. Structures anterior to it (eyelid skin, pre-septal fat) can be infected without danger to vision or life. Structures posterior to it (orbital fat, extraocular muscles, optic nerve) cannot. The Chandler classification of orbital infections grades severity: Class I (inflammatory oedema) through Class V (cavernous sinus thrombosis). Class II upwards (subperiosteal abscess or beyond) requires IV antibiotics and urgent imaging. The cardinal signs of orbital (vs pre-septal) disease are: proptosis (forward displacement of the globe), ophthalmoplegia (restricted eye movement from myositis or mechanical tethering), pain on eye movement, and reduced visual acuity. Any one of these in a patient with periorbital swelling mandates same-day hospital assessment.

The swinging torch test (RAPD assessment) is one of the most important and underused clinical skills in primary care — it can be performed with any bright torch in 30 seconds and directly tests optic nerve function. A positive RAPD means that the affected optic nerve is conducting fewer signals to the brain despite being illuminated — the brain therefore interprets the affected eye as "seeing" less light, and drives pupillary dilation rather than constriction when the light swings to that eye. In the context of periorbital swelling, an RAPD indicates optic nerve compression from an orbital abscess, tumour, or the swollen tissues of severe thyroid eye disease. It is a genuine ocular emergency. Visual acuity measurement with a Snellen chart should be performed at every GP consultation involving an eye complaint — it takes 60 seconds, requires no equipment beyond the chart on the wall or a phone app (Peek Acuity), and provides objective baseline data. Many cases of delayed diagnosis of serious ophthalmic conditions involve the absence of documented visual acuity at the primary care presentation.

Recurrent chalazion deserves specific clinical attention — a chalazion that recurs at the same site should raise suspicion for sebaceous gland carcinoma (SGC), a rare but aggressive eyelid malignancy that classically masquerades as a recurrent or treatment-resistant chalazion. SGC arises from the meibomian glands or Zeis glands and has a pagetoid spread pattern (intraepithelial spread along the conjunctiva without visible mass). It accounts for up to 5% of all eyelid malignancies and carries a metastatic mortality of 15–25%. The clinical rule is: any chalazion that recurs at the same site after incision and curettage must be sent for histological examination. GPs should document chalazion location and refer any recurrence to ophthalmology for biopsy. Thyroid eye disease (TED) is the most common cause of bilateral proptosis and unilateral proptosis in adults — it is directly caused by TSH receptor antibody-driven glycosaminoglycan deposition in the orbital fat and extraocular muscles, causing volume expansion within the rigid bony orbit. Importantly, TED can occur when thyroid function tests are normal (euthyroid Graves' disease) — TFTs alone do not exclude TED as the cause of proptosis.

CT orbits with contrast is the definitive investigation for orbital cellulitis — it identifies subperiosteal abscess (collection between the bony orbital wall and periorbita, typically medial from ethmoid sinusitis), intraorbital abscess, and intracranial extension. The decision to proceed to CT should be made in hospital after initial clinical assessment, not in primary care (where CT is not available). The key primary care role is to recognise the clinical signs of orbital involvement and refer urgently — attempting to definitively exclude orbital cellulitis in primary care by clinical examination alone is unsafe and is associated with delayed diagnosis. The TSH receptor antibody (TRAb) test is the most specific investigation for thyroid eye disease — it is positive in 95% of active TED cases, even when TFTs are normal (euthyroid Graves'). A positive TRAb with periorbital swelling and any of the features of TED (lid retraction, proptosis, restricted eye movement) confirms the diagnosis and mandates urgent ophthalmology referral. TRAb is not routinely available in all GP laboratories — check local availability and if unavailable, refer on clinical suspicion.

Thyroid eye disease requires dual-specialty management — ophthalmology for the orbital component and endocrinology for thyroid function optimisation. The relationship between thyroid disease activity and TED activity is imperfect — TED can be active when thyroid function is controlled, and can progress after radioactive iodine treatment (RAI). EUGOGO (European Group on Graves' Orbitopathy) guidelines note that RAI treatment worsens TED in smokers and those with moderate-severe TED — in these patients, thyroidectomy or antithyroid drugs are preferred over RAI. Smoking is the most powerful modifiable risk factor for TED severity — smokers have a 7.7-fold increased risk of TED compared to non-smokers, and active smoking is associated with a 2–3-fold higher risk of sight-threatening TED. Smoking cessation is the single most important intervention a GP can make for a patient with Graves' disease to reduce TED risk. This should be documented at every consultation. Dacryocystitis is a condition that GPs frequently under-refer — acute dacryocystitis (lacrimal sac infection) causes medial canthal swelling and can track both pre-septally into the eyelid and post-septally into the orbit. All acute dacryocystitis in adults should have same-day or urgent ophthalmology assessment.

The warm compress technique for chalazion and blepharitis works by liquefying the solidified meibum (meibomian gland secretion) that blocks the gland orifice — normal meibum melts at approximately 28–32°C, but in meibomian gland dysfunction the composition changes and the melting point rises. A compress heated to approximately 40°C applied for 5–10 minutes softens the secretions, and the subsequent lid massage physically expresses them. The EyeBag (reusable microwaveable eye mask) maintains consistent heat for 10 minutes and is superior to flannels. Studies show temperature-controlled warm compresses produce significantly better symptom improvement than flannels alone. Doxycycline for blepharitis works not via antibiotic activity but via its anti-inflammatory and anti-lipase properties — it inhibits bacterial lipase enzymes (from Staphylococcus and Demodex) that break down meibum into toxic free fatty acids. It also reduces matrix metalloproteinase activity. The clinical response takes 6–8 weeks — patients must be counselled to continue for the full 12-week course before assessing efficacy.

The selenium supplementation evidence for mild TED (EUGOGO Selenium trial, NEJM 2011) is one of the few RCT-proven treatments for mild active TED — it demonstrated that selenium 200 mcg daily for 6 months significantly improved mild active TED compared to placebo (quality of life score improvement, reduction in lid swelling and proptosis, and reduced progression to moderate-severe disease). The number needed to treat was approximately 5 for meaningful improvement. Selenium is thought to work by reducing reactive oxygen species that stimulate orbital fibroblast proliferation and glycosaminoglycan deposition. It is inexpensive, safe, and available OTC — GPs can recommend it for all patients with mild TED while awaiting ophthalmology review. The correct dose is 200 mcg daily (L-selenomethionine or sodium selenite formulation). Higher doses are potentially toxic — selenium toxicity causes hair loss, nail changes, and neurological symptoms above 400 mcg/day. The 6-month course is important — extending beyond 6 months has not shown additional benefit.

Orbital mucormycosis is an extremely rare but virtually always fatal condition if not recognised early — it occurs in immunocompromised patients (classically diabetic ketoacidosis, but also haematological malignancy, organ transplant). The Mucorales fungi (Rhizopus, Mucor) invade the sinonasal mucosa and rapidly extend through the bony walls of the sinuses into the orbit and intracranially. It presents as rapidly progressive periorbital or facial swelling with black eschar on nasal or palatal mucosa — pathognomonic of tissue necrosis from vascular invasion. Any periorbital swelling in a diabetic who is unwell or acidotic requires same-day assessment as a matter of urgency. Treatment is IV liposomal amphotericin B + aggressive surgical debridement — mortality remains 50–80% even with treatment. The 20-20-20 rule for screen users is supported by optometric guidance and is one of the most effective simple interventions for screen-related dry eye — reduced blink rate during screen use leads to tear film evaporation and meibomian gland hyperstimulation, directly contributing to the blepharitis-meibomian gland dysfunction spectrum.

The 48-hour review rule for pre-septal cellulitis is one of the most important safety-netting steps in GP ophthalmology — it identifies the minority of cases that progress to orbital involvement and allows early escalation before complications develop. Documenting the cellulitis margin with a pen (marking the erythema boundary with a date and time) at the first consultation provides an objective comparator at the 48-hour review — the margin should be receding or static, not advancing. Advancing margin at 48 hours = treatment failure = same-day hospital. This rule applies regardless of whether the patient feels systemically better — local progression can occur without worsening systemic symptoms. Written safety-netting instructions (either a leaflet or EyeSafe-style written advice) given to every patient with periorbital swelling should include: "If you develop pain when moving your eye, difficulty moving your eye in any direction, your eye appears to be bulging forward, your vision changes, or you develop a severe headache with fever, call 999 or attend A&E immediately." This instruction must be documented in the clinical records to meet safety-netting standards.