In a patient six weeks after vitrectomy who now has a fixed, dilated, non‑reactive pupil, what is posterior synechiae of the pupil, its causes, clinical findings, and recommended management?

Posterior Synechiae of the Pupil: Definition and Post-Vitrectomy Management

In a patient six weeks after vitrectomy presenting with a fixed, dilated, non-reactive pupil, posterior synechiae are adhesions between the iris and structures behind it (lens capsule, intraocular lens, or anterior vitreous), and this patient requires immediate examination with pupillary dilation prevention strategies and consideration of surgical lysis if vision or intraocular pressure is compromised.

What Are Posterior Synechiae?

Posterior synechiae are adhesions that form between the posterior surface of the iris and anterior structures behind it—typically the anterior lens capsule, an intraocular lens, or in aphakic/vitrectomized eyes, the anterior vitreous face 1. When extensive (360 degrees), this creates "iris bombé" where aqueous cannot flow through the pupil, causing secondary angle closure 1.

Causes in Post-Vitrectomy Patients

The development of posterior synechiae after vitrectomy occurs through several mechanisms:

• Postoperative inflammation (iritis/iridocyclitis): The primary driver after intraocular surgery 2, 3
• Gas tamponade: Significantly increases risk (28.1% vs 10.1% without gas) by promoting iris-lens contact 2
• Fibrin deposition: Particularly in diabetic patients with proliferative diabetic retinopathy (30.8% incidence) 2
• Immobility of iris: Prolonged pupillary dilation or miotic use creates static iris-lens apposition 4
• Aqueous bypass: After iridectomy or in aphakic states, aqueous bypasses the pupil, reducing iris movement 4

Clinical Findings

Your patient's presentation of a fixed, dilated, non-reactive pupil six weeks post-vitrectomy indicates:

• Complete 360-degree posterior synechiae preventing any pupillary movement
• The pupil is "fixed" because the iris cannot move away from posterior structures
• "Dilated" suggests the synechiae formed while the pupil was pharmacologically dilated
• "Non-reactive" confirms complete loss of pupillary light reflex

Critical examination findings to assess:

• Intraocular pressure: Check for secondary angle closure from iris bombé 1
• Anterior chamber depth: Shallow chamber suggests forward iris displacement 1
• Gonioscopy: Evaluate for peripheral anterior synechiae and angle closure 1, 5
• Slit-lamp examination: Document extent of synechiae (in clock hours), iris bombé, corneal edema 1
• Fundus examination: Rule out other causes of fixed pupil (though less likely given timing)

Management Algorithm

Immediate Assessment (Week 6 Post-Op):

1. Measure IOP immediately: Elevated pressure requires urgent intervention 1, 5
2. Perform gonioscopy: Assess angle status and degree of closure 1, 5
3. Document synechiae extent: Grade in clock hours (0-12) or degrees (0-360°) 6

Prevention (Should Have Been Done):

The evidence strongly supports that this complication was potentially preventable 1, 3, 6:

• Short-acting mydriatics preferred: Studies show 9.4% synechiae rate with short-acting vs 29.7% with long-acting agents (P=0.036) 3
• Postoperative dilation protocol: Guidelines recommend dilating the pupil after iridotomy/surgery to decrease posterior synechiae risk 1
• Avoid pilocarpine: Miotics dramatically increase synechiae formation by creating small, immobile pupils and increasing iris-lens contact 4

Current Treatment Options:

For asymptomatic synechiae without elevated IOP:

• Observation with regular IOP monitoring
• Patient education about angle closure symptoms

For elevated IOP or symptomatic cases:

1. Medical management first 1, 5:

   • Topical corticosteroids to reduce inflammation
   • Ocular hypotensive agents if IOP elevated
   • Avoid miotics (pilocarpine worsens synechiae) 4

2. Laser peripheral iridotomy (LPI) if not already present 1, 5:

   • Creates alternative aqueous pathway
   • Prevents pupillary block mechanism
   • Must confirm patency by visualizing anterior lens capsule 1, 5

3. Surgical synechiolysis 7:

   • Combined limbal and pars plana approach can lyse posterior synechiae
   • Results in partial pupillary constriction restoration
   • Indicated when medical management fails or cosmetic concerns significant
   • Best performed within 6-12 months of formation 1, 5

4. Goniosynechialysis if peripheral anterior synechiae present 1, 5:

   • Most effective within 6-12 months of acute angle closure
   • May improve aqueous outflow

Critical Pitfalls to Avoid

• Do NOT use pilocarpine or other miotics: This worsens synechiae by creating iris-lens apposition and immobility 4
• Do NOT delay IOP assessment: Secondary angle closure can cause irreversible optic nerve damage 1, 5
• Do NOT assume the pupil will spontaneously improve: Six weeks post-op with complete synechiae requires intervention
• Do NOT forget fellow eye examination: Many vitreoretinal conditions are bilateral; assess risk factors

Prognosis

• Cosmetic outcome: Surgical lysis can achieve partial pupillary constriction but rarely complete restoration 7
• Visual function: Usually preserved unless secondary glaucoma develops
• IOP control: Requires ongoing monitoring; may need chronic medical therapy or filtering surgery 1, 5

The key message: This patient needs immediate IOP measurement and gonioscopy to rule out secondary angle closure, followed by a treatment plan that avoids miotics and considers surgical lysis if vision-threatening complications develop 1, 7, 2.