Introduction
Neuropathy is damage to a nerve or a group of nerves that results in loss of function of the particular nerve or nerves. This impairment in the function of the nerves has an adverse effect on the muscles supplied by those nerves. The optic nerve is the second cranial nerve (nerves that arise from the spinal cord). The main function of the optic nerve is to enable vision. It transmits the visual information recorded in the retina of the eye to the brain, where it is processed. The optic nerve is the only nerve that leaves the cranial cavity and can be visualized clinically. It is formed by the axons (head of the nerve cell) that arise from the retinal ganglion cells at the back of the eyeball.
What Is Infectious Optic Neuropathy?
When the optic nerve is subjected to damage caused by a viral, bacterial, parasitic, or fungal infection, it may be termed an infectious optic neuropathy. Optic neuritis is the most common form of optic neuropathy. In addition, various infections can cause optic neuritis in young individuals. Optic neuritis is characterized by swelling of the optic nerve. The swelling and inflammation combined to damage the optic nerve and result in pain and temporary loss of vision.
Which Infections Can Result in Optic Neuropathies?
The following infections are associated with optic neuropathy -
1. Viral Infections -
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Herpes Simplex Virus (HSV Type 1 and 2) - When the herpes simplex virus affects the brain, it is known as herpes simplex encephalitis. Optic neuritis is a common complication of herpes simplex encephalitis. It is known as acute retinal necrosis syndrome. It causes retina degeneration, resulting in rapid and severe loss of vision. Oral corticosteroid therapy with Acyclovir.
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Varicella Zoster Virus - This virus is responsible for developing varicella zoster in children and herpes zoster in adults. Varicella causes the primary infection that occurs in childhood, and herpes zoster is the reactivation of infection that occurs later on in life. Papillitis occurs commonly after the appearance of a rash in children. It is also known as optic neuritis and is caused by the deterioration and inflammation of a particular portion of the optic nerve known as the optic disc. Antiviral drugs are of little to no importance in treating the optic neuropathies caused by this virus. However, systemic corticosteroid therapy is effective as it helps recover vision loss.
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Cytomegalovirus - It causes optic neuritis, papillitis, and neuroretinitis (a condition characterized by inflammation of the optic nerve head and retina).
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Epstein Barr Virus - This virus causes infectious mononucleosis in children and young adults. In this disorder, there is a bilateral involvement of the optic nerve. There is inflammation of the optic nerve resulting in optic neuritis. Treatment with corticosteroids is proven to help recover vision.
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Chikungunya Virus - This virus is transmitted through the infected Aedes mosquito. Optic neuropathy is the leading cause of blindness in patients with chikungunya. The prognosis of this condition is good, provided it is treated in time with corticosteroids.
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Dengue Virus - Dengue fever-associated optic neuritis is a severe complication of the dengue virus. It may cause permanent loss of vision. The optic neuritis may be unilateral or bilateral. There are cases where patients have recovered spontaneously, while in some cases, the vision has not recovered.
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Mumps virus - Mumps is an acute contagious viral infection that is characterized by swelling and inflammation of the parotid gland that is situated on the side of the cheek in front of the ear. It usually affects children and causes fever and pain in the affected areas. There is bilateral involvement of the optic nerve, and symptoms appear two to five weeks after the appearance of swelling on the parotid glands.
2. Bacterial Infections - Invasive bacterial infections have been observed in the orbital and periorbital areas. Systemic broad-spectrum antibiotics help manage this condition caused by bacterial infections.
3. Fungal Infections - Various types of fungi cause fungal infections and can affect different parts of the body, including the eyes. Fungal optic neuropathies are relatively rare but can occur in individuals with compromised immune systems or those exposed to specific fungi. Examples of fungi that can cause optic neuropathy include Aspergillus, Candida, and Histoplasma.
4. Parasitic Infections - Parasitic infections occur when organisms called parasites invade the body and establish themselves at the expense of the host. Certain parasites can also affect the optic nerve, causing optic neuropathy.
What Is the Pathophysiology of Infectious Optic Neuropathies?
Optic neuritis is a condition that occurs due to inflammation and infections associated with the optic nerve. The inflammation caused by the infections affects the protective covering of the neurons called the myelin sheath. Inflammatory degeneration of the myelin sheath is known as demyelination. The immune system of the body mediates the demyelination of neurons. It is the body’s response to an infection.
What Are the Symptoms of Infectious Optic Neuropathies?
Optic neuritis is an acute demyelination of the neurons that results in the following symptoms: pain and monocular vision loss (vision loss in one eye due to the involvement of the optic nerve on one side). Even after the recovery of vision, there may be certain signs that may persist. The chronic signs of optic neuritis are as follows -
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Afferent Pupillary Defect - It occurs as a result of asymmetric involvement of the optic nerve. It is characterized by an asymmetric reaction of the pupils to light. When the eye on one side is exposed to direct light, there will be a constriction of the pupil of that eye. The direction of the light source is then changed and directed toward the other eyeball. The examiner will observe changes in the pupil of the other eye. This test is carried out in a dim room. The differences in the pupil diameter of both eyes will confirm the presence of optic neuropathy in one eye.
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Color Desaturation - The colors appear dull and muted rather than vivid and bright.
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Optic Atrophy - The cells that constitute the optic nerve start dying. The death of the retinal ganglion cells will result in the impaired transmission of signals from the eye to the brain, causing problems with vision.
How Are Infectious Optic Neuropathies Diagnosed?
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Appropriate clinical diagnosis and laboratory findings play a critical role in the diagnosis of infectious optic neuropathies.
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Cranio-orbital high-resolution computed tomography is a gold standard technique for diagnosing orbital and periorbital optic neuropathies.
How Are Infectious Optic Neuropathies Treated?
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Medications for treating the infections and corticosteroid medications to suppress the inflammation caused by these infections will be the primary goal of treatment.
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Antibacterial drugs are useful for treating bacterial infection-induced optic neuropathy.
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Corticosteroid medications are recommended for almost all optic neuropathies as they help in vision recovery and prevent the worsening of the condition.
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Severe cases may require surgical intervention.
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Laser therapy for repairing nerve damage is also recommended in some cases.
What Is the Prognosis for Infectious Optic Neuropathies?
The prognosis is good in cases of optic neuropathies, provided the condition is identified and treated in time. Immediate intervention may preserve vision. However, severe vision loss is irreversible, and there is progressive optic atrophy in untreated cases.
Conclusion
Various bacteria, fungi, parasites, and viruses can cause optic neuropathies. Ocular findings depend upon the proper clinical examination and laboratory findings. The recovery of lost vision depends upon how early the treatment was initiated for the patient. The outcome is good for almost all cases of optic neuropathy, provided the treatment is started early.
