What are the causes of bilateral proptosis?

Causes of Bilateral Proptosis

Thyroid eye disease (TED) is the most common cause of bilateral proptosis in adults, accounting for the majority of cases, and should be the primary diagnostic consideration when both eyes are affected. 1, 2

Primary Etiologic Categories

Thyroid Eye Disease (Most Common)

• TED presents with bilateral proptosis that is often asymmetric, accompanied by eyelid retraction, upward gaze limitation from inferior rectus restriction, and exposure symptoms from corneal drying. 1, 2
• Patients may be hyperthyroid, hypothyroid, or euthyroid at presentation, so normal thyroid function does not exclude the diagnosis. 1
• Imaging reveals tendon-sparing enlargement of extraocular muscles (most commonly inferior and medial recti) with proptosis, which confirms the diagnosis. 1, 2
• Exophthalmometry should be performed to quantify the degree of proptosis and establish a baseline for monitoring disease progression. 1, 2

Inflammatory Disorders

• IgG4-related orbital disease can present with bilateral proptosis and requires serum IgG4 levels and inflammatory markers (ESR, CRP) for diagnosis. 3, 4
• Idiopathic orbital inflammatory syndrome (orbital pseudotumor) may cause bilateral involvement, though unilateral presentation is more typical. 3, 5
• These conditions may have initially subtle or normal imaging findings, necessitating high clinical suspicion. 3

Vascular Abnormalities

• Bilateral carotid-cavernous fistulas, though rare, can present with bilateral proptosis and require urgent MRA or CTA to identify vascular congestion and dilated superior ophthalmic veins. 3, 4
• Vascular malformations may occasionally present bilaterally. 6

Neoplastic Causes

• Orbital lymphoma can present bilaterally, typically appearing as painless "salmon-patch" conjunctival lesions extending into the orbit. 3
• Metastatic disease (particularly from breast carcinoma) may rarely cause bilateral orbital involvement. 7

Diagnostic Algorithm

Initial Clinical Assessment

• Measure proptosis with exophthalmometry—values exceeding 21 mm from the orbital rim confirm true proptosis rather than pseudoproptosis. 2, 8
• Assess for restrictive strabismus with forced duction testing to differentiate inflammatory from fibrotic muscle restriction. 1, 2
• Screen immediately for compressive optic neuropathy by checking visual acuity, color vision, visual fields, pupillary reactions, and fundoscopy for disc edema or optic atrophy. 1, 2

Imaging Strategy

• Obtain contrast-enhanced CT or MRI of the orbits to evaluate extraocular muscle size, orbital fat volume, and degree of orbital apex crowding. 1, 2
• CT provides superior osseous detail and can reveal calcifications in neoplastic lesions. 3
• MRI better characterizes soft tissue and can determine disease activity in TED using T2 and STIR sequences. 1
• If proptosis is present with pulsatile quality or inferior dystopia, add urgent MRA or CTA to exclude carotid-cavernous fistula. 3, 4

Laboratory Evaluation

• Order thyroid function tests (TSH, free T4) and thyroid-associated antibodies (TSI, anti-TPO) to assess for TED. 1, 2
• Obtain serum IgG4 levels and inflammatory markers (ESR, CRP) if IgG4-related disease or orbital inflammatory syndrome is suspected. 3

Critical Management Pitfalls

• Do not dismiss bilateral proptosis as benign—a thorough etiologic workup is mandatory even when TED seems clinically obvious, as other serious conditions can coexist. 3
• Standard brain MRI protocols may miss orbital pathology; dedicated orbital sequences with fat suppression must be employed. 3
• Never initiate corticosteroids before completing infectious and neoplastic workup, as steroids can mask diagnoses and worsen outcomes. 3, 4
• Smoking cessation is non-negotiable in TED patients, as smoking significantly worsens orbitopathy progression and reduces treatment response. 2
• Aggressive ocular lubrication with preservative-free artificial tears during the day and ointment at night is essential to prevent exposure keratopathy from proptosis and lid retraction. 1, 2

Monitoring for Complications

• Serial assessment of visual acuity, color vision, pupillary reactions, and visual fields is essential to detect compressive optic neuropathy, which requires urgent intervention. 1, 3, 2
• Optical coherence tomography and automated threshold visual fields provide ancillary screening for optic nerve compression. 1
• Orbital apex crowding and optic nerve compression on imaging signify high risk for vision loss and mandate urgent ophthalmology referral. 2