What are the indications for lamellar keratoplasty (LK) versus amniotic membrane transplantation (AMT) in cases of keratitis?

Lamellar Keratoplasty vs AMT in Keratitis

Amniotic membrane transplantation (AMT) should be used first-line for keratitis complicated by corneal thinning or small perforations to stabilize the ocular surface, while lamellar keratoplasty is reserved for post-infectious stromal scarring after the infection has been eradicated or for medically unresponsive cases requiring removal of infected tissue. 1

AMT Indications in Active Keratitis

AMT serves as the primary surgical intervention during active infection when complications develop:

• Corneal stromal thinning without frank perforation to decrease inflammation and stabilize the ocular surface 1
• Small corneal perforations where the goal is to seal the defect and restore globe integrity 1
• Impending perforation in eyes with progressive corneal melting 1
• Cases refractory to medical treatment where you want to avoid emergency keratoplasty and improve prognosis of subsequent elective keratoplasty 1

AMT Outcomes in Infectious Keratitis

AMT demonstrates clear therapeutic benefit when used as adjuvant therapy:

• Promotes faster corneal healing by 4.08 days compared to antimicrobial treatment alone in moderate-to-severe bacterial and fungal keratitis 2
• Improves visual outcomes with better uncorrected visual acuity at 1 month (0.26 logMAR improvement) 2
• Effective in promoting epithelialization in 75% of active fungal keratitis cases and 100% in inactive cases 3
• Prevents corneal perforations without risk of rejection 3

Critical AMT Caveat

Risk of persistent or recurrent infection necessitates continued antifungal/antimicrobial treatment throughout and after AMT - this is particularly important as persistent fungal keratitis occurred in 8.7% of cases despite AMT 3. Repeated cultures should be performed immediately before AMT to guide ongoing therapy 3.

Lamellar Keratoplasty Indications in Keratitis Context

Lamellar keratoplasty serves different roles depending on timing and disease severity:

Therapeutic DALK During Active Infection

Deep anterior lamellar keratoplasty (DALK) is indicated for medically unresponsive infectious keratitis requiring complete removal of infected stromal tissue 1, 4:

• Medical treatment failure where infection cannot be controlled with antimicrobial therapy alone 1
• Deep stromal involvement where infection extends through corneal layers but spares Descemet's membrane and endothelium 1
• Acanthamoeba keratitis unresponsive to intensive antimicrobial therapy - DALK with total stromal removal down to Descemet's membrane can achieve complete eradication without anterior chamber entry 5

The key advantage of therapeutic DALK over penetrating keratoplasty is less risk of intraocular entry of infectious organisms at surgery and improved graft survival from reduced endothelial rejection 5, 4.

Important Limitation in Acanthamoeba

When DALK is used for Acanthamoeba keratitis, obtaining a big bubble is more difficult, recurrent infection occurs in 20% of cases, and graft failure rates reach 60% (mostly from poor endothelial function) 6. Therefore, use confocal microscopy and create large margins around involved cornea to minimize recurrence 6.

DALK for Post-Infectious Scarring

After infection has been eradicated, DALK is the preferred approach for visual rehabilitation:

• Stromal scarring from resolved keratitis that does NOT involve Descemet's membrane or endothelium 1
• Post-infectious scars have excellent 10-year survival rates (second only to keratoconus) 6
• Avoids endothelial rejection (though stromal rejection still occurs in 2-12% of cases, usually reversible with topical corticosteroids) 1, 6

Therapeutic ALTK for Peripheral/Melting Disorders

Automated lamellar therapeutic keratoplasty (ALTK) provides tectonic support after melting disorders including central corneal ulcers and peripheral ulcerative keratitis, particularly if excessive thinning or descemetocele develops 6:

• Central grafts are circular; peripheral pathology may require concentric donut or partial crescentic grafts 6
• Donor tissue may be partial-thickness irradiated, glycerin preserved, or fresh tissue 6

When to Escalate to Penetrating Keratoplasty

Large corneal perforations that cannot be sealed with AMT or tissue adhesive require penetrating keratoplasty 1:

• Large perforations exceeding AMT sealing capacity 1
• Therapeutic indication for unresponsive microbial infection requiring removal of infected tissue when DALK is not feasible 1
• Deep stromal involvement extending to endothelium where lamellar dissection is impossible 1

Critical Pitfalls to Avoid

• Do NOT use corticosteroids during active infection, especially with fungal or Nocardia keratitis - this worsens outcomes 1, 7
• Send all removed corneal tissue for pathologic and microbiologic analysis during therapeutic keratoplasty 1
• Continue antimicrobial therapy throughout AMT and monitor for persistent infection 3
• Recognize that deeper stromal scars increase risk of Descemet's membrane perforation during DALK, potentially requiring conversion to penetrating keratoplasty 6

Adjunctive Measures

• Tissue adhesive with Tenon's patch graft and cyanoacrylate glue can manage larger perforations as alternative to immediate penetrating keratoplasty 1
• Oral tetracyclines (doxycycline/minocycline) or N-acetylcysteine to inhibit matrix metalloproteinases and counteract stromal thinning, though limited data exists 1