What is Optic Nerve Sheath Meningioma?
Optic nerve sheath meningiomas (ONSM) are uncommon non-cancerous growths in the central nervous system, which controls most functions of the body and mind. Despite growing at a slow and steady rate, their position is problematic as they directly impact the part of the eye responsible for vision, potentially leading to severe vision loss. Treating this condition is still up for discussion because the tumor is located near the optic nerve, making its complete removal challenging without causing further complications.
What Causes Optic Nerve Sheath Meningioma?
Optic nerve sheath meningioma (ONSM) typically occurs in the inside part of the optic nerve that’s either within the eye socket or in the canal that the optic nerve passes through. The tumor originates from certain cells in the protective cover (or sheath) around this nerve. As per statistics, the part inside the eye socket is the most common location for this condition (affecting 92% of cases). In almost all cases (95%), the tumor only affects one eye. However, if there are tumors in both eyes, it’s usually in people who have a certain genetic disorder called neurofibromatosis type 2.
The exact cause of most tumors isn’t clear, but genetics likely plays a part. In the case of meningiomas (a particular type of tumor), there may be a link with exposure to certain types of radiation and hormonal changes. However, for ONSM specifically, these links are not firmly established.
Risk Factors and Frequency for Optic Nerve Sheath Meningioma
Optic nerve sheath meningiomas (ONSM) are a type of tumor found in the area around the eye. They make up about 2% of all tumors in this area and 1% of all cases of a type of tumor called meningiomas. Mainly middle-aged women are affected by these tumors. People usually notice these tumors at a younger age compared to other types of meningiomas because vision problems cause them to see a doctor earlier than they otherwise would.
These tumors can also occur in children, although this is rare, and they tend to be more aggressive in children. The frequency of ONSM in children ranges from 1 in 95,000 to 1 in 525,000. About 5-6% of ONSM cases affect both eyes and are linked to a condition known as neurofibromatosis type 2.
When it comes to meningiomas that affect the orbit (the eye socket), only 10% start in the orbit itself. The rest start in the brain and extend into the orbit.
Signs and Symptoms of Optic Nerve Sheath Meningioma
Optic Nerve Sheath Meningioma (ONSM) is a type of tumor that gradually causes painless vision loss in the affected eye. If not treated, it can result in total blindness. The classic symptoms of ONSM comprise vision loss, optic atrophy (damage to the eye’s optic nerve), and optociliary shunt vessels (abnormal blood vessels on the optic disc of the eye), though it’s rare for all three to be present. Other signs can include bulging of the eye and resistance to pressing the eye back into its socket, but these are not always present. Some patients may also report temporary vision loss linked to eye movement, also known as gaze-evoked amaurosis.
In the initial phases, the patient may experience chronic optic nerve swelling as the tumor presses on the optic nerve within the eye socket. As the swelling lessens over time, discoloration of the optic nerve can occur. Around 30% of patients may develop optociliary shunt vessels as the optic disc swelling resolves. While these shunt vessels are indicative of long-term blockage of the central retinal vein like in ONSM, they can also be a symptom of other conditions like central retinal vein occlusion, optic nerve glioma, and sphenoid wing meningioma. Vision impairment in ONSM often results in a relative afferent pupillary deficit in the affected eye, meaning the pupil doesn’t respond to light properly. Defects in the visual field (area in which objects can be seen when the eyes are fixed in one position) of the affected eye are also common, which can appear as partial vision loss, generalized narrowing of the visual field, or enlarged blind spots.
Testing for Optic Nerve Sheath Meningioma
To confirm a diagnosis of Optical Nerve Sheath Meningioma (ONSM), a test known as a magnetic resonance imaging (MRI) is essential. This MRI test, which uses a special agent known as gadolinium to improve image clarity, has become the go-to method for diagnosing ONSM. Its effectiveness has eliminated the need for a tissue biopsy, which is a more invasive procedure that involves taking a small sample of tissue for examination.
On the MRI test, ONSM appears to have the same intensity as grey matter—brain tissue involved in controlling your muscles, sensations and speech—on both T1 and T2 scans, different types of imaging that provide varying levels of detail. ONSM is particularly sensitive to the gadolinium contrast, which makes it illuminate vividly against the optic nerve that doesn’t enhance. This results in a characteristic “tram-track” sign on MRI axial images and a “doughnut” or “non-enhancing dot” sign on coronal images. These terms refer to specific patterns observed in the scans which help identify ONSM.
There are typical ways ONSMs appear on these scans, including tubular expansion (62%), globular (23%), fusiform (11%), and focal enlargement (4%) of the layers surrounding the optic nerve. In contrast, an optic nerve glioma—another type of optic nerve tumor—will show uniform enhancement of the optic nerve and will not possess the distinctive tram track sign.
Computed Tomographic (CT) scans of the orbit and head can provide further details about the bone structure and potential changes in bone tissue caused by secondary tumors. CT scans can also highlight the existence of calcifications within the ONSM and can reaffirm the presence of the characteristic tram track sign.
Treatment Options for Optic Nerve Sheath Meningioma
If your vision is not affected or remains stable, especially if your eyesight is still pretty clear (a measure referred to as ’20/50′ vision or better), your medical team may choose to simply watch the condition closely rather than take immediate action. This approach is usually led by a specific kind of eye specialist known as a neuro-ophthalmologist. They’ll do detailed check-ups, which include tracking how your visual fields (the scope of what you can see) change over time and using a special imaging technology to assess the health of the nerve fibers in your eye. They might also recommend an MRI scan every year.
If a tumor is present, it can be very difficult to remove without causing significant vision loss because of the tumor’s close relationship with the optic nerve. But in some cases, surgery might be considered, especially if the tumor causes noticeable bulging of the eye, significant vision loss, or if it starts to spread into the brain. Surgeons might also opt for observation if the tumor is confined to the orbit (the bony socket containing the eyeball) in the case of a blind eye, because these tumors tend to grow very slowly. However, If the tumor and nerve extend into the brain, both should be removed. Some experts lean towards surgery to not only halt the disease progression, reduce the risk of future vision loss but also, to a degree, restore visual deficits that have already occurred.
A new approach includes transnasal endoscopic optic nerve decompression, which is a minimally invasive surgery to relieve pressure on the optic nerve. This has shown to stop the disease progression and, in some cases, even improve the vision.
Doctors have been using conventional radiotherapy before and after surgery for many years. More recently, they’ve started using a precise form of radiotherapy known as stereotactic radiotherapy (STR) as an alternative to surgery. When vision starts to worsen, STR has become a preferred treatment option because it delivers a precise dose of radiation to the tumor without harming the surrounding tissue. However, there are risks associated with it, such as potential damage to the retina or optic nerve due to radiation exposure.
What else can Optic Nerve Sheath Meningioma be?
When a doctor suspects a problem with your eyes, they would consider different potential diagnoses. These might include:
- Adult optic neuritis, an inflammation of the optic nerve
- Optic nerve glioma, a type of brain tumor
- Orbital lymphoma, a cancer of the white blood cells in the eye
- Orbital metastases, the organ where cancer spreads to
- Sarcoidosis, a disease that causes inflammation in the lungs, skin, or lymph nodes
- Idiopathic orbital inflammatory disease (also known as orbital pseudotumor), which is an inflammation of the eye muscles and tissues.
What to expect with Optic Nerve Sheath Meningioma
The risk of death from ONSM (Optic Nerve Sheath Meningioma), a type of tumor that affects the optic nerve, is virtually non-existent. However, if this tumor goes untreated, it will eventually lead to a steady loss of vision. This occurs because the tumor puts pressure on the optic nerve, impairs its blood supply, and in some cases, spreads into the nerve fluid itself.
In most cases, the patient’s blindness in that eye will gradually increase. At this stage, the doctor will consider whether surgery is the best course of action. Even when performed by an expert eye surgeon, there is a 33% – 50% chance that vision could further deteriorate post-surgery.
Possible Complications When Diagnosed with Optic Nerve Sheath Meningioma
Possible Complications:
- Permanent loss of vision
- Bulging or protruding eyes
- Brain injury after surgery
- Leakage of the fluid around the brain and spinal cord
- Tumor spread into the chiasm (the part of the brain where optic nerve fibers cross)
Recovery from Optic Nerve Sheath Meningioma
It’s recommended that you regularly get your eye health and vision checked by a specialist in neuro-ophthalmology. They will test your visual sharpness and your ability to see the entire scope around you.
You should also have an MRI scan every year. This type of scan uses a magnetic field and radio waves to create detailed images of the inside of your body. A special dye called a contrast is used during the scan to help the areas of your body show up more clearly. This helps check whether the disease has grown or come back.
Preventing Optic Nerve Sheath Meningioma
It’s essential for patients to understand how to manage and the likely outcomes of ONSMs, which are tumors that generally grow slowly over time. Such knowledge equips patients to make informed decisions regarding their tumor situation. It’s interesting to note that some individuals show no severe symptoms even when their tumor is growing, while others might experience rapid loss of vision without any noticeable increase in the size of their tumor. Therefore, it’s highly recommended that all patients have regular health check-ups for close monitoring.
