Does the pupil constrict or dilate in lens subluxation?

Pupillary Response in Lens Subluxation

In lens subluxation, the pupil typically dilates, and this dilation can precipitate serious complications including anterior lens dislocation and pupillary block glaucoma.

Mechanism and Clinical Presentation

The pupil dilates in lens subluxation due to loss of zonular support, and this dilation can worsen the subluxation by allowing the lens to move more freely through the enlarged pupillary space 1, 2. When the crystalline lens loses its zonular attachments, pupillary dilation removes the mechanical barrier that normally keeps the lens in position 1.

The most critical clinical scenario occurs when a dilated pupil allows the subluxated lens to dislocate anteriorly into the pupil space or anterior chamber, causing pupillary block and acute angle-closure glaucoma 1, 2. This represents an ophthalmic emergency requiring urgent surgical intervention 1.

Key Clinical Findings

Physical Examination Signs

• Iridodonesis (tremulous iris movement) is the pathognomonic clinical sign of lens subluxation 3
• The pupil may be mid-dilated, asymmetric, or oval in the affected eye 4
• A fixed, dilated pupil (up to 8mm) can occur with anterior lens dislocation and pupillary block 2
• Pupillary reactivity may be poor or nonreactive depending on the degree of dislocation 4

Position-Dependent Changes

• The lens position can vary dramatically based on patient posture—a lens may appear normally positioned when upright but drop posteriorly into the vitreous when supine 5
• Examination in both erect and supine positions is essential for accurate preoperative assessment 5

Critical Management Principles

Prevention of Further Dislocation After Diagnostic Dilation

If you dilate a patient and discover lens subluxation, immediately reverse the dilation before the patient leaves your office 1. The specific protocol is:

1. Place the patient supine to allow gravity to move the lens posteriorly if it has moved anteriorly 1
2. Administer 1% pilocarpine to both eyes to constrict the pupil and trap the lens in the posterior segment 1
3. Wait until the pupil constricts before allowing the patient to ambulate 1

This approach prevents the catastrophic complication of anterior lens dislocation with pupillary block that occurred in one reported case where a patient was sent home after dilation without pupillary constriction, resulting in complete anterior dislocation requiring urgent pars plana lensectomy 1.

Examination Technique Considerations

• When examining angles in patients with suspected lens issues, perform gonioscopy in a dark room with a bright, short beam that does not pass through the pupil to avoid inducing pupillary constriction that could artificially widen the angle 4
• However, this must be balanced against the risk of allowing pupillary dilation in a subluxated lens 1

Common Pitfalls

• Never send a patient home after dilated examination if lens subluxation is discovered without first constricting the pupil 1
• Do not assume a lens is stable just because it appears well-positioned in one body position—always examine supine and upright 5
• Recognize that YAG iridotomy or peripheral iridectomy are rarely effective for controlling IOP in lens subluxation with glaucoma; definitive lens removal is usually required 3

Associated Glaucoma Patterns

• Short eyes (axial length ~21.5mm) with subluxated lens typically present with acute angle-closure glaucoma requiring urgent intervention 3
• Longer eyes (axial length ~24.4mm) more commonly develop chronic glaucoma 3
• The mechanism involves ciliolenticular block rather than simple pupillary block 3