What is the diagnosis for a patient with a fixed semi-dilated pupil in the left eye and difficulty visualizing the left fundus, with a normal cranial nerve (CN) examination?

Diagnosis: Adie's Tonic Pupil (Most Likely)

The most likely diagnosis is Adie's tonic pupil, a benign condition affecting the parasympathetic innervation to the pupil and ciliary muscle, which explains both the fixed semi-dilated pupil and difficulty visualizing the fundus (due to impaired accommodation). 1

Clinical Reasoning

Key Distinguishing Features Present in This Case

• Fixed semi-dilated pupil with normal cranial nerve examination effectively rules out third nerve palsy, which would show additional extraocular muscle dysfunction 2
• Difficulty visualizing the fundus suggests impaired accommodation from ciliary muscle dysfunction, a hallmark of Adie's pupil 1
• Absence of other neurological signs excludes compressive lesions, aneurysms, or intracranial pathology 2, 3

Why This Is NOT a Third Nerve Palsy

A pupil-involving third nerve palsy would present with:

• Ptosis (complete or partial) from levator muscle involvement 2
• Ophthalmoplegia affecting medial rectus, superior rectus, inferior rectus, and/or inferior oblique muscles 2
• Urgent concern for posterior communicating artery aneurysm requiring immediate neuroimaging with MRA or CTA 2

Since the cranial nerve examination is normal (no ptosis, no extraocular muscle dysfunction), third nerve palsy is excluded. 2

Adie's Tonic Pupil Characteristics

• Unilateral dilated pupil that reacts poorly or slowly to light 1
• Impaired accommodation causing blurred near vision and difficulty with fundoscopy 1
• Light-near dissociation (better response to near stimulus than light) may be present 1
• Segmental iris palsy on slit-lamp examination (vermiform movements) is pathognomonic 1
• Benign condition requiring no urgent intervention 1

Diagnostic Confirmation

Pharmacologic Testing

• Dilute pilocarpine (0.125%) will constrict an Adie's pupil due to denervation hypersensitivity, but not a normal pupil 4
• This test distinguishes Adie's from pharmacologic mydriasis (which would not respond to pilocarpine) 4

Alternative Diagnoses to Exclude

Pharmacologic mydriasis from:

• Inadvertent exposure to anticholinergic medications (ipratropium, atropine) 4
• Would not respond to pilocarpine 4
• History of medication exposure would be present 4

Acute angle-closure glaucoma:

• Would present with severe eye pain, headache, nausea, corneal edema, and elevated intraocular pressure 3
• Fundus would show optic disc changes, not just difficulty visualizing 3
• This is excluded by the clinical presentation 3

Critical Pitfall to Avoid

Do not mistake anisocoria with exotropia for third nerve palsy. 1 The combination of diplopia, exotropia, and dilated pupil can mislead clinicians into suspecting oculomotor nerve palsy, triggering unnecessary urgent neuroimaging. 1 The key is that true third nerve palsy always has additional motor findings (ptosis and/or ophthalmoplegia). 2

Management

• Reassure the patient that this is a benign condition 1
• Reading glasses may help with near vision if accommodation is significantly impaired 1
• No urgent intervention required 1
• Monitor for bilateral involvement as Adie's pupils can become bilateral over time 1