Neurotrophic keratitis (NK) is a degenerative disease of the cornea caused by damage of the trigeminal nerve, which results in impairment of corneal sensitivity, spontaneous corneal epithelium breakdown, poor corneal healing and development of corneal ulceration, melting and perforation. This is because, in addition to the primary sensory role, the nerve also plays a role maintaining the integrity of the cornea by supplying it with trophic  factors and regulating tissue metabolism.Neurotrophic keratitis is classified as a rare disease, with an estimated prevalence of less than 5 in 10,000 people in Europe. It has been recorded that on average, 6% of herpetic keratitis cases may evolve to this disease, with a peak of 12.8% of cases of keratitis due to varicella zoster virus.The diagnosis, and particularly the treatment of neurotrophic keratitis are the most complex and challenging aspects of this disease, as a satisfactory therapeutic approach is not yet available. Causes
The cornea lacks blood vessels and is among the most densely innervated structures of the human body. Corneal nerves are responsible for maintaining the anatomical and functional integrity of the cornea, conveying tactile, temperature and pain sensations, playing a role in the blink reflex, in wound healing and in the production and secretion of tears.Most corneal nerve fibres are sensory in origin and are derived from the ophthalmic branch of the trigeminal nerve.
Corneal sensitivity test: performed by placing a cotton wad or cotton thread in contact with the corneal surface: this only allows to determine whether corneal sensitivity is normal, reduced or absent; or using an esthesiometer that allows to assess corneal sensitivity. Tear film function test, such as Schirmers test, and tear film break-up time. Fluorescein eye stain test, which shows any damage to the corneal and conjunctival epithelium

Classification
According to Mackies classification, neurotrophic keratitis can be divided into three stages based on severity:

Stage I: characterized by alterations of the corneal epithelium, which is dry and opaque, with superficial punctate keratopathy and corneal oedema. Long-lasting neurotrophic keratitis may also cause hyperplasia of the epithelium, stromal scarring and neovascularization of the cornea. Stage II: characterized by development of epithelial defects, often in the area near the centre of the cornea. Stage III: characterized by ulcers of the cornea accompanied by stromal oedema and/or melting that may result in corneal perforation. Treatment
Early diagnosis, targeted treatment according to the severity of the disease, and regular monitoring of patients with neurotrophic keratitis are critical to prevent damage progression and the occurrence of corneal ulcers, especially considering that the deterioration of the condition is often poorly symptomatic.The purpose of treatment is to prevent the progression of corneal damage and promote healing of the corneal epithelium. The treatment should always be personalized according to the severity of the disease.