Optic neuropathy is damage to the optic nerve from any cause. The optic nerve is a bundle of millions of fibers in the retina that sends visual signals to the brain.
Damage and death of these nerve cells, or neurons, leads to characteristic features of optic neuropathy. The main symptom is loss of vision, with colors appearing subtly washed out in the affected eye. A pale disc is characteristic of long-standing optic neuropathy. In many cases, only one eye is affected and patients may not be aware of the loss of color vision until the doctor asks them to cover the healthy eye.
Optic neuropathy is often called optic atrophy, to describe the loss of some or most of the fibers of the optic nerve.
| Optic neuropathy |
In ischemic optic neuropathies, there is insufficient blood flow (ischemia) to the optic nerve. The anterior optic nerve is supplied by the short posterior ciliary artery and choroidal circulation, while the retrobulbar optic nerve is supplied intraorbitally by a pial plexus, which arises from the ophthalmic artery, internal carotid artery, anterior cerebral artery, and anterior communicating arteries. Ischemic optic neuropathies are classified based on the location of the damage and the cause of reduced blood flow, if known.[1]
Optic neuritis is inflammation of the optic nerve, which is associated with swelling and destruction of the myelin sheath covering the optic nerve. Young adults, usually females, are most commonly affected. Symptoms of optic neuritis in the affected eye include pain on eye movement, sudden loss of vision, and decrease in color vision (especially reds). Optic neuritis, when combined with the presence of multiple demyelinating white matter brain lesions on MRI, is suspicious for multiple sclerosis.
Several causes and clinical courses are possible for the optic neuritis. It can be classified in:
Medical examination of the optic nerve with an ophthalmoscope may reveal a swollen optic nerve, but the nerve may also appear normal. Presence of an afferent pupillary defect, decreased color vision, and visual field loss (often central) are suggestive of optic neuritis. Recovery of visual function is expected within 10 weeks. However, attacks may lead to permanent axonal loss and thinning of the retinal nerve fiber layer.
Tumors, infections, and inflammatory processes can cause lesions within the orbit and, less commonly, the optic canal. These lesions may compress the optic nerve, resulting optic disc swelling and progressive visual loss. Implicated orbital disorders include optic gliomas, meningiomas, hemangiomas, lymphangiomas, dermoid cysts, carcinoma, lymphoma, multiple myeloma, inflammatory orbital pseudotumor, and thyroid ophthalmopathy. Patients often have bulging out of the eye (proptosis) with mild color deficits and almost normal vision with disc swelling.
The optic nerve can be infiltrated by a variety of processes, including tumors, inflammation, and infections. Tumors that can infiltrate the optic nerve can be primary (optic gliomas, capillary hemangiomas, and cavernous hemangiomas) or secondary (metastatic carcinoma, nasopharyngeal carcinoma, lymphoma, and leukemia). The most common inflammatory disorder that infiltrates the optic nerve is sarcoidosis. Opportunistic fungi, viruses, and bacteria may also infiltrate the optic nerve. The optic nerve may be elevated if the infiltration occurs in the proximal portion of the nerve. The appearance of the nerve on examination depends on the portion of the nerve that is affected.
The optic nerve can be damaged when exposed to direct or indirect injury. Direct optic nerve injuries are caused by trauma to the head or orbit that crosses normal tissue planes and disrupts the anatomy and function of the optic nerve; e.g., a bullet or forceps that physically injures the optic nerve. Indirect injuries, like blunt trauma to the forehead during a motor vehicle accident, transmit force to the optic nerve without transgressing tissue planes. This type of force causes the optic nerve to absorb excess energy at the time of impact. The most common site of injury of the optic nerve is the intracanalicular portion of the nerve. Deceleration injuries from motor vehicle or bicycle accidents account for 17 to 63 percent of cases. Falls are also a common cause, and optic neuropathy most commonly occurs when there is a loss of consciousness associated with multi-system trauma and serious brain injury. In less than three percent of patients, an orbital hemorrhage after an injection behind the eye (retrobulbar block) can cause injury to the optic nerve, but this is readily manageable if it does not involve direct optic nerve injury and is caught early. The role of high-dose steroids and orbital decompression in treating these patients is controversial and, if administered, must be done very soon after injury with minimal effects. In patients with an orbital fracture, vomiting or nose blowing can force air into the orbit and possibly compromise the integrity of the optic nerve.
Mitochondria play a central role in maintaining the life cycle of retinal ganglion cells because of their high energy dependence. Mitochondria are made within the central somata of the retinal ganglion cell, transported down axons, and distributed where they are needed. Genetic mutations in mitochondrial DNA, vitamin depletion, alcohol and tobacco abuse, and use of certain drugs can cause derangements in efficient transport of mitochondria, which can cause a primary or secondary optic neuropathy.[3]
A nutritional optic neuropathy may be present in a patient with obvious evidence of under-nutrition (weight loss and wasting). Months of depletion are usually necessary to deplete body stores of most nutrients. Undernourished patients often have many vitamin and nutrient deficiencies and have low serum protein levels. However, the optic neuropathy associated with pernicious anemia and vitamin B12 deficiency can even be seen in well-nourished individuals. Gastric bypass surgery may also cause a vitamin B12 deficiency from poor absorption.
Patients who have nutritional optic neuropathy may notice that colors are not as vivid or bright as before and that the color red is washed out. This normally occurs in both eyes at the same time and is not associated with any eye pain. They might initially notice a blur or fog, followed by a drop in vision. While vision loss may be rapid, progression to blindness is unusual. These patients tend to have blind spots in the center of their vision with preserved peripheral vision. In most cases, the pupils continue to respond normally to light.
The most recognized cause of toxic optic neuropathy is methanol poisoning. This can be a life-threatening event that normally accidentally occurs when the person mistook or substituted, methanol for ethyl alcohol. The patient initially has nausea and vomiting, followed by respiratory distress, headache, and visual loss 18–48 hours after consumption. Without treatment, patients can go blind, and their pupils will dilate and stop reacting to light.
The inherited optic neuropathies typically manifest asa symmetric bilateral central visual loss. Optic nerve damage in most inherited optic neuropathies is permanent and progressive.
See main article: Optic nerve. The optic nerve contains axons of nerve cells that emerge from the retina, leave the eye at the optic disc, and go to the visual cortex where input from the eye is processed into vision. There are 1.2 million optic nerve fibers that derive from the retinal ganglion cells of the inner retina.[6]
While optic neuropathy cannot be outright cured, there are surgical options to alleviate pain and symptoms associated with such diseases. The Endoscopic Endonasal Approach method (EEA) is a preferred method of relieving pressure associated with tumors formed in the brain that press upon the optic nerve. It is a minimally invasive surgery. However, due to the genetic and developmental nature of most causes of optic neuropathy, no other surgeries have been proven to cure these diseases.