Surface healing, amnion and cenegermin

4. Corneal Wound Healing & Regenerative Therapies

Autologous Serum Tears (AST) & Platelet-Rich Plasma (PRP)

Conventional artificial lubricants lack the biological, epitheliotropic, and neurotrophic factors present in human tears. Autologous Serum Tears (AST) are prepared from the patient's own peripheral blood (centrifuged after clotting, separated, and diluted to $20\%$, $50\%$, or $100\%$ in sterile balanced salt solution without preservatives):

• Biochemical Composition: Supplies critical factors essential for epithelial proliferation and wound healing:
  • Epidermal Growth Factor (EGF): Accelerates epithelial cell migration and mitosis.
  • Transforming Growth Factor-$\beta$ (TGF-$\beta$): Regulates cell differentiation and stromal repair.
  • Fibronectin: Provides an extracellular matrix scaffold promoting epithelial adhesion.
  • Vitamin A & Substance P: Prevent squamous metaplasia and promote corneal nerve maintenance.
  • Lysozyme & serum immunoglobulins (predominantly IgG): Confer antimicrobial protection.
• Indications: Persistent epithelial defects (PED), severe dry eye disease (Sjögren syndrome), neurotrophic keratitis, ocular graft-versus-host disease (GvHD), and chemical burns.
• Platelet-Rich Plasma (PRP): Centrifuged from anti-coagulated whole blood to concentrate platelets 3- to 5-fold, delivering even higher concentrations of PDGF, VEGF, and FGF.

Amniotic Membrane Transplantation (AMT)

The human amniotic membrane—the innermost layer of the fetal placenta—possesses unique biological properties that promote ocular surface reconstruction:

• Histological Structure: An avascular stroma, a thick collagenous basement membrane (rich in laminin-5, type IV, V, and VII collagens, and fibronectin), and an epithelial layer whose cell viability and biological properties depend on processing.

Biological Properties & Molecular Mechanisms

1. Anti-Scarring / Anti-Fibrotic: Upregulates basic fibroblast growth factor (bFGF) while suppressing TGF-$\beta 1$, TGF-$\beta 2$, and TGF-$\beta$ receptor II expression in stromal keratocytes, inhibiting transformation into myofibroblasts and reducing corneal haze.
2. Anti-Inflammatory: Contains Heavy Chain-Hyaluronan / Pentraxin 3 (HC-HA/PTX3) complexes that entrap inflammatory cells, induce rapid apoptosis of activated neutrophils, and downregulate pro-inflammatory cytokines (IL-1$\alpha$, IL-1$\beta$, IL-6, TNF-$\alpha$).
3. Anti-Angiogenic: Synthesises tissue inhibitors of metalloproteinases (TIMP-1, TIMP-2, TIMP-3, TIMP-4), endostatin, and thrombospondin-1, potently suppressing pathological corneal neovascularisation.
4. Scaffold for Epithelial Migration: Serves as a surrogate basement membrane, providing adhesion sites for migrating limbal epithelial stem cells.

Clinical Preparation Modalities & Surgical Techniques

• Cryopreserved Amniotic Membrane: Stored at $-80^\circ\text{C}$ in Dulbecco's Modified Eagle Medium (DMEM) with glycerol/DMSO. Commercial sutureless devices (Prokera) mount cryopreserved amniotic membrane between two polycarbonate rings, inserted directly into the fornices under topical anaesthesia.
• Dehydrated / Lyophilised Amniotic Membrane: Ambient-temperature stable, vacuum-dried sheets (AmbioDisk, Omnigen) rehydrated intraoperatively with saline and covered by a bandage contact lens.
• Surgical Techniques:
  • Inlay ("Graft") Technique: Sized to match the stromal ulcer bed, placed epithelial/basement membrane side facing up (stromal side down) and sutured with 10-0 nylon or secured with fibrin glue. Host epithelial cells migrate over the membrane, which incorporates permanently into the corneal stroma.
  • Overlay ("Patch") Technique: Placed across the entire cornea and limbus, epithelial side facing down, functioning as a biological bandage lens to protect underlying tissue. The patch sheds or is removed after 1 to 2 weeks.

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5. Neurotrophic Keratitis (NK) & Cenegermin

Neurotrophic keratitis is a degenerative corneal disease triggered by impairment of corneal sensory innervation from the ophthalmic division of the trigeminal nerve ($V_1$). Common etiologies include Herpes Simplex Keratitis, Herpes Zoster Ophthalmicus, acoustic neuroma (vestibular schwannoma) resection, diabetic neuropathy, and chronic topical drug toxicity (preservatives, beta-blockers, NSAIDs).

Pathophysiological Cascade

Sensory trigeminal denervation abolishes the protective blink reflex and sensory-evoked tearing. Crucially, denervation depletes essential trophic sensory neuropeptides—predominantly Substance P, Calcitonin Gene-Related Peptide (CGRP), and acetylcholine. Without these trophic factors, corneal epithelial cellular mitosis is arrested, microvilli vanish, cell-matrix adhesion fails, and spontaneous breakdown occurs without pain.

Mackie Staging System of Neurotrophic Keratitis

Cenegermin (Recombinant Human Nerve Growth Factor)

Until recently, neurotrophic keratitis management was purely supportive. The approval of Cenegermin 20 $\mu\text{g}$/mL (Oxervate) introduced the first targeted neuro-regenerative pharmacotherapy for moderate and severe neurotrophic keratitis (Mackie Stages 2 and 3).

• Molecular Biology: Cenegermin is a recombinant human nerve growth factor (rhNGF) produced in Escherichia coli, identical in amino acid sequence to endogenous human NGF.
• Receptor Targets: Binds with high affinity to the Tropomyosin receptor kinase A (TrkA) receptor and with low affinity to the p75 neurotrophin receptor (p75NTR) expressed on corneal epithelial cells, stromal keratocytes, and sub-basal sensory nerve fibers.
• Downstream Cellular Effects:
  • Directly stimulates corneal epithelial cell proliferation, migration, and differentiation.
  • Upregulates synthesis of adhesion molecules, promoting rapid basement membrane re-anchoring.
  • Promotes survival, sprouting, and elongation of damaged trigeminal sensory nerve endings, supporting corneal epithelial healing; sensory recovery is variable.
• EU indication: Oxervate is indicated for moderate or severe neurotrophic keratitis in adults (stages 2 and 3). Epithelial healing improves in trials but is not assured in every patient, and recurrence or persisting sensory loss may need further treatment.
• Administration: One drop six times daily at two-hour intervals for eight weeks, using the supplied daily-vial and pipette system.
• Storage: Pharmacy frozen storage is different from patient storage. Patients refrigerate the weekly carton at 2–8°C according to the EU leaflet, and follow its daily-vial handling and discard instructions; do not instruct patients to freeze the medicine.

See the EMA product information. Check the live local leaflet because handling instructions may differ between jurisdictions.

Surgical Interventions: Tarsorrhaphy & Corneal Neurotisation

• Tarsorrhaphy: Surgical or chemical (botulinum toxin A injection into the levator palpebrae superioris muscle) closure of the eyelid aperture protects the denervated cornea from mechanical micro-trauma and evaporative desiccation, facilitating epithelial closure.
• Corneal Neurotisation: For irreversible, complete trigeminal sensory loss, surgical neurotisation restores corneal innervation by rerouting a functional sensory donor nerve—such as the contralateral or ipsilateral supraorbital, supratrochlear, or great auricular nerve—directly or via an interpositional sural nerve graft to the perilimbal subconjunctival space. Over 6 to 12 months, axon sprouting restores corneal sensation, trophic support, and epithelial durability.