What is Toxic and Nutritional Optic Neuropathy?
Toxic and nutritional optic neuropathies are conditions that affect the eyes and they often show up as progressive vision loss in both eyes, decreased color vision, and blind spots on formal visual field tests. The reason why these blind spots do not affect the pupils’ reaction to light is that the optic nerve, which delivers visual information from the eyes to the brain, is affected equally on both sides.
Usually, vision loss in these cases is a slow process that happens over several months. Taking a careful medical history is very important for diagnosing the cause. Potential risk factors like exposure to harmful substances, alcohol and tobacco use, diet, and work environment should be examined if someone experiences slow vision loss in both eyes.
In the early stages of these conditions, the optic nerves, which are responsible for carrying visual information from the eyes to the brain, usually look normal or possibly a little red and swollen. But if someone continues to be exposed to harmful substances or lacks certain nutrients, the optic nerves can gradually show signs of damage, specifically in the part that carries information from the center of the visual field. Over time, this could lead to a pale appearance of the entire optic disc, which is the circular area at the back of the eye where the optic nerve connects. If the usual visual field test does not reveal an obvious defect, a more focused test can be used to detect blind spots in the center or near-center of the visual field.
What Causes Toxic and Nutritional Optic Neuropathy?
Nutritional optic neuropathy is a condition that usually occurs at random, but can become widespread during periods of war or famine. It is mostly caused by the body not getting enough nutrients like vitamin B12, folic acid, and copper. There is no direct evidence that deficiencies of other vitamins like thiamine, niacin, riboflavin, and pyridoxine cause this condition.
Lack of vitamin B12, the most common cause of nutritional optic neuropathies, can occur due to pernicious anemia, old age, acid reduction treatments for acid reflux, past history of stomach surgery, conditions that affect the gut like celiac disease, tapeworm infestations, alcoholism and its resulting nutrition deficiencies and stomach disorders. It can also be caused by nitrous oxide toxicity, or by adhering strictly to a vegan diet.
Folate deficiency, on the other hand, could be a result of poor diet, increased demand during pregnancy and illnesses related to fast cell multiplication, malabsorption due to alcoholism and gastrointestinal diseases that impair absorption of dietary folic acids, or due to use of certain medications which contraindicate folates.
Copper deficiency is usually caused by stomach surgeries and the resulting problems with nutrient absorption, gastrointestinal (GI) disease, some specific kinds of nutrition received through a tube or drip, or even due to a dietary deficiency. Excessive consumption of zinc can also lead to this.
Toxic optic neuropathy can occur due to the use of harmful medications and even by inhaling toxic substances. When suspected, it’s important to test for presence and levels of toxins in the blood, urine or hair. Certain medications and substances can pose a risk.
Ethambutol, an antimicrobial drug is a common cause, though it depends on the dosage and duration of drug use. There is a higher risk for people over 65, those with low body mass, increased filtration by kidneys and HIV patients. Stopping the drug immediately after toxicity has been recognized often can lead to improvement of vision.
Methanol toxicity is a type of toxic optic neuropathy that happens suddenly after ingestion or exposure to the toxin which is commonly found in home-distilled alcoholic beverages and other substances. Symptoms start with nausea and vomiting and can escalate quickly to confusion, visual loss and even coma.
Amiodarone, an antiarrhythmic drug, is sometimes associated with optic neuropathy. Signs to look out for are gradually progressive vision loss, optic disc swelling and the halt in visual loss when the drug is stopped.
Alcohol and tobacco are no longer directly linked to causing toxic optic neuropathy, but they do increase the risk of deficiencies that may cause optic neuropathy. Smoking, particularly pipe or cigars, is a diagnosis to consider when no other explanation is found for the neuropathy. Other conditions that require investigation include genetic diseases such as Leber’s hereditary optic neuropathy (LHON).
There was a notable occurrence in the 1990s in Cuba, where people had symptoms similar to toxic/nutritional optic neuropathies due to famine. While the presence of mitochondrial mutations was suspected, most patients were not found to have LHON mutations. This suggests the condition might have been due to an as-yet-unknown mitochondrial DVA mutation or due to malnutrition exacerbating underlying mitochondrial DNA mutations.
Risk Factors and Frequency for Toxic and Nutritional Optic Neuropathy
Nutritional optic neuropathy, a vision problem related to poor diet, happens more often during times of war and famine. It’s more common among individuals who might not consume enough nutrients, such as those dealing with alcoholism or those who are socially isolated.
The frequency of toxic optic neuropathy, another vision condition caused by harmful substances, varies based on the specific toxin. For example, in one study of 857 people taking a drug called ethambutol, 1.5% developed vision problems due to the drug. In another study of 837 people taking a drug named amiodarone, the highest yearly rate for losing vision in both eyes due to the drug was 0.13%.
Signs and Symptoms of Toxic and Nutritional Optic Neuropathy
Toxic and nutritional optic neuropathies, conditions caused by toxins or diet-related deficiencies, often lead to gradual and symmetric loss of vision in both eyes. The only exception is when the condition is caused by methanol, in which case vision loss usually occurs after methanol is ingested. Common reports from patients include decreased vision, trouble seeing within their field of view, difficulty seeing in the dark, and diminished color perception.
Early in the disease, the optic disc – part of the eye visible during a standard eye exam – usually appears normal. However, as the disease progresses, a pale optic disc and thinning of the retinal nerve fiber layer, two signs of deteriorating eye health, can be observed.
Testing for Toxic and Nutritional Optic Neuropathy
If you have symptoms pointing to a condition called optic neuropathy, which affects both eyes equally, your doctor will likely recommend a number of tests. The goal of these tests is to check various factors in your blood that could give clues about what’s causing your condition.
Here are some of the tests you might be asked to do:
1. Blood tests to check the levels of vitamin B12 and a compound called serum cobalamin. These are both related to nerve health.
2. Tests to measure the levels of substances that get produced when your body uses vitamin B12. These substances are called “vitamin B12 metabolites” and include serum methylmalonic acid and plasma total homocysteine.
3. A complete blood count, also known as a CBC. This test looks at different types of cells in your blood. It’s often used to check for conditions like anemia, which involves not having enough healthy red blood cells. Along with the CBC, a smear analysis will also be done to look for specific changes in the cells that could indicate a problem.
4. Tests to measure the levels of two types of antibodies, called intrinsic factor and parietal cell antibodies. These tests can help to rule out a condition called pernicious anemia, which affects your body’s ability to absorb vitamin B12.
5. Measuring the levels of a type of vitamin B9 called folate present in the red blood cells. It is a more reliable way to check the body’s supply of this nutrient instead of checking the levels in the blood (serum folate levels).
6. Testing the total level of homocysteine in the plasma (the liquid part of your blood).
Additionally, your doctor might want to see if a copper deficiency could be the cause of your symptoms. To do so, they will check the levels of copper in your blood and urine. They will also look for certain signs in your blood, such as lower than normal number of red blood cells (anemia) or a specific type of white blood cells (neutropenia), and the presence of untypical ‘immature’ cells and ringed sideroblasts – these are signs of problems in red cell production.
You may also have your levels of four vitamins – niacin, riboflavin, pyridoxine, and thiamine – checked to see if any deficiencies might be related to your symptoms.
Treatment Options for Toxic and Nutritional Optic Neuropathy
The treatment for optic neuropathy, a condition that affects the optic nerve in your eye, depends on what caused it in the first place.
If it’s nutritional optic neuropathy, which is usually caused by a lack of certain nutrients, you would need nutritional supplements. If your body can’t absorb these supplements in pill or liquid form, you might need to get them through an injection.
If the optic nerve damage is caused by medication toxicity, meaning a certain medication is harming your optic nerve, that medication should be stopped unless the benefits of taking it are greater than the increased risk of further damaging your optic nerve. In that case, you should consult the doctor who prescribed the medication, as they can recommend alternative treatments.
Lastly, if you consume alcohol or use tobacco products, you may want to limit or stop these habits. This is because alcohol and tobacco can increase your risk of optic neuropathy getting worse.
What else can Toxic and Nutritional Optic Neuropathy be?
When a patient comes in with a double-sided, slowly worsening vision loss, doctors need to check for conditions affecting the macula, a part of the eye responsible for sharp, central vision. This can typically be done with tests like intravenous fluorescence angiography and a type of eye test known as a multifocal electroretinogram (ERG).
Genetic eye diseases, such as dominant optic atrophy and Leber’s hereditary optic neuropathy, can present with similar symptoms as toxic or nutritional optic neuropathies. Therefore, it’s recommended to perform genetic tests to rule these out if toxic/nutritional optic neuropathies are suspected.
If the doctor suspects the vision loss may be caused by issues like demyelination, compression, infiltration, or lack of blood supply (ischemia), additional tests may need to be performed. These could include a brain and orbital MRI with contrast administration, targeted blood tests, and in some cases, a lumbar puncture. It is important to note that the vision loss with these causes is typically sudden and not symmetric.
In cases where the optic nerves appear normal, there’s no thinning of the layer around the optic disc, and the ganglion cells in the eye are functioning normally, yet the patient has decreased central vision, no relative afferent pupillary defect, and narrowed visual fields, it may indicate that the vision loss isn’t due to an organic cause.
What to expect with Toxic and Nutritional Optic Neuropathy
The healing process and outcome for toxic and nutritional optic neuropathy, a disease that affects the optic nerve (the nerve responsible for carrying visual information from the eye to the brain) is not the same for everyone and depends a lot on the cause and initial damage to the optic nerve.
One study found that in patients whose optic neuropathy was caused by a B12 deficiency, over 50% saw an improvement in their eyesight after taking oral B12 supplements. However, there were still a small number of patients who lost their eyesight completely.
In cases where optic neuropathy was caused by methanol poisoning, most patients found that their visual disturbances went away after two to three weeks. However, about a third or more of the patients ended up with lasting visual defects.
On the other hand, nutritional optic neuropathy that resulted from folate deficiency generally had good outcomes if treated within several months after symptoms started showing up. The situation of optic neuropathy caused by tobacco and alcohol use could vary; generally, less severe symptoms and shorter duration of these symptoms meant a better outcome.
Preventing Toxic and Nutritional Optic Neuropathy
Patients who are starting to take a drug called ethambutol need to be aware of any potential risk for damage to the optic nerve, a condition called toxic optic neuropathy. This information is also essential for people who regularly consume alcohol or use tobacco and are starting to take a medication known as amiodarone. They should be informed that these habits can increase the risk for damage to the optic nerve.
There’s another category of patients who are at risk for developing nutritional optic neuropathy. Those patients, especially those who are considering undergoing gastric bypass surgery, starting a medication therapy involving methotrexate, or being diagnosed with a condition called pernicious anemia, should be briefed about the potential for vision loss and the importance of regular check-ups with an eye specialist (ophthalmologist).
Optic neuropathy can often be the result of certain vitamin deficiencies. One way to prevent these deficiencies is by taking nutritional supplements right from the start of the treatment or lifestyle change.
