Is the pupil spared in myasthenia gravis (MG)?

Pupil Involvement in Myasthenia Gravis

Yes, the pupil is typically spared in myasthenia gravis, and any significant pupillary involvement should prompt immediate consideration of alternative diagnoses, particularly third nerve palsy. 1

Key Diagnostic Feature

Pupils are characteristically not affected in myasthenia gravis, which serves as an important distinguishing feature from other causes of ophthalmoplegia and ptosis. 1 This pupil-sparing pattern is highly consistent across the literature and represents a critical diagnostic clue when evaluating patients with suspected myasthenia.

Rare Exceptions

While pupillary sparing is the rule, rare cases may demonstrate:

• Impaired or slow pupillary responses in exceptional circumstances 1
• These cases are distinctly uncommon and should not be considered typical presentations

Critical Clinical Implication

The presence of pupillary involvement should immediately alert the clinician to carefully consider other etiologies, particularly third nerve palsy. 1 This is emphasized in the American Academy of Ophthalmology guidelines as a key red flag that argues against myasthenia gravis as the primary diagnosis.

Pathophysiologic Basis

The pupil-sparing pattern in myasthenia gravis makes pathophysiologic sense:

• Myasthenia affects the neuromuscular junction of voluntary skeletal muscles, not the autonomic nervous system 1
• Acetylcholine receptor antibodies target nicotinic receptors at the neuromuscular junction of extraocular muscles, levator palpebrae, and orbicularis oculi 1
• Pupillary function depends on autonomic innervation (parasympathetic and sympathetic), which operates through different receptor systems not affected by the autoimmune process in myasthenia 1

Practical Diagnostic Algorithm

When evaluating a patient with ptosis and ophthalmoplegia:

1. Immediately assess pupillary function as part of the initial examination 1
2. If pupils are abnormal (dilated, poorly reactive, or asymmetric): Prioritize evaluation for third nerve palsy, Horner syndrome, or other neurologic causes over myasthenia gravis 1
3. If pupils are normal and symmetric: Proceed with myasthenia workup including ice pack test, fatigability assessment, and serologic testing 1, 2, 3

Common Pitfall to Avoid

Do not dismiss the diagnosis of third nerve palsy in favor of myasthenia gravis when pupillary abnormalities are present, even if other features seem consistent with myasthenia. 1 The pupil-sparing nature of myasthenia is so characteristic that deviation from this pattern demands alternative explanation, potentially including life-threatening causes such as posterior communicating artery aneurysm in the case of pupil-involved third nerve palsy.