Herpes simplex keratitis

Herpes simplex keratitis is a serious medical condition involving the cornea. The cornea is the clear, protective front layer of your eye. This disease is caused by the herpes simplex virus (HSV), which is a double-stranded DNA virus. It has a specific shape called an icosahedral capsid. This virus is very common among humans across the globe. It is estimated that one third of the world's population carries the virus for recurrent infections. Because of this, HSV infections represent a major worldwide public health problem. In developed nations, HSV is the most common cause of blindness derived from the cornea.

The way the virus behaves in the body is quite complex. It often starts with a primary infection of the epithelial cells on the eye's surface. The virus can also move through a process called retrograde infection. This means it travels backward along the nerves that serve the cornea. The virus then hides in a sensory ganglion, which is a cluster of nerve cells. Later, the virus can reactivate and travel down the nerve axon. It moves toward the sensory nerve endings to infect the ocular surface again. This infection is typically unilateral, meaning it affects only one eye at a time.

Doctors classify the different types of keratitis to understand how they affect the eye. One common type is epithelial keratitis, which involves dendritic ulcers. A dendritic ulcer is a linear, branching sore on the cornea. Doctors often use a blue dye called fluorescein to see these defects during an exam. If the ulcer becomes very large, it is called a geographic ulcer. This can happen to patients who are immune deficient or using corticosteroids. Another type is disciform keratitis, also known as endothelial keratitis. This involves inflammation of the corneal endothelial layer, which is the inner layer of the cornea. This can cause swelling called edema and lead to permanent scarring.

There are other, less common forms of the disease as well. A metaherpetic ulcer occurs when the corneal surface is simply unable to heal. This is not caused by a live virus itself. There is also necrotizing keratitis and keratouveitis. Keratouveitis is a type of inflammation that often shows large precipitates on the corneal endothelium. Some patients may also experience a loss of sensation in the affected areas. With repeated recurrences, this can lead to generalized corneal anaesthesia, where the eye loses much of its feeling.

In some cases, the body's own defense system causes the most damage. If a persistent infection allows viral DNA to reach a critical limit, a massive immune response can occur. This happens when antibodies react against the viral antigen expression in the stroma. The stroma is the thick middle layer of the cornea. This immune-mediated stromal keratitis can destroy the corneal stroma. This destruction often leads to opacification, which is when the cornea becomes cloudy. This cloudiness is a major reason why people lose their vision from this disease. The global incidence of herpes keratitis is roughly 1.5 million cases every year. This includes about 40,000 new cases of severe visual impairment or blindness annually.

Medical professionals use several methods to diagnose the specific type of infection. Ophthalmologists and optometrists can often identify the disease by looking at characteristic clinical features. In complicated cases, they may use laboratory tests. They can perform a Tzanck smear, which uses Papanicolaou staining to look for multinucleated giant cells. While this test is useful, it has low sensitivity and specificity. DNA testing is much more rapid, sensitive, and specific, but its high cost limits it to research centers. Doctors can also use viral culture to demonstrate the presence of the virus.

Treatment depends entirely on which part of the cornea is affected. For epithelial keratitis caused by a live virus, doctors use topical or oral antivirals. Aciclovir is a common oral medication used because it does not cause toxicity on the eye surface. Other effective options include ganciclovir and brivudine. However, doctors must be careful with valacyclovir in patients who are severely immunocompromised. For stromal keratitis, which is an immune response, treatment is different. Doctors may use prednisolone drops to reduce inflammation, but they must use a prophylactic antiviral at the same time. For metaherpetic ulcers, treatments like artificial tears, lubricants, or even amniotic membrane transplants help the surface heal.