Proptosis — Differential & Clinical Correlation

Localizing Orbital Lesions by Direction of Proptosis

Not every asymmetry is true proptosis. Pseudoproptosis describes an eye that looks pushed forward without an orbital mass behind it: a high-myopic globe with a long axial length simply occupies more space and reads a higher Hertel value, a normal eye can appear falsely proptotic next to a fellow eye with enophthalmos from an old floor fracture, and asymmetric orbital bone volume or eyelid position can produce the same illusion. Ruling out these mimics first keeps the localization exercise below from being pointed at the wrong problem.

The direction the globe is pushed gives a first clue to where an orbital mass sits, before any imaging is obtained. Axial proptosis, forward displacement straight along the visual axis with no lateral shift, points to a lesion within the muscle cone (intraconal): an optic nerve glioma or cavernous hemangioma are the classic examples, since a centrally placed mass has nowhere to go but straight ahead. An eccentric, extraconal mass instead pushes the globe away from itself. A mass sitting superiorly in the orbit, for instance, displaces the globe downward.

Vertical displacement has its own vocabulary. Hyperglobus is superior displacement of the globe, typically from an inferior orbital (floor) mass pushing upward; hypoglobus is inferior displacement, from a superior orbital mass pushing down. Measuring this dystopia, the inter-pupillary height difference in primary gaze, helps localize the mass and guides surgical planning.

Thyroid Eye Disease

Thyroid eye disease (TED) is by far the most common cause of bilateral proptosis in adults. TSH-receptor antibody-mediated activation of orbital fibroblasts drives glycosaminoglycan accumulation, which expands orbital fat and enlarges the extraocular muscles.

Disease activity is tracked separately from anatomical severity. The clinical activity score (CAS), scored out of 7, captures inflammatory activity: spontaneous retrobulbar pain, pain on eye movement, eyelid erythema, eyelid edema, conjunctival erythema, chemosis, and caruncle or plica inflammation. A score of 3 or higher indicates active inflammation warranting immunosuppressive treatment. Proptosis on Hertel measurement is not a CAS component, since it reflects structural volume change rather than active inflammation. The two can be discordant: a patient can have marked proptosis from established fibrofatty expansion with a low CAS, or the reverse.

The sight-threatening complication to screen for is dysthyroid optic neuropathy (DON), caused by optic nerve compression at the crowded orbital apex from enlarged extraocular muscle bellies. MRI coronal fat-suppressed sequences show this apical crowding directly; OCT of the retinal nerve fiber layer detects the resulting axonal thinning. Reduced color vision and a relative afferent pupillary defect are the bedside correlates, and either should prompt urgent treatment.

Carotid-Cavernous Fistula

Pulsatile proptosis, proptosis with a visible pulsation synchronous with the heartbeat, accompanied by an orbital bruit on auscultation and dilated, arterialized (corkscrew) conjunctival vessels, is highly suggestive of a carotid-cavernous fistula (CCF): an abnormal connection between the carotid artery and the cavernous sinus. Direct CCFs, usually post-traumatic, produce high-flow shunting with dramatic, rapid-onset signs. Indirect dural CCFs are more subtle, with a gradual, low-grade pulsatile proptosis. Diagnosis is confirmed with MRA or CTA; treatment is endovascular embolization.

Orbital Cellulitis

Orbital cellulitis presents with acute, typically unilateral proptosis, fever, pain on eye movement, and conjunctival chemosis. Infection most often spreads into the orbital soft tissues from adjacent sinusitis, and requires urgent antibiotic treatment to prevent vision- and life-threatening intracranial complications.

Orbital Varix

Orbital varices are low-flow venous malformations that engorge with raised intraorbital venous pressure. The Valsalva maneuver (bearing down), a head-down posture, or straining worsens the proptosis, producing an intermittent, position-dependent finding that is pathognomonic for an orbital varix once other causes have been excluded.

Blowout Fracture and Enophthalmos

Where the diagnoses above cause proptosis, orbital trauma more often causes the opposite finding: enophthalmos, posterior displacement of the globe. A blowout fracture of the orbital floor (herniating into the maxillary sinus) or medial wall (into the ethmoid sinus) lets orbital fat and extraocular muscle herniate through the defect, reducing effective orbital volume and letting the globe sink posteriorly. The inferior rectus can become trapped in the process, producing diplopia in upgaze.

High-Yield Pearls

• Axial proptosis (no lateral shift) → think intraconal lesion; an eccentric mass pushes the globe away from itself, not toward it.
• TED is the most common cause of bilateral proptosis; CAS ≥3/7 indicates active inflammation needing immunosuppression, and Hertel proptosis is not part of that score.
• Pulsatile proptosis + bruit + arterialized conjunctival vessels = carotid-cavernous fistula.
• Proptosis that worsens with Valsalva/bearing down = orbital varix.
• A "mild-looking" pediatric orbital injury with severe upgaze restriction is a red flag for a white-eyed trapdoor fracture, not reassurance that the injury is minor.

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