How do you differentiate and manage tonic pupil versus third nerve palsy?

Differentiating and Managing Tonic Pupil vs. Third Nerve Palsy

The key to differentiating tonic pupil from third nerve palsy lies in the associated ocular motility deficits and ptosis present in third nerve palsy, while tonic pupil presents with pupillary abnormalities alone without motility defects or ptosis. 1

Clinical Differentiation

Third Nerve Palsy

• Presentation:

  • Ptosis (drooping eyelid)
  • Eye positioned "down and out" (abducted and infraducted)
  • Impaired adduction, elevation, and depression of the eye
  • Pupil may or may not be involved (critical diagnostic feature)
  • Diplopia (double vision)

• Pupil Assessment:

  • Pupil-involving: Suggests compressive lesion (aneurysm, tumor)
  • Pupil-sparing: Often indicates microvascular cause (diabetes, hypertension)
  • Note: Pupil involvement is not 100% reliable; 17% of microvascular cases can have pupil involvement 2

Tonic Pupil

• Presentation:
  • Pupillary abnormality without ptosis or extraocular muscle weakness
  • Dilated pupil with poor light reaction but preserved near response
  • Segmental iris constriction may be visible
  • No associated diplopia or eye movement limitations
  • Often unilateral

Diagnostic Approach

For Suspected Third Nerve Palsy:

1. Comprehensive eye examination 1:

   • Assess pupil size, shape, and reactivity
   • Evaluate extraocular movements
   • Check for ptosis
   • Perform fundus examination for papilledema/optic atrophy

2. Urgent imaging for pupil-involving third nerve palsy 3, 1:

   • MRI with gadolinium
   • MRA or CTA to rule out aneurysm (especially posterior communicating artery)
   • Consider catheter angiogram if high suspicion despite normal MRA/CTA

3. For pupil-sparing third nerve palsy:

   • If complete ptosis and complete motility dysfunction: Likely microvascular
   • If partial involvement: Still consider imaging to rule out compressive lesion 3
   • Check for vascular risk factors (diabetes, hypertension, hyperlipidemia)

4. Additional tests based on clinical suspicion:

   • ESR/CRP if giant cell arteritis suspected (especially in elderly with headache/jaw pain) 4
   • Evaluate for other neurological signs that help localize the lesion 3

For Suspected Tonic Pupil:

1. Pharmacological testing:

   • Dilute pilocarpine test (0.125%): Tonic pupil will constrict due to denervation hypersensitivity
   • Third nerve palsy pupil will not respond to dilute pilocarpine

2. Rule out other causes of mydriasis:

   • Medication effects
   • Trauma
   • Angle closure glaucoma

Management

Third Nerve Palsy:

1. Treat underlying cause 1:

   • Urgent neurosurgical intervention for aneurysms
   • Control vascular risk factors for microvascular causes
   • Appropriate treatment for tumors or other compressive lesions

2. Symptomatic management 1:

   • Occlusion therapy (eye patch, occlusive contact lens) for diplopia
   • Prism therapy if appropriate
   • Botulinum toxin to antagonist muscles as temporary measure

3. Surgical intervention 3, 1:

   • Consider only after 6-12 months if no spontaneous recovery
   • May include recession of lateral rectus + resection of medial rectus
   • Goal: Eliminate diplopia in primary position

Tonic Pupil:

• Usually benign and doesn't require specific treatment
• Consider low-concentration pilocarpine drops for cosmetic concerns or photophobia
• Reassurance about benign nature

Prognosis

Third Nerve Palsy:

• Microvascular causes: Complete recovery typically within 3 months 1
• Compressive causes: Depends on underlying etiology and treatment
• Persistent symptoms beyond 3 months warrant reconsideration of diagnosis 1

Tonic Pupil:

• Usually stable or slowly progressive
• Generally good prognosis without significant visual or health consequences

Common Pitfalls and Caveats

1. Incomplete third nerve palsy may be subtle and easily missed 5
2. Pupil involvement is not 100% reliable for differentiating causes 2:
   • Some microvascular third nerve palsies can have pupil involvement
   • Some compressive lesions may spare the pupil
3. Giant cell arteritis can present with third nerve palsy - consider in elderly patients 4
4. Unusual causes of third nerve palsy to consider 6:
   • Schwannomas
   • Meningiomas
   • Cyclic oculomotor paresis
   • Neuromyotonia

Regular follow-up is essential to monitor for improvement and document recovery, especially in cases of presumed microvascular third nerve palsy 1.