Interventional procedure overview of nerve graft for corneal denervation
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Description of the procedure
Indications and current treatment
The cornea is innervated by the ophthalmic branch of the trigeminal nerve. This innervation maintains the health of the cornea. It does this by providing trophic factors to the corneal cells, activating protective blink reflexes, and stimulating tear production.
Damage to the trigeminal nerve can result in a decrease or loss of corneal sensation. The trigeminal nerve can be damaged by various diseases, chemical burns, physical injuries, or by surgery. Loss of innervation to the cornea can result in neurotrophic keratitis (also known as neurotrophic keratopathy). People with neurotrophic keratitis typically have corneal epithelium defects, poor corneal healing, and can develop sight loss. They are also prone to corneal infections. Neurotrophic keratitis is often defined by 3 stages of severity (Mackie classification). See Outcome measures for a description of the Mackie classification.
Current treatment for neurotrophic keratitis aims to stop progression to later stages of the disease and promote regeneration of the epithelium. This can include topical lubricants and artificial tears. Antibiotic tear drops may be needed to prevent infections. Options for severe disease include lateral tarsorrhaphy (using sutures to partially or fully close the eyelids), topical nerve growth factor, topical collagenase inhibitors, and amniotic membrane grafting.
What the procedure involves
Nerve graft to restore corneal sensation is done under general anaesthesia. This overview presents evidence on autografts, when the graft is taken from the person having the procedure, and allografts, when the graft is a processed nerve from a deceased donor. This overview presents evidence for 4 different graft types:
Autografts:
Sural nerve, harvested from the lower leg.
Lateral antebrachial cutaneous nerve, harvested from the forearm.
Great auricular nerve, harvested from below the ear.
Allograft: processed nerve from a deceased donor.
The nerve graft is harvested or prepared. At the same time, an incision is made on the contralateral side. This is to access an orbital nerve (the supratrochlear, supraorbital, or infraorbital nerve) of the eye that still has normal sensation (the 'donor' nerve). In some people, the ipsilateral supratrochlear, supraorbital, or infraorbital nerve, or the ipsilateral great auricular nerve is used as a donor nerve. The nerve graft is attached to the donor nerve and then subcutaneously tunnelled to the perilimbal area of the affected eye. The nerve fascicles can either be placed around the entire limbal circumference and secured to the sclera or are inserted into corneoscleral tunnels. The nerve fascicles are secured with sutures or fibrin glue, or both. The conjunctiva is closed and a temporary lateral tarsorrhaphy may be placed. A patch and topical lubricants may be prescribed after surgery. Over time, new nerve endings grow into the cornea. A corneal transplant may be needed to fully restore sight in people with loss of corneal clarity.
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This procedure is distinct from 'direct' corneal neurotisation, where the supratrochlear, supraorbital, or infraorbital nerve is dissected and directly transferred to the affected eye without the use of an interpositional nerve graft.
Outcome measures
Mackie classification
The Mackie classification grades neurotrophic keratitis as follows:
Stage 1: punctate keratopathy, positive Bengal staining of the inferior palpebral conjunctival surface (indicative of damage to the corneal epithelium), a decrease in tear breakup time, and an increase in tear viscosity.
Stage 2: epithelial breakdown with recurrent or persistent epithelial defects surrounded by loose hazy epithelium.
Stage 3: corneal ulceration that can progress to melting and perforation.
Corneal sensation
Corneal sensation is typically measured using a handheld Cochet-Bonnet aesthesiometer that contains a thin, retractable, nylon monofilament. Variable pressure can be applied by the device by adjusting the monofilament length (up to 60mm). The filament is first fully extended and applied to the cornea. The filament is then incrementally reduced until the person being tested can feel it. The shorter the length of the filament, the greater the pressure applied on the cornea, and the lower the sensation of the cornea.
Best-corrected visual acuity (BCVA)
BCVA is the measurement of the best visual acuity achieved through use of corrective lenses (that is, glasses or contact lenses). Visual acuity is often measured by a Snellen chart. The Snellen chart consists of multiple lines of letters. The letters in each line are smaller than the letters in the previous line. Standing at 6 metres (20 feet) and covering 1 eye, the participant reads from the top of the chart downwards. The smallest row that can be read accurately indicates the visual acuity of the eye being tested. Visual acuity is expressed in relation to 'standard' vision. For example:
6/6 or 20/20 – 'standard' vision.
6/12 or 20/40 – the participant sees the same level of detail at 6 metres (20 feet) as someone with 'standard' vision sees at 12 metres (40 feet).
6/60 or 20/200 – the participant sees the same level of detail at 6 metres (20 feet) as someone with 'standard' vision sees at 60 metres (200 feet).
A newer test of visual acuity is the Logarithm of the Minimum Angle of Resolution (LogMAR) chart. This chart uses similar principles to the Snellen chart but is more accurate. Higher LogMAR visual acuity scores indicate worse vision. A person with 'standard' vision can resolve 1 minute of visual angle and scores LogMAR 0. A person with 6/12 vision on the Snellen chart would score approximately LogMAR 0.3.
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