The ocular surface is an extremely sensitive and dynamic structure, the health of which is crucial for the optimal functioning of the eye. Any mechanical or chemical insult to it can result in anatomical, physiological and optical dysfunction of the eye as a whole.1
Human amniotic membrane is a unique collagenous membrane derived from the innermost submucosa of the placenta, the area in which the human foetus grows and develops within the mother’s uterus.
The innermost layer, the amnion faces the foetus, the chorion facing the uterus. The innate properties of amniotic membrane make it ideal for wound healing/tissue regeneration and in eye care, the management of corneal and conjunctiva disease/reconstruction.2,3 While available in an NHS ophthalmology setting, this article will aim to briefly review this sutureless technology and its growing application in optometric settings, particularly in the United States.
Amniotic membrane (AM) is harvested in a sterile environment from placental tissue obtained during elective caesarean sections from medically cleared donors, and prepared for future use in a variety of ways to retain its biologic properties.2,4 AM was first used therapeutically for skin transplantation in 1910.5
The first ophthalmic application came in 1940 where it was used as a biological dressing for the management of a conjunctival defect.6 The use of amniotic membrane therapy in eyecare became more widespread from the 1990s as the ability to process the tissue improved, so retaining the biological properties of the amnion.2
WHY IS AM SPECIAL?
Amniotic membrane shares a resemblance to the cornea and conjunctiva in its anatomy and physiological make up, containing a variety of collagen types, essential active growth factors and specialised proteins such as fibronectin, laminins, proteoglycans, and glycosaminoglycans. 2,3
MECHANISM OF ACTION
Several mechanisms of action are attributed to the AM’s ability to suppress stromal inflammation, angiogenesis, scarring, and promoting re-epithelialisation, helping in the healing and reconstruction of the ocular surface.
Mechanical
The AM acts as a biological bandage contact lens,1,2 shielding the regenerating conjunctival and corneal epithelium from external agents as well as reducing symptoms of pain and discomfort 7,8 caused by frictional forces during blinking.9
Promotion of epithelialisation
The collagen composition of the AM basement membrane resembles that of the conjunctiva and cornea. It serves as a substrate on which epithelial cells can grow easily, influencing the regenerating corneal epithelium by:
- Facilitation epithelial cell migration10,11
- Reinforcing epithelial basal cell adhesion12-14
- Promoting epithelial cell differentiation15-17
- Prevention of apoptosis (programmed or regulated triggered cell death)2,3,18,19
These properties render it suitable for use in cases of non-healing or persistent epithelial defects of the ocular surface, especially that of the cornea.
Anti-fibrotic properties
Foetal hyaluronic acid is an important constituent of the stromal matrix of the AM. This helps to suppress certain growth factors (TGF ß) and inhibits proliferation and differentiation of corneal, limbal and conjunctival fibroblasts, thereby reducing scarring.20
Anti-inflammatory properties
AM works on the basis of ‘down regulation’ of the immune system, through inhibiting the expression of pro-inflammatory cytokines from the damaged ocular surface.21 This works by:
- Suppression of pro-inflammatory cytokines (IL6 and IL8)
- Down regulation of macrophages and CD4 helper cells through the action of hyaluronic acid with PTX3 (3,5)
Anti-angiogenic properties
In addition to the anti-inflammatory properties that retard new vessel proliferation, a specific anti-angiogenic effect has also been described.22,23,24
Anti-microbial properties
Although some debate exists as to the exact mechanism, AM may reduce the risk of infection through the combination of the AM acting as a physical barrier against infection, with the presence of anti-microbial factors from the amniotic fluid.25,26,27
Donor Capability
AM is universally tolerated due to its lack of histocompatibility antigens HLA-A, B, or DR. It can be implanted without concern for rejection.3,28,29
Additionally, the basement membrane of amnion can also support the expansion of progenitor cells, making it useful for the treatment of partial limbal stem cell deficiency.2
TYPES OF AMNIOTIC MEMBRANE
Currently there are two main types of AM commercially available for in-clinic use:
- Cryopreserved - Prokera, (Biotissue)
- Dehydrated - examples include AmbioDisk (IOP Ophthalmics) and BioDOptix (BioD)3
Both types come in a variety of tissue thicknesses and sizes, depending on clinical needs.2,4,30,31,32 Once in contact with the corneal surface, the AM slowly disintegrates, at a rate related to the severity of the condition and level of inflammation.2,4 Both forms of AM are successful in a variety of ocular surface disorders.33
Cryopreserved AM (Prokera)
Cryopreservation of AM involves slow freezing (at -80°C) using DMEM/glycerol preservation media, containing ciprofloxacin and amphotericin B to allow for slow-rate freezing without ice formation.34 This preservation technique retains the extracellular matrix components, such as heavy-chain hyaluronic acids, growth factors, fibronectin, and collagen, all of which promote anti-inflammatory effects and healing. The tissue is stored in a freezer and brought to room temperature when needed for use.35
Dehydrated AM (AmbioDisk)
Dehydrated AM is preserved using a vacuum with low temperature heat to retain devitalized cellular components. In the case of AmbioDisk, an ‘IOP’ orientation is embossed onto the epithelial surface of the graft during the dehydration process. Unlike cryopreserved tissue, dehydrated AM is kept at room temperature, but it must be rehydrated for clinical use.4,32
CLINICAL USE OF CRYOPRESERVED AM (Prokera)
Prokera (BioTissue) gained FDA approval in 2005, and is a popular choice for the clinical setting as it is pre-packaged and does not require assembly. The AM is secured around a polycarbonate ring system (inner diameter 16mm, outer diameter 21mm) that can be applied without the need for sutures (Figure 1).3,36
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The design of the device ensures the stromal side of the AM is in contact with the cornea. Greater use in optometry coincided with extending the family in 2013 to include:
- Prokera (thickness ~100µm)
- Prokera Slim (thickness ~100µm)
- Prokera Plus (~200µm thick)
The recommended thickness of the tissue and the polycarbonate ring specification is based on the severity of the corneal defect being treated – the more severe the condition, the thicker the AM (and polycarbonate ring) must be. Prokera Slim is considered for mild to moderate indications (such as recurrent corneal erosion and dry eye) and has a lower profile and comfort ring resulting in improved patient comfort. Prokera is recommended in moderate to severe indications (such as neurotrophic epithelial defect) and Prokera Plus is for very severe indications (such as chemical burns) where the degree of corneal compromise that would require a more aggressive healing response.2,3,30,37 Table 1 offers a non-exhaustive list of possible uses.
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For the majority of optometric indications, Prokera Slim appears to be the lens of choice, with successful outcomes and patient comfort.32 Prokera is contraindicated in fungal infections, patients with a glaucoma drainage device or patients with sensitivity to the constituents of the preservation media: ciprofloxacin or amphotericin B.2,36 An advantage of the Prokera is it allows for topical therapeutic agents to be applied over the device when active infection is present. Topical absorption of medicines is not affected by the presence of the AM.2,4,35 Also, it is safe to use with a fluorescein stain to monitor efficacy.35
Prokera in a clinical setting
To maintain its therapeutic and healing effects, Prokera needs to be stored in fridge for up to three months, or in a freezer for up to one year. Once the decision to use an AM has been made, remove the Prokera from the fridge/ freezer, allowing the unopened package to sit at room temperature for 15 minutes.34,35 The device is packaged in an anti-infective glycerol storage medium requiring thorough rinsing with saline. Although the procedure is not fully sterile, application and removal is generally performed using gloves.3 The device is applied in a one-step process by having the patient look down while the upper lid is retracted, and the device pushed in and slid down under the lower lid, ensuring correct position of the membrane over the cornea (Figure 2).32,34
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Although not necessary, some clinicians apply a tape tarsorrhaphy over the lateral canthus to help keep the Prokera centred and to minimise discomfort.
Follow-up depends on the ocular condition. For most conditions in optometric practice, Prokera is generally on the ocular surface for five to seven days, but has been FDA approved to remain on the eye for a maximum of eight weeks.3 Once the AM has dissolved, remove the polycarbonate ring. If the AM is still intact, but the ocular condition has fully healed, then remove the AM/polycarbonate device (see Figure 3).32,34
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CLINICAL USE OF A DEHYDRATED AMNIOTIC MEMBRANE
Two of the most common dehydrated AMS are the AmbioDisk (IOP Ophthalmics – see Figure 4) and BioDOptix (BioD). They are flat discs of tissue that can be stored in a controlled room environment.2,32 The AmbioDisk family includes:
• Ambio 2 (35µm thick, diameter 15mm)
• Ambio 5 (100µm thick, diameter 15mm)38
BioD Optyx comes in two disc sizes (12mm and 15mm) with a central thickness of 40 to 60 microns.30 Insertion requires some dexterity as they lack a stabilizing outer ring, requiring a lid speculum to aid in applying the AM to the cornea. After the dehydrated amniotic membrane is placed on the eye, a soft bandage contact lens is used to secure its placement. Special care must be taken when removing the lid speculum in order not to disrupt the graft by bumping it or the contact lens.30,32
INDICATIONS
The usefulness of AM in its anti-inflammatory, anti-fibrotic, anti-vascularization, anti-scarring effects and ability to enhance epithelial healing is now being used by optometrists most commonly in;
- Dry eye and exposure keratopathy 2
- Recurrent corneal erosion 30
Other optometric indications include; 39-44
- Neurotrophic corneal epithelial defects
- Shielding of ulcers
- Corneal abrasions
- Corneal ulcers
- Corneal burns
- Filamentary keratitis
- Salzmann’s nodular degeneration
- Chemical and thermal burns
- Post-infectious keratitis
Of particular interest is the ability of AM to offer another option for optometrists when treating dry eye. Use of AM is based on the DEWS severity grading level (see Table 2) 45. Most clinicians consider AM generally at level 3 and 4,46 although some clinicians consider AM as early as level 2, before corneal healing becomes an issue, or before considering punctal plugs and chronic steroid use.47
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Patients use the AM for four to seven days, though resolution of signs and symptoms may continue for three to 12 months depending on the initial severity. Patients are educated about expectations and the likelihood that the procedure may need to be repeated.46
CONCLUSION
AM is changing the way clinicians view the treatment and management of ocular surface disorders. AM possesses components that support the growth of ocular surface epithelium and facilitate epithelial cell migration, creating a non-inflammatory environment for the ocular surface to regenerate and heal with the least likelihood of scarring. The increasing choice of commercially available sutureless amnion graft designs and membrane thicknesses offers the opportunity for optometrists to become involved in the management of patients suffering from a variety of painful and sight-threatening ocular surface conditions where conventional therapy may not be sufficient.
Dr Rohit Narayan is a therapeutic optometrist practicing in the Midlands (UK)
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