Can proptosis be seen in myasthenia gravis?

Proptosis in Myasthenia Gravis

Proptosis (exophthalmos) is NOT a feature of myasthenia gravis and its presence should immediately redirect your diagnostic evaluation toward alternative etiologies, most notably thyroid eye disease, orbital masses, or vascular malformations. 1, 2

Why Proptosis Does Not Occur in Myasthenia Gravis

Myasthenia gravis affects the neuromuscular junction of voluntary skeletal muscles, causing weakness and fatigability—not mechanical displacement or enlargement of orbital structures. 2, 3 The pathophysiology involves:

• Autoantibodies blocking acetylcholine receptors at the neuromuscular junction, preventing normal nerve signal transmission to extraocular muscles, levator palpebrae, and orbicularis oculi 2, 3
• This produces functional weakness (ptosis, diplopia, variable strabismus) without any mass effect, inflammation, or structural changes that would push the globe forward 2, 4
• The pupils remain characteristically unaffected because myasthenia does not involve the autonomic nervous system 2

Classic Ocular Manifestations of Myasthenia Gravis

When myasthenia gravis presents with ocular symptoms, you will see:

• Variable, fatigable ptosis that worsens with sustained upgaze and improves with rest 2, 5, 4
• Fluctuating diplopia with strabismus patterns that change during prolonged examination due to muscle fatigue 2, 4
• Cogan lid-twitch sign and slow saccadic eye movements 2, 5
• Positive ice pack test: applying ice over closed eyes for 2 minutes produces ≥2 mm improvement in ptosis 2, 5
• Pupil-sparing ophthalmoplegia—pupillary involvement excludes myasthenia gravis 2

Critical Differential Diagnosis When Proptosis Is Present

If your patient presents with ptosis, diplopia, AND proptosis, you must immediately consider:

Thyroid Eye Disease (Most Common Cause)

• Produces restrictive myopathy with mechanical limitation rather than neuromuscular junction dysfunction 2
• Key distinguishing features include proptosis, eyelid retraction, and fixation on forced duction testing (mechanical restriction versus fatigable weakness in myasthenia) 2
• Orbital imaging shows tendon-sparing muscle enlargement in thyroid eye disease, which is absent in myasthenia 2

Orbital Masses or Vascular Malformations

• Unilateral or asymmetric proptosis suggests an underlying mass intrinsic to the globe, optic nerve, extraocular muscles, lacrimal glands, or adjacent soft-tissue structures 1
• Vascular malformations (including carotid-cavernous fistula) may present with proptosis, orbital congestion, and chemosis 1
• MRI of the orbits with contrast is the optimal imaging modality to localize and characterize orbital lesions 1, 5

Bilateral Proptosis

• Indicates an underlying systemic or diffuse condition, most commonly thyroid eye disease 1
• IgG4-related disease and idiopathic orbital inflammatory syndrome can also present with bilateral proptosis 1

Diagnostic Algorithm When Proptosis Is Present

Step 1: Measure proptosis objectively using exophthalmometry (normal range varies by race; typically <21 mm in Caucasians) 1

Step 2: Assess for thyroid eye disease features:

• Eyelid retraction, lid lag, restrictive strabismus on forced duction testing 2
• Order thyroid function tests and thyroid-stimulating immunoglobulin 2

Step 3: Obtain orbital imaging:

• MRI orbits with contrast is the preferred modality for soft tissue characterization and evaluating orbital masses 1, 5
• CT orbits with contrast is complementary for assessing bony anatomy and orbital volumes, particularly when orbital decompression is being considered for thyroid eye disease 1

Step 4: If myasthenia gravis is still suspected despite proptosis:

• This combination is extremely rare and would suggest two separate conditions (e.g., concurrent thyroid eye disease and myasthenia gravis, which can co-occur given shared autoimmune predisposition) 2, 4
• Proceed with myasthenia workup: acetylcholine receptor antibodies (positive in 40-77% of ocular myasthenia), ice pack test, and single-fiber EMG 2, 4

Common Pitfall to Avoid

Do not attribute proptosis to myasthenia gravis simply because the patient also has ptosis and diplopia. 1, 2 The presence of proptosis mandates a thorough search for structural orbital pathology or thyroid eye disease, even if myasthenia gravis is ultimately confirmed as a concurrent diagnosis. 2 Missing thyroid eye disease is particularly problematic because performing strabismus surgery before orbital decompression can worsen ocular alignment. 5