Clinic
If an optic nerve is completely destroyed, the affected eye is blind. However, if only part of the fibers are destroyed, for example in the optic chiasm, i.e. the crossing of the fibers of the right and left eye, the patient suffers from heteronymous hemianopsia. This means that the nasal fibers of both eyes fall out, which leads to a restriction of the visual field of both eyes on the temporal side (temporal portion).
A contralateral hemianopsy is performed when an optic tract is affected. In this case, the temporal parts of the affected side and the nasal parts of the opposite side are no longer functional. Furthermore, the optic nerve may be inflamed (neuritis nervi optici).
This results in an increasing loss of visual acuity (loss of visual acuity) and possibly a scotoma (selective loss of visual field). The cause of such an inflammation is usually demyelinating diseases.Multiple sclerosis in particular can manifest itself with neuritis nervi optici. Due to the inability of the optic nerve to regenerate, a restoration of vision is very unlikely.
Diagnostics
The optic nerve papilla, i.e. the point where the optic nerve exits the eyeball, can be viewed directly by an ophthalmologist using an eye mirror. Edema in this area indicates severe damage to the nerve and the threat of blindness. The determination of the visual field (perimetry) is often used to differentiate other diseases at different points of the visual path.
In this way, failures of the visual field, such as nasal failures, can be detected in both eyes and thus damage to the crossed fibers in the optic chiasm can be diagnosed. With the help of visually evoked potentials (VEP), the nerve conduction velocity of the optic nerve can be determined. Ultrasound (sonography), magnetic resonance imaging (MRI) and computed tomography (CT) are used to image the nerve and its course.
Summary
The optic nerve is the second cranial nerve and, historically, does not belong to the peripheral nerves like almost all other cranial nerves, but directly to the brain. It is composed of millions of small nerve fibers in the retina, from where it runs to the visual cortex in the brain. On its way through the eye socket, the sphenoid bone and the subarachnoid space into the brain, it is surrounded by a myelin layer and the three meninges.
In the brain, the nasal nerve fibers of both eyes cross each other, and then run further in the brain as the optic tractus. After passing through the Corpus geniculatum laterale, the nerve fibers end in the primary visual cortex (Area 17) at the back of the head (occipital pole). The further processing of information then takes place in the secondary visual cortex (Area 18) and the other higher visual cortex areas.
On its way, the optic nerve can be damaged in many places by bleeding, tumors or other diseases. Since the optic nerve is not able to regenerate, recovery of vision is often unlikely. Diagnosis of optic nerve diseases is done by visual field determination, direct assessment of the optic nerve papilla at the exit point by means of an eye mirror or by imaging. The nerve conduction velocity can be measured using the visually evoked potentials.
