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.