Optic Neuritis and its Occurence with Chiari Malformations and MS
Optic neuritis is associated with inflammation of the optic nerve. It can cause pain, blurred vision and blindness. Chiari malformations may play a role in optic neuritis. A Chiari malformation occurs when the brainstem sinks in the cranial vault.
Humans are susceptible to Chiari malformations, which squeeze the brainstem, typically the cerebellum, into the foramen magnum thereby compressing it resulting in a variety of neurolgical and vascular problems in the brain.
As the brainstem sinks toward the foramen magnum it pulls other parts of the brain down along with it toward the base of the skull. This raises the risk of similar compressions of cranial nerves within the openings of the base of the skull, especially the larger openings. The nerves and blood vessels that pass through these openings can suffer a similar fate to the brainstem and its circulatory routes in a Chiari malformation; that is, the cranial nerves and blood vessles can get squeezed into the openings in the base of the skull and get compressed. One of the most vulnerable of the cranial nerves in this regard, appears to be the optic nerve.
Optic neuritis is closely associated with multiple sclerosis, so much so, that it is the first sign of MS in twenty to thirty percent of MS cases, and fifty percent of MS patients will eventually suffer from it. Furthermore, approximately thirty percent of the patients who get optic neuritis as the initial sign will eventually get MS within in three to five years.
Optic neuritis is also one of the hallmarks of optic-spinal MS seen in Asian races and Devic's disease, which affects African-Americans who otherwise, similar to Asians, have a much lower incidence of MS than European races. Both conditions are, for the most part, clinically identical in that neither are associated with classic MS lesions typically seen in the brain. Instead, both are associated with transverse myelitis, which affects myelinated tracts in the cord. In any case, in all three conditions, the cause of optic neuritis is unknown.
The cell bodies of the optic nerve are located in the retina at the back of the eye. Thus the cell bodies are in the eye which is located in the eye socket outside the cranial vault and brain. Because the nerve cell bodies are outside the cranial vault, technically speaking, it means the optic nerve isn't a true cranial nerve. On the other hand, the optic nerve is an outgrowth of the hypothalamus of the brain and is wrapped in the meninges of the brain. In fact, the white part of the visible part of the eye, called the sclera, is actually part of the dura mater, or outer coat of the brain. Lastly,cerebrospinal fluid (CSF) flows through perivascular spaces of the optic nerve the same as it does in the perivascular spaces of the brain.
To get from the eyeball in the eye socket to the brain, the optic nerve passes through the optic canal. It is called the optic foramen in the picture above but canal is more appropriate due to it's length. You can see in the picture, the optic canal passes beneath the lesser wing of the sphenoid bone in the anterior fossa of the cranial vault. The sphenoid bone was so named because it is shaped like a bat. The greater wings of the sphenoid bone are the large wing-like looking structures that make up the middle fossa of the brain. The eye sockets sit in front of the middle fossa. The lesser wings of the sphenoid are behind the ethmoid bone, which is for the passage of nerves from the brain to the nose. The ethmoid bone and lesser wings of the sphenoid make up the floor of the bottom of the skull in the forehead area known as the anterior fossa.
The lesser wing of the sphenoid creates a slight cover over the entrance to the canal so that you can't see through it by looking straight down at the floor of the cranial vault. You have to tilt the head to the side to look through the canal. In some cases, however, the cover is smaller and it doesn't take much head tilt to see through it.
Once inside the cranial vault the optic nerve passes alongside the sella turcica, which houses the pituitary gland. The front part of the sella turcica is located between the two lesser wings of the sphenoid bone. See the picture above.
The optic nerve fibers terminate in the lateral geniculate body of the thalamus and the superior colliculus of the midbrain.
Blood supply to the eye comes from the internal carotid artery. The internal carotid artery enters the skull through the carotid canal just in front of the jugular foramen, which is the opening for the jugular veins. The cavernous sinuses, likewise, pass alongside the sella turcica beneath the optic nerve.
The cavernous sinuses are part of the dural sinus drainage system of the brain. The carotid canal is a relatively large and long canal compared to other passages in the cranial vault. The internal carotid arteries pass through the canal and enter the cranial vault through the foramen lacerum next to the sella turcica. The internal carotid arteries then pass through the cavernous sinuses on their way to the brain.
The first branch of the internal carotid artery is the opthalmic artery. The opthalmic artery supplies the eye and is one of the chief alternate circulatory routes for brain circulation in the event of reduced flow from other routes. The opthalmic arteries leave the cavernous sinuses and enter the optic canals along with the optic nerves on their way to supply arterial blood to the eyes.
Causes of Optic Neuritis
Optic neuritis is attributed to inflammation of the optic nerve. There are many different causes of optic neuritis. Three causes may be related to upright posture in certain circumstances. The first one is compressive optic neuropathy.
Compressive neuropathy can be due to many causes such as tumors, inflammation and infections associated with swelling and edmema that can press on the nerve. Currently, most compressive neuropathies of the optic nerve are associated with conditions inside the orbit and much less frequently from within the optic canal.
The second type of optic neuropathy is ischemic neuropathy. Ischemic neuropathy occurs when blood flow to the nerve is reduced to the point that it causes irritation and damage to the nerve. There are several key types. Two types in particular are anterior ischemic optic neuritis and posterior ischemic optic neuritis depending on whether the problem originates inside or outside the cranial vault. In either case they have similar consequences, which is loss of or impaired vision.
The third type of optic neuropathy of interest to upright posture is traumatic optic neuropathy. There are two types of traumatic optic neuropathy. One is direct trauma to the tissues of the eye. The other is indirect trauma. Interestingly, most types of indirect trauma occur inside the optic canal.
In violent head trauma and whiplash injuries, the brain bounces around inside the cranial vault, which can cause bruises that show up on the opposite side of the trauma. Technically, it is called a contrecoup injury. Cerebrospinal fluid also goes through acceleration and deceleration. Lastly, according to Dr. Franz Schelling venous blood from the body, causes back jets into the brain. All these forces can be destructive to brain tissues.
Furthermore, whiplash injuries generate tension and compression forces that can cause the brain to herniate into structures in the brain and in the skull. In particular, whiplash injuries have been shown to cause Chiari malformations where the brainstem, herniates, that is, gets pulled by tensile forces into the foramen magnum. Likewise, the optic nerve gets tugged by tensile forces that cause it to get pulled back and forth within the optic canal along with the displacement of the brain inside the cranial vault. Thus tensile forces may similarly cause it to become lodged or injured inside the canal similar to a Chiari malformation. Compression of the optic nerve into the optic canal could result in optic neuritis.
Additionally, structural problems such as genetic design issues can lead to undersized spaces and openings in the base of the skull called hypoplasia, which may affect the dimensions of the optic canal. Furthermore, misalignments of the upper cervical spine, exaggerated head tilts and scoliosis of the spine can displace the brain within the cranial vault and increase pressure on the optic nerve and opthalmic artery in the optic canal.
Lastly, the brain is kept floating above the base of the skull and its openings by CSF, which is mostly water, contained in cisterns strategically placed for protection. The chiasmatic cistern is one of the highest cisterns. Its sits beneath the optic nerve and provides protection from the hard bones of the base of the skull.
When CSF volume in the brain decreases, the brain sinks in the cranial vault. Among other things, when the brain sinks in the vault it can compress the brainstem and cranial nerves.
Due to its frequent association with MS and other similar conditions, it appears that one of the most vulnerable cranial nerves is the optic nerve, as well as the opthalmic artery that travels with it through the optic canal. In certain cases, shifts in the position of the brain may be the cause of compressive optic neuritis.
For additional information see cranial nerves.
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