Decoding Eye Pain For the Residents

Eye pain is one of the most alarming symptoms a patient can present with. It can range from a minor irritation to a sign of a sight-threatening or even life-threatening emergency. As a resident, I used to get puzzled often, and even now, some causes of eye pain puzzle me in a routine eye OPD. My residents often ask me how to differentiate a case of ocular  orperiocular pain. So, this write-up is an overview of ocular pain, irrespective of my area of subspecialty (uveitis). Rather, it is a guide for postgraduates and residents in ophthalmology on how to differentiate a case of ocular or periocular pain.

As an ophthalmologist, while evaluating a case of eye pain, your goal is to rapidly decide: Is this dangerous? Does the patient need an ophthalmologist right now?

The eye is richly innervated by the ophthalmic division of the trigeminal nerve (V1). The cornea alone has the densest sensory innervation of any surface in the human body, being about 300–600 times more sensitive than skin. This is why even a tiny corneal scratch causes excruciating pain. However, many sight-threatening conditions (open-angle glaucoma, retinal detachment, cataract) are entirely painless. Remember one important truth: “Pain does not always mean danger, and no pain does not mean safety.”

Quick Pain Symptom Guide

Always pay attention to the patient’s version, but don’t jump to conclusions based only on the symptoms.

Corneal Abrasion   

Pathology / Mechanism of Ocular Pain

The corneal epithelium is disrupted — by a fingernail, dust, contact lens, or foreign body. This exposes densely packed C-fibre polymodal nociceptors (300-600x more sensitive than skin). The nerve endings fire continuously with every blink, producing pain completely out of proportion to the injury size.

What the Patient Says

• Severe sharp pain, worse with blinking
• "Something is in my eye — I can't blink it away"
• Excessive tearing and photophobia
• Pain relieved by closing the eye or instilling topical anaesthetic (proparacaine)

What You See on Exam

• Conjunctival injection, excessive lacrimation
• Fluorescein staining under cobalt blue light — reveals epithelial defect (green patch)
• Evert the upper lid — look for subtarsal foreign body

Bacterial / Fungal Keratitis (Corneal Ulcer)   

Pathology / Mechanism of Ocular Pain

Microbial invasion of the corneal stroma through a break in epithelium (e.g., contact lens wear, trauma, dry eye). Bacteria (Pseudomonas in contact lens users, Staph in general) or fungi (Aspergillus, Fusarium — after vegetative trauma) colonise the stroma. Intense neutrophilic infiltration causes stromal necrosis and ulceration.

What the Patient Says

• Severe pain with photophobia and watering
• Reduced or blurred vision
• History of recent ocular trauma, contact lens use (overnight wear is high risk), or topical steroid use

What You See on Exam

• White or grey stromal infiltrate on slit lamp — may have a surrounding ring or satellite lesions (fungal)
• Mucopurulent discharge
• Hypopyon (pus in anterior chamber) in severe cases — a sign of serious infection
• Fluorescein shows overlying epithelial defect

Conjunctivitis   

Pathology / Mechanism of Ocular Pain

Inflammation of the conjunctiva. Causes include: bacterial (Staph, Strep, Haemophilus in children; Neisseria gonorrhoeae — rapidly destructive), viral (adenovirus — most common, highly contagious; HSV), allergic (Type I IgE-mediated hypersensitivity — eosinophilic response), and chemical/toxic.

What the Patient Says

• Redness, gritty/burning sensation — described as "like sand in my eye"
• Watery discharge (viral/allergic) or mucopurulent discharge (bacterial)
• Itching is the hallmark of allergic conjunctivitis
• "My eyelids were stuck together when I woke up" — bacterial conjunctivitis

What You See on Exam

• Diffuse conjunctival hyperaemia — brick-red appearance
• Follicles (viral) or papillae (allergic/bacterial) on palpebral conjunctiva
• Pre-auricular lymphadenopathy = viral (adenovirus) — almost never seen in bacterial
• Chemosis in allergic — lid oedema and stringy mucous discharge

Acute Angle-Closure Glaucoma   

Pathology / Mechanism of Ocular Pain

The drainage angle between the iris and cornea suddenly closes, blocking outflow of aqueous humour. Intraocular pressure (IOP) can spike to 40-70 mmHg (normal 10-21 mmHg) within minutes. This causes corneal epithelial oedema (cloudy cornea), ischaemia of the iris, and if untreated, rapid irreversible optic nerve damage. Triggers include dim lighting or anything that dilates the pupil (anticholinergics, adrenergic drugs, stress). Risk factors: hyperopia, female sex, Asian ethnicity, advancing age, shallow anterior chamber.

What the Patient Says

• Sudden severe eye pain + severe headache
• Nausea and vomiting (mimics GI emergency!)
• "Halos around lights" — due to corneal oedema
• Rapidly decreasing vision
• Pain often starts in the evening (dim light → pupil dilation → angle closure)

What You See on Exam

• Red eye with circumcorneal (ciliary) flush
• "Steamy" or hazy cornea — due to epithelial oedema
• Fixed, semi-dilated pupil (mid-dilated, oval, non-reactive)
• IOP markedly elevated on tonometry
• Shallow anterior chamber on slit lamp
• Hard eye on gentle palpation

Anterior Uveitis

Pathology / Mechanism of Ocular Pain

The breakdown of the blood-aqueous barrier allows protein and inflammatory cells to spill into the anterior chamber. Causes include: idiopathic (50%), HLA-B27-associated conditions (ankylosing spondylitis, reactive arthritis, IBD, psoriatic arthritis), infections (HSV, CMV, toxoplasma, TB, syphilis, Lyme), sarcoidosis, Behcet disease, and juvenile idiopathic arthritis (JIA).

What the Patient Says

• Dull, aching periorbital pain — worse in bright light (photophobia)
• Redness — especially around the corneal limbus (ciliary flush/perilimbal injection)
• Blurred vision
• "My eye hurts more when you shine a light in the other eye" — consensual photophobia

What You See on Exam

• Perilimbal (ciliary) flush — violaceous injection around the cornea
• Keratic precipitates (KPs): inflammatory cells on corneal endothelium
• Aqueous flare and cells on slit lamp ("dusty" anterior chamber)
• Posterior synechiae (iris stuck to lens)
• Hypopyon in severe or infectious cases (Behcet — hypopyon uveitis)
• RAPD if posterior involvement affects optic nerve

Scleritis   

Pathology / Mechanism

Inflammation of the sclera. Pain arises because the sclera has a rich innervation via short posterior ciliary nerves. Scleritis is classified as anterior (most common — nodular, diffuse, or necrotising) or posterior. 50% of cases are associated with systemic autoimmune disease: rheumatoid arthritis (most common), granulomatosis with polyangiitis (Wegener), SLE, relapsing polychondritis, IBD, and giant cell arteritis. Infections (herpes zoster, TB, syphilis) also cause scleritis.

What the Patient Says

• SEVERE, deep, boring, constant pain — often described as aching into the jaw, temple, or forehead
• Pain characteristically WAKES the patient at night
• Eye redness that does not improve with vasoconstrictor drops
• Tearing and photophobia

What You See on Exam

• Deep violaceous (bluish-red) discolouration of the sclera
• Scleral tenderness on gentle palpation with a cotton tip
• Scleral oedema and engorgement of deep episcleral vessels
• Vessels do NOT blanch with 2.5% phenylephrine (blanching = episcleritis)
• Posterior scleritis: proptosis, disc oedema, exudative RD (no red eye!)

Optic Neuritis  

Pathology / Mechanism

Inflammation of the optic nerve — demyelinating (most common, associated with multiple sclerosis), or less commonly infectious/granulomatous. In MS, activated T-cells cross the blood-brain barrier and attack myelin surrounding the optic nerve. This slows nerve conduction causing visual loss. Pain occurs because the inflamed nerve and surrounding dura are pulled on with eye movements. 20% of MS patients present with optic neuritis as the first symptom; 70% develop it at some point.

What the Patient Says

• Acute unilateral visual loss (develops over days, nadir at 1-7 days)
• Pain behind the eye — worse with eye movement (classic and pathognomonic)
• Colours look "washed out" — dyschromatopsia (especially red desaturation)
• Uhthoff's phenomenon: vision worsens in hot weather or after exercise (Lenz's sign)

What You See on Exam

• Reduced visual acuity
• Impaired colour vision — ask patient to compare red target in both eyes
• RAPD (Relative Afferent Pupillary Defect) — swinging flashlight test shows affected pupil dilating when light swung to it
• Disc may be normal (retrobulbar neuritis — 65%) or swollen (papillitis)
• Visual field defect (central scotoma most common)
• Optic pallor develops after 4-6 weeks

Endophthalmitis   

Pathology / Mechanism

Bacteria or fungi enter via surgery, trauma, or hematogenous spread (endogenous), breaching the blood-ocular barrier to colonize the posterior segment. Virulence factors like bacterial toxins (e.g., hemolysins, phospholipases from Bacillus cereus), quorum-sensing regulators (PlcR), and capsules (Streptococcus pneumoniae) drive rapid growth and tissue destruction. Host neutrophils release chemokines (e.g., CXCL1) and proteases, causing bystander retinal necrosis despite aiding microbial clearance. The vitreous has no immune privilege — bacteria replicate rapidly and destroy retinal tissue.

What the Patient Says

• Severe pain, usually starting 1-7 days after intraocular surgery or trauma
• Rapidly worsening vision
• Red eye, swollen eyelids
• Fever may be present in endogenous endophthalmitis

What You See on Exam

• Red eye with severe chemosis
• Hypopyon (pus in anterior chamber) — often large and rapidly increasing
• Hazy vitreous on slit lamp / indirect ophthalmoscopy
• Absent or severely diminished fundal red reflex
• Corneal oedema

Cluster Headache   

Pathology / Mechanism of Ocular Pain

Cluster headache pain arises from activation of the trigeminovascular system and trigeminal-autonomic reflex, driven by hypothalamic dysregulation, producing unilateral periorbital agony. This is a neurovascular headache where neuronal firing, not primary vascular changes, initiates the attack. The posterior hypothalamus acts as a “biological clock,” showing activation on PET scans during bouts and structural changes on voxel-based morphometry; it triggers central disinhibition of trigeminal nociceptors. Pain signals from dural vessels and dura traverse the ophthalmic (V1) trigeminal branch to the trigeminocervical complex (TCC), releasing CGRP, substance P, and neurokinin A, elevated in attacks, to cause vasodilation and neurogenic inflammation. Predominantly affects middle-aged men (M:F = 5:1). Triggers include alcohol, nitroglycerin, histamines, altitude, and disrupted sleep.

What the Patient Says

• Excruciating, strictly unilateral, retroorbital or periorbital boring/stabbing pain
• Attacks last 45-90 minutes, occurring 1-3x per day (often waking at night)
• Patient is RESTLESS and paces around (contrast to migraine where patient lies still in dark room)
• "The worst pain I have ever felt" — sometimes called "suicide headaches"

What You See on Exam

• Ipsilateral Horner syndrome: ptosis + miosis + anhidrosis
• Ipsilateral lacrimation, rhinorrhea, nasal congestion
• Conjunctival injection and facial flushing on the side of headache
• Normal optic disc and eye exam between attacks

Tolosa-Hunt Syndrome   

Pathology / Mechanism of Ocular Pain

Nonspecific granulomatous inflammation of the cavernous sinus or superior orbital fissure. This region contains CNs III, IV, VI (oculomotor, trochlear, abducens — control eye movements) and the ophthalmic branch of V (sensation). Inflammation compresses these nerves causing painful ophthalmoplegia. The exact cause is unknown — likely autoimmune. It responds dramatically to corticosteroids (a diagnostic-therapeutic trial).

What the Patient Says

• Severe, constant, unilateral retroorbital pain — often the first symptom, preceding diplopia
• Double vision (diplopia) — due to cranial nerve palsies
• Numbness or tingling of the forehead (V1 involvement)
• Drooping of the eyelid (ptosis — CN III or sympathetic involvement)

What You See on Exam

• Painful ophthalmoplegia (limited eye movements in any combination of CN III, IV, VI palsy)
• Ptosis
• Pupillary abnormalities (mydriasis if CN III; miosis if sympathetic)
• Reduced corneal sensation (V1 involvement)
• Proptosis may be present

Orbital Cellulitis   

Pathology / Mechanism of Ocular Pain

Orbital cellulitis is infection posterior to the orbital septum — a MEDICAL AND SURGICAL EMERGENCY. Orbital cellulitis pain stems from intense acute inflammation in the orbital fat and extraocular muscles, triggered by bacterial invasion (often Staphylococcus aureus, Streptococcus spp.) via sinusitis, trauma, or skin spread. This causes tissue edema, proptosis, and mechanical stretch on fascial planes and nerves, exacerbated by cytokine-mediated nociceptor sensitization .Pathogens produce virulence factors like toxins (S. aureus exotoxins), enzymes (e.g., streptokinase), and adhesins, promoting rapid spread across the orbital septum. The host mounts a vigorous neutrophilic response with cytokine release (IL-1, TNF-α), leading to edema, fibrin deposition, and vascular congestion—elevating IOP and compressing tissues. Pain intensifies with eye movements due to inflamed extraocular muscle stretch and periorbital nerve irritation.

What the Patient Says

• Painful, red, swollen eyelids (rapidly worsening)
• Fever and malaise
• Diplopia and pain on eye movement (orbital cellulitis — not preseptal)
• Visual deterioration (orbital cellulitis — suggests optic nerve compression)
• Headache, nausea (may suggest intracranial extension)

What You See on Exam

• Preseptal: Lid oedema and erythema, NORMAL eye movement, NORMAL vision, NO proptosis
• Orbital: Proptosis, restricted painful eye movement, chemosis, vision may be reduced, RAPD
• Fever, raised WBC
• Signs of sinusitis (tenderness over sinuses, nasal discharge)

Giant Cell Arteritis (Temporal Arteritis) 

Pathology / Mechanism of Ocular Pain

A granulomatous vasculitis of medium and large vessels — predominantly affecting the external carotid branches (temporal, ophthalmic arteries) in patients over 50 years. Giant cells infiltrate and destroy the elastic layer of the vessel wall, causing intimal thickening, stenosis, thrombosis, and ischaemia. Ischaemia of the ophthalmic artery or posterior ciliary arteries causes anterior ischaemic optic neuropathy (AION) — the most common cause of sudden visual loss in elderly patients in developed countries.

The pain arises from ischemic inflammation of cranial arteries, particularly the superficial temporal and ophthalmic arteries, due to granulomatous vasculitis. This triggers vessel wall distension, perivascular nerve irritation, and cytokine-mediated nociceptor sensitization

What the Patient Says

• New onset temporal headache in a patient over 50 years
• "Jaw claudication" — jaw pain while chewing (pathognomonic — masseter ischaemia)
• Scalp tenderness — "it hurts to comb my hair or rest my head on a pillow"
• Sudden painless visual loss — if AION has occurred
• Polymyalgia rheumatica symptoms (shoulder and hip girdle stiffness in the morning)

What You See on Exam

• Tender, thickened, nodular, non-pulsatile temporal artery on palpation
• Sudden visual loss — RAPD, pale swollen disc (anterior AION)
• Scalp tenderness
• Occasionally diplopia (ischaemia of extraocular muscles or cranial nerves)

Herpes Zoster Ophthalmicus (HZO)   

Pathology / Mechanism of Ocular Pain

Reactivation of dormant varicella-zoster virus (VZV) in the trigeminal ganglion — specifically the ophthalmic division (V1). After primary chickenpox, VZV lies latent in sensory ganglia. Immunosuppression (age, HIV, steroids, malignancy) triggers reactivation. The virus travels down the nerve causing skin eruption (dermatomal vesicular rash) and neural inflammation. Ocular involvement occurs in 50-72% of HZO cases.

Viral replication causes axonal necrosis, demyelination, and edema, sensitizing nociceptors via inflammatory mediators (e.g., cytokines from immune response). Perineural inflammation (perineuritis) directly irritates sensory fibers, producing allodynia/hyperalgesia

What the Patient Says

• Prodrome: burning, tingling, itching pain over the forehead/scalp 2-3 days before rash
• Painful vesicular rash over V1 dermatome (forehead, scalp, tip of nose)
• "Hutchinson's sign" — vesicles on the TIP of the nose indicate nasociliary nerve involvement and predict ocular involvement
• Eye pain, redness, photophobia after rash appears

What You See on Exam

• Unilateral vesicular rash in V1 distribution — does NOT cross midline
• Hutchinson's sign: vesicles on tip or side of nose
• Conjunctivitis, keratitis (dendritic or geographic ulcers), iritis, scleritis, or retinitis
• Corneal hypoaesthesia (reduced sensation) — risk of neurotrophic keratopathy
• Ptosis, ophthalmoplegia if cranial nerve involvement

Dry Eye Disease & Neuropathic Ocular Pain   

Pathology / Mechanism of Ocular Pain

The cornea is the most densely innervated surface in the body. Dry eye disease  involves insufficient tear film (aqueous deficiency: Sjogren's, post-radiation; evaporative: Meibomian gland dysfunction) causing ocular surface inflammation and epithelial breakdown. Repeated desiccation and inflammation damage corneal nerve fibres, leading to peripheral sensitisation and eventually CENTRAL sensitisation — where pain persists even after the original stimulus resolves. This is neuropathic ocular pain . Risk factors: Sjogren's syndrome, GVHD, post-LASIK, fibromyalgia, PTSD, migraine, female sex.

What the Patient Says

• Burning, stinging, dryness, foreign body sensation
• Symptoms worse in air-conditioned environments, with screen use, or wind
• Pain out of proportion to clinical signs — a hallmark of neuropathic component
• Paradoxical tearing (reflex lacrimation from desiccation)

What You See on Exam

• Reduced tear meniscus, rapid tear break-up time (TBUT < 10 seconds)
• Punctate epithelial erosions (PEE) on fluorescein staining
• Reduced Schirmer test (< 5 mm/5 min = aqueous deficiency)
• In NOP: symptoms disproportionate to clinical findings — exam may appear near-normal
• Confocal microscopy: reduced corneal nerve density, nerve tortuosity

 At-A-Glance Differential Diagnosis

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