Optic Neuritis, Papillitis, and Neuronal Retinopathy

THE word "neurone" in ancient Greece, in addition to signifying "nerve", also meant "a bow-string".The toughness of the optic nerve might be regarded as a vindication of this etymology.It is not uncommon to find the optic nerve reduced to a mere thread by a tumour or aneurysm at a time when vision is still intact.But the optic nerve is not, in fact, a nerve, but a tract of white matter.This, and the viability of this nerve, are preliminaries to any consideration of the optic neuropathies. The Optic NerveAs a tract of white matter of the central nervous system, the optic nerve is unique from the morphological point of view, in having its elements-the myelinated axons -separated into fascicles by abundant inward septa of pia mater.Within these septa are contained connective tissue elements (fibroblasts and collagen) and blood vessels.These vessels can be traced to originate from the arterial pial plexus sur- rounding the nerve.Collateral branches to the optic nerve from the internal carotid, ophthalmic, and central retinal arteries, and the recurrent pial branches of the circle of Zinn all contribute to the formation of the pial arterial plexus.Injection experi- ments indicate the very extensive nature of this anastomosis.Singh and Dass (1960a) found that these anastomoses were usually large enough to allow blood to flow from one of the contributing vessels to another.These authors also claim to have established, in a few specimens, an anastomosis between vessels within the substance of the nerve.In addition to the arterial pial plexus-reminiscent of the arterial vasocorona of the spinal cord-they recorded inconstant and variable branching of the central retinal artery within the optic nerve.As seen by the electron microscope, the pia mater enveloping the optic nerve is a basement membrane which, fused with the underlying glial membrane, forms the pia-glia.Vessels which come into contact with the pia-glia have, upon entering the subarachnoid space, acquired an arachnoid cuff.Branches of these vessels with their surrounding arachnoid "tubes" form the core of the numerous pia-glia septa of the optic nerve. Optic NeuritisIn the 19th century, during which the term was in wide use, its precise nosographic significance underwent many mutations, and its continued use may be questioned.Virologists widely agree that among the neurotropic viruses none has been identified as causing primary damage to the white matter.Infective agents are injurious to the