NK is a rare degenerative eye disease that affects the cornea of the eye. The cornea is the thin, transparent structure in front of the iris and pupil. It plays a critical role in vision.

In someone with NK, the nerves that send signals to the cornea do not function properly. This makes the cornea susceptible to damage.

Prompt treatment can help slow the progression of NK and reduce the risk of complications, such as corneal scarring and vision loss.

This article explores NK’s symptoms, causes, and risk factors. It also describes how doctors diagnose and treat the condition, plus some related health issues that can cause similar symptoms.

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NK is a rare degenerative eye disease. In the United States, it develops in fewer than 5 per 10,000 people. NK affects the cornea, and in order to understand the disease, it is important to understand the cornea’s function.

The cornea is a thin, transparent structure at the front of the eye. It bends light so that the light can enter the retina at the back of the eye. The retina contains specialized nerve cells that convert the light into nerve impulses. These impulses then travel along the optic nerve to the brain, which interprets them as images.

The cornea has nerves that allow it to function properly. In NK, the nerves that send signals to the cornea are dysfunctional.

This means that the cornea cannot respond appropriately to light stimulation or sensations of pressure, and the result is a breakdown and thinning of the outer layer of the cornea, called the epithelium.

In someone with severe NK, the inner layer of the cornea, the stroma, may also thin and break down. Medical professionals refer to this as “stromal melting.” When this is advanced, the stroma may become so thin that a hole forms. This can lead to complications such as infections and vision loss.

The nerve damage of NK may cause a loss of sensation in the cornea, so in the early stages, a person may not notice any symptoms.

When symptoms do appear, they may include:

As NK progresses, the cornea becomes more susceptible to injury, scarring, and infection. Without prompt treatment, the cornea may become severely damaged, and this may lead to permanent vision loss.

The main cause of NK is damage to the trigeminal nerve leading to the cornea. This damage can result from:

  • blunt force injury
  • surgery
  • pressure from an acoustic neuroma, a noncancerous tumor that grows on the main nerve connecting the brain and inner ear

Other medical conditions can cause or increase the risk of NK. The American Academy of Ophthalmology, for example, lists herpes simplex and herpes zoster as leading causes.

Diabetes and multiple sclerosis can also lead to NK.

An early diagnosis is essential in protecting the cornea from damage. However, people usually experience few, if any, symptoms in the early stages, so the diagnosis is often delayed.

To diagnose NK, a doctor:

  • asks about symptoms
  • takes a full medical history
  • takes a family history
  • conducts a full eye examination

The examination may involve:

  • using a device called an esthesiometer to measure corneal sensitivity
  • staining the cornea with dye to help detect changes
  • taking high-resolution pictures of the cornea and its cells
  • using lacrimal function tests to check how well the eyes make tears
  • taking viral cultures to check for herpes simplex or herpes zoster

The treatment for NK aims to relieve the symptoms, protect the cornea, and slow the damage and progression of the disease.

Some typical treatment options include:

Artificial tears

Doctors often prescribe these to keep the cornea moist and clean. This helps prevent corneal injury.

Topical antibiotic eye drops

These help prevent and treat bacterial infections that may develop after injury to the cornea.

Additional medication

In 2018, the Food and Drug Administration (FDA) approved Oxervate topical eye drops for the treatment of NK. The medication helps promote corneal healing.

The FDA describe two 8-week randomized clinical trials in which researchers studied the safety and effectiveness of these drops.

In total, 151 participants with NK took part in the trials. The eye drops were administered six times a day in the affected eye or eyes for 8 weeks. Across both studies, 70% of the participants who used Oxervate had complete corneal healing, compared with 28% who did not receive this medication.

Protective contact lenses

Doctors may prescribe dome-shaped contacts to help protect the cornea.

For people with mild NK, the lenses contain sterile saline, which keeps the eye moist. For people with more severe NK, the lenses contain an advanced lubricant to protect the cornea. Both types help keep the eyes wet and help oxygen reach the cornea.

Surgery may be necessary when severe NK does not respond to more conservative treatments.

The most common surgery is a partial or total tarsorrhaphy. This involves reducing the space between the upper and lower eyelids by partially sewing the lids together. This helps corneal lesions heal. The surgery may be temporary or permanent.

Other health conditions that affect the cornea can have similar symptoms, including dryness, and they can also involve corneal damage.

Some of these include:

  • Corneal dystrophies: This group of genetic eye diseases result from a buildup of abnormal material in the outer layer of the cornea. Like NK, corneal dystrophies are progressive and can lead to vision loss.
  • Keratoconjunctivitis sicca: This condition is also known as dry eye syndrome, and it stems from having a lack of tears or tears of a poor quality. The symptoms are similar to those of NK. A person may also have eye irritation or a gritty sensation in the eyes.
  • Sjögren’s disease: This autoimmune disease can affect the entire body, but one of the most common symptoms is dry eyes. Although corneal damage is not an initial symptom, the dryness can increase the risk.

NK is a rare degenerative eye condition caused by damage to the nerves that send signals to the cornea. The result is reduced corneal sensitivity, which can lead to damage.

A person with NK is susceptible to dry eyes and corneal injury. If it progresses to an advanced stage, eye infections and vision loss may occur.

Treatment promotes corneal healing and includes lubricating eye drops and protective lenses, among other approaches. If a person has any symptoms of this illness, they should consult an eye doctor right away because prompt treatment is key.