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OP2.6 | Proptosis: Causes, Differentiation and Management — SDL Guide (Part 2)

Clinical Examination: Measuring and Characterising Proptosis

A structured orbital examination generates the data points that distinguish benign from malignant, inflammatory from neoplastic, and treatable from immediately urgent. No single finding is pathognomonic, but the combination is usually diagnostic.

1. Hertel exophthalmometer measurement: The Hertel exophthalmometer is the standard instrument for measuring the forward protrusion of the globe, using the lateral orbital rim as the fixed reference point. The instrument has two mirror-angled notches that rest on the lateral orbital margins; the observer reads the millimetric scale while viewing the corneal apex in the mirrors. The normal upper limit is approximately 20 mm (some references cite 21 mm); individual variation is wide, so the side-to-side asymmetry >2 mm is the more clinically significant criterion. Measurements should be recorded at each visit — progressive increase over serial visits is often more informative than a single absolute value. The base (lateral orbital rim to lateral orbital rim width) must also be recorded, because the same measurement at a different base yields a different value.

2. Direction of proptosis: After measurement, note whether the globe is displaced axially (straight forward) or eccentrically. Eccentric displacement indicates the direction of the pushing lesion (opposite side of displacement = lesion origin).

3. Reducibility: Ask the patient to look straight ahead and gently apply posterior pressure to the globe with a fingertip. A reducible proptosis suggests a vascular lesion (orbital varices, lymphangioma) or significant orbital fat. Non-reducible proptosis is typical of firm orbital masses.

4. Pulsation: Place a finger on each closed eyelid simultaneously and feel for a pulse coinciding with the heartbeat. A pulsatile proptosis suggests an arteriovenous communication (carotid-cavernous fistula, orbital AV malformation) or transmission of CSF pulsations through a bony defect (encephalocele, sphenoidal wing meningioma with bone erosion). Auscultate over the closed lids and temple — a bruit (rushing sound) is heard in high-flow carotid-cavernous fistula.

5. Lid signs: Lid retraction (scleral show above the limbus in primary gaze) points strongly to thyroid eye disease. Lid lag (the upper lid lags behind the globe on downgaze) is also a TED sign. Assess for lagophthalmos (incomplete eyelid closure) — if present, corneal exposure must be managed urgently.

6. Visual acuity and colour vision: Compressive optic neuropathy (CON) is the single most sight-threatening complication of proptosis. Because the parvocellular (colour) fibres are more vulnerable to compression than achromatic channels, colour desaturation precedes visual acuity loss in CON. Test colour vision with an Ishihara chart at every visit; a patient who reports colours look 'washed out' in the proptotic eye requires urgent intervention. Test visual acuity with Snellen chart.

7. IOP in upgaze: In thyroid eye disease, the enlarged inferior rectus muscle restricts upgaze. When the patient looks up, the restricted muscle acts like a tourniquet on the vortex veins, elevating IOP by 5–10 mmHg. An IOP rise in upgaze of >5 mmHg is a supportive finding for TED-related mechanical restriction.

Investigations for Proptosis

Investigations are targeted based on the clinical assessment — the direction of proptosis, the time course, the patient's age, and associated systemic findings guide the choice of imaging and laboratory tests. No investigation should be requested without a working differential, because the sensitivity of any test depends on what disease is being excluded. A young woman with bilateral axial proptosis, lid retraction, and chemosis almost certainly has thyroid eye disease — the investigation is thyroid function and anti-TSHR antibodies, and CT will confirm EOM enlargement. By contrast, a middle-aged man with rapid unilateral proptosis and pain needs CT orbit urgently to exclude a subperiosteal abscess or orbital tumour. Ordering "CT orbit" without this clinical reasoning produces low-yield investigations and delays diagnosis.

Orbital imaging is the cornerstone of proptosis investigation. The choice between CT and MRI depends on the clinical question:

• CT orbit (axial and coronal cuts): Superior for bony anatomy — detects bony erosion, expansion of the orbital walls, sinus mucocele extension, calcification within a mass (cavernous haemangioma: phleboliths; optic nerve sheath meningioma: 'tram-track' calcification), and foreign bodies. CT is the first-line investigation for acute traumatic proptosis (rule out haemorrhage, fracture) and for sinus-related disease. The 'Cola-can sign' on coronal CT (enlarged fusiform muscle bellies with sparing of the tendons) is classic for TED-related myositis — a key differentiator from idiopathic inflammatory myositis where tendons are involved.
• MRI orbit: Superior for soft-tissue characterisation and optic nerve detail. T1 with gadolinium shows vascularity; T2 shows fluid/oedema. Preferred for suspected optic nerve lesions (glioma: fusiform T2-bright expansion of the nerve; meningioma: 'tram-track' enhancement around the nerve), orbital apex lesions, and intracranial extension assessment.

Laboratory investigations:
- Thyroid function tests (TFTs) + TSH receptor antibodies (TRAb): Mandatory in any adult with unexplained proptosis, especially bilateral. TRAb are the most specific marker for TED even in euthyroid patients.
- Full blood count + LDH + serum protein electrophoresis: For suspected orbital lymphoma.
- Chest X-ray: To detect pulmonary primary (metastatic proptosis) or mediastinal lymphadenopathy.
- Blood culture / inflammatory markers: In orbital cellulitis or suspected cavernous sinus thrombosis.

Biopsy: Incisional or excisional biopsy is required when the diagnosis is uncertain and malignancy cannot be excluded on imaging alone. Rhabdomyosarcoma in a child is a surgical emergency — urgent biopsy under general anaesthetic and immediate paediatric oncology referral. Lacrimal gland lesions: if a pleomorphic adenoma is suspected (benign, decades history, no pain), incisional biopsy is contraindicated (risks seeding) — planned total excision with intact capsule is the approach.

Differential Diagnosis: Key Distinguishing Features

The differential diagnosis of proptosis is constructed by integrating the direction, laterality, time course, and examination findings gathered in the preceding steps. The approach differs by patient age and laterality.

Step 1 — Confirm true proptosis, exclude pseudoproptosis. Measure both eyes with the Hertel exophthalmometer. If the 'proptotic' eye measures within normal limits (<20 mm) but there is marked asymmetry, look at the contralateral eye for enophthalmos (sunken eye after old trauma or silent orbital floor fracture), or at the ipsilateral lids for retraction. A patient with bilateral lid retraction in one eye will appear to have proptosis compared to the fellow eye even when the globe position is normal.

Systematic clinical reasoning, not a shotgun of investigations, is the gateway to the correct diagnosis. Adult unilateral proptosis differential:
- Thyroid eye disease — most common overall; axial; typically bilateral but 10% initially unilateral; lid retraction + lag; TRAb positive; CT: fusiform muscle belly enlargement sparing tendons.
- Cavernous haemangioma — commonest benign intraconal tumour in adults; slowly progressive axial proptosis over years; no pain; MRI: T1 iso, T2 bright, gradual 'fill-in' enhancement on dynamic gadolinium imaging; surgical excision curative.
- Orbital lymphoma — typically 5th–6th decade; can be bilateral; may present as 'salmon-patch' subconjunctival lesion; CT: moulded enhancing lesion conforming to orbital anatomy; requires systemic staging.
- Orbital metastasis — breast (women), lung, prostate; rapid onset; pain; enophthalmos rather than proptosis can occur with scirrhous breast metastasis (fibrosis retracted the orbit).
- Mucocele — from frontal or ethmoid sinus; pushes the globe down and laterally (frontal) or inferolaterally (ethmoid); CT shows cystic expansion of sinus with bony remodelling.
- Lacrimal gland pleomorphic adenoma — superolateral, inferomedial globe displacement; firm, non-tender; CT shows well-defined mass remodelling (not eroding) the lacrimal fossa.
- Lacrimal gland adenoid cystic carcinoma — similar location but painful, bone-eroding (perineural invasion), progresses faster.

Adult bilateral proptosis: TED is the diagnosis until proved otherwise — it accounts for >80% of bilateral orbital disease in adults.

Childhood proptosis differential:
- Rhabdomyosarcoma — URGENT. Commonest primary orbital malignancy in children; age <10 years typically; rapidly progressive eccentric proptosis over days to weeks; may have bruising mimicking trauma; CT: irregular heterogeneous mass, may erode bone. Urgent biopsy + oncology.
- Capillary haemangioma — presents in first year of life; increases with crying (Valsalva); spontaneously involutes by age 7–10; strawberry surface; MRI: lobulated T2-bright mass. Treatment only if causing amblyopia.
- Dermoid cyst — at bony suture lines (frontozygomatic, frontolacrimal); smooth, firm, non-tender; CT: well-defined lucency expanding the suture. Rupture causes intense inflammatory reaction — elective excision.
- Optic nerve glioma — associated with neurofibromatosis type 1 (NF-1) in 30–50% of childhood cases; axial proptosis + relative afferent pupillary defect + visual loss; MRI: fusiform T2-bright expansion of the optic nerve.

Red flag signs requiring urgent same-day/next-day assessment: rapid onset, severe pain, fever, reduced vision, afferent pupillary defect, colour desaturation, IOP >25 mmHg in primary gaze.