Peripheral Anterior Synechiae (PAS): Gonioscopy Signs

Peripheral anterior synechiae (PAS) are adhesions that bind the peripheral iris to the angle wall, usually at or above the trabecular meshwork.

Unlike appositional contact, they block outflow permanently.

Therefore, indentation gonioscopy is the bedside test that separates the two.

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What Peripheral Anterior Synechiae Are

The iris normally inserts behind the scleral spur and leaves the meshwork exposed.

With PAS, the iris root adheres to structures anterior to the spur.

As a result, the examiner loses sight of the spur, the meshwork and sometimes Schwalbe line.

Why Peripheral Anterior Synechiae Raise Pressure

Each clock hour of PAS removes a segment of filtering meshwork.

However, extensive synechial closure reduces outflow facility and raises intraocular pressure.

The obstruction also persists after the original trigger settles.

Indentation Gonioscopy Technique

A four-mirror lens smaller than the cornea allows dynamic examination (see gonioscopy technique).

First, examine the angle in a dark room with a short, narrow beam.

Next, press gently on the central cornea.

The pressure forces aqueous into the angle recess and pushes the peripheral iris backward.

Then, compare each quadrant before and during indentation.

Reading the Response to Indentation

In appositional closure, the iris falls back and the meshwork reappears.

By contrast, PAS hold the iris to the wall, so that segment stays closed.

Too much pressure distorts the angle, while too little misses appositional segments.

Peripheral Anterior Synechiae Versus Iris Processes

Iris processes are normal, fine strands that bridge the angle recess.

They look lacy, follow the angle curve and let the meshwork show through.

In addition, they rarely extend beyond the posterior meshwork.

PAS, on the other hand, form solid sheets that hide the structures beneath them.

Morphology in Primary Angle Closure

Primary angle closure often produces broad PAS that start superiorly, where the angle is narrowest.

Some eyes, for instance, show creeping closure, with the iris insertion advancing slowly up the angle wall.

Because the process is gradual, the patient may notice nothing until the optic nerve suffers.

Morphology After Uveitis

Inflammatory PAS tend to look irregular, tented or patchy.

They often form inferiorly, where inflammatory cells and fibrin settle.

Coexisting posterior synechiae can also push the iris forward into bombe (see posterior synechiae and iris bombe).

Neovascular and ICE-Related Peripheral Anterior Synechiae

In neovascular glaucoma, a contracting fibrovascular membrane pulls the iris onto the meshwork.

The result is a smooth, zipper-like closure line, often with new vessels crossing the spur.

Iridocorneal endothelial syndrome, by comparison, causes high, broad PAS that may pass Schwalbe line.

Therefore, adhesions beyond Schwalbe line warrant a careful endothelial exam.

Other Causes to Check

• Flat chamber after filtering or cataract surgery
• Blunt or penetrating trauma
• Argon laser trabeculoplasty spots placed too posteriorly
• Plateau iris with long-standing apposition
• Intumescent lens pushing the iris forward
• Posterior segment tumors or large choroidal detachments

Documenting Peripheral Anterior Synechiae

Record the extent in clock hours or degrees for each quadrant.

Also note the height of each adhesion, for example meshwork or Schwalbe line.

A simple angle drawing helps the next examiner judge progression.

In addition, serial gonioscopy shows whether closure is stable or advancing.

Peripheral Anterior Synechiae in Angle-Closure Classification

Foster and colleagues listed PAS among the signs that separate primary angle closure from a closure suspect.

An occludable angle with PAS, raised pressure or both defines primary angle closure.

Glaucomatous optic neuropathy turns this into primary angle-closure glaucoma.

As a result, even a few PAS move the patient into a higher risk group.

Peripheral Anterior Synechiae and Laser Iridotomy

Laser iridotomy relieves pupillary block and can widen appositional segments (see laser peripheral iridotomy).

However, it cannot reopen angle already sealed by synechiae.

In the ZAP trial, iridotomy in angle-closure suspects lowered the risk of progression, although absolute benefit was small.

Its primary endpoint also included new PAS, which shows the weight trials give this sign.

Lens Extraction and Goniosynechialysis

The EAGLE trial favored clear-lens extraction over laser iridotomy in selected patients with angle closure.

Removing the lens deepens the chamber and relieves crowding at the angle.

Finally, goniosynechialysis strips adhesions from the angle wall, often during phacoemulsification.

Results tend to be better with recent PAS, because older adhesions may cover scarred meshwork.

Counseling the Patient

Patients cannot feel PAS, so describe them as scars that block the eye’s drain.

Treatment aims to stop new scarring and control pressure.

However, existing adhesions usually stay, and some patients still need drops or surgery.

Therefore, stress regular gonioscopy and optic nerve review even when vision feels normal.

Pitfalls to Avoid

• Calling an angle closed without indentation
• Mistaking iris processes for PAS
• Ignoring inferior tented PAS from past uveitis
• Assuming iridotomy reverses synechial closure

For the latest studies, see the PubMed results for peripheral anterior synechiae.

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References

1. Foster PJ, Buhrmann R, Quigley HA, Johnson GJ. The definition and classification of glaucoma in prevalence surveys. Br J Ophthalmol. 2002;86:238-242.
2. Forbes M. Gonioscopy with corneal indentation: a method for distinguishing between appositional closure and synechial closure. Arch Ophthalmol. 1966;76:488-492.
3. Campbell DG, Vela A. Modern goniosynechialysis for the treatment of synechial angle-closure glaucoma. Ophthalmology. 1984;91:1052-1060.
4. He M, Jiang Y, Huang S, et al. Laser peripheral iridotomy for the prevention of angle closure: a single-centre, randomised controlled trial. Lancet. 2019;393:1609-1618.
5. Azuara-Blanco A, Burr J, Ramsay C, et al. Effectiveness of early lens extraction for the treatment of primary angle-closure glaucoma (EAGLE): a randomised controlled trial. Lancet. 2016;388:1389-1397.