RCEM Basic Science Curriculum
A competent knowledge of the anatomical basis of vision is important. Knowledge is expected of the key components of the visual axis and the role of each in the generation of normal vision.
- Retinal bipolar and ganglion cells
- Optic nerve
- Optic chiasm
- Optic tracts
- Optic radiation and geniculate bodies
- Visual cortex
- Blood supply of the optic tract, chiasm and nerves.
An appreciation of likely causes of disease or injury, the common clinical effects, and the anatomical rationale for these. In particular, knowledge of the effects of interruption of the optic nerve along its course from retina to radiation is required.
Visual Tract Revision
The optic nerve is formed by the convergence of axons from the retinal ganglion cells, which receive impulses from the photoreceptors of the eye. The optic nerve leaves the bony orbit via the optic canal, a passageway through the sphenoid bone. It enters the cranial cavity, running along the surface of the middle cranial fossa (in close proximity to the pituitary gland).
Within the middle cranial fossa, the optic nerves from each eye unite to form the optic chiasm. At the chiasm, fibres from the medial (nasal) half of each retina cross over, forming the optic tracts. Each optic tract travels to its corresponding cerebral hemisphere to reach its lateral geniculate nucleus (LGN) located in the thalamus where the fibres synapse.
Axons from the LGN then carry visual information via the optic radiation to the visual cortex in the occipital lobe; the upper optic radiation carries fibres from the superior retinal quadrants (corresponding to the inferior visual field quadrants) and travels through the parietal lobe to reach the visual cortex whereas the lower optic radiation carries fibres from the inferior retinal quadrants (corresponding to the superior visual field quadrants) and travels through the temporal lobe to reach the visual cortex of the occipital lobe.
- The visual cortex is supplied by the posterior cerebral artery.
- The optic radiations are supplied by the middle and posterior cerebral arteries.
- The LGN is supplied by the anterior and posterior choroidal arteries.
- The optic tract is supplied mainly by the anterior choroidal artery (branch of internal carotid).
- The intracranial portion of the optic nerve and optic chiasm is supplied by the internal carotid, anterior cerebral and anterior communicating arteries.
- The retinal and orbital portion of the optic nerve is supplied by branches of the ophthalmic artery (first branch of the internal carotid artery).
Visual Field Defects
Lesions peripheral to the optic chiasm (i.e. the optic nerve) result in loss of vision in the ipsilateral eye. For example:
- optic neuritis in multiple sclerosis or secondary to measles or mumps
- optic nerve compression secondary to orbital cellulitis, glaucoma or ocular tumours
- optic nerve toxicity secondary to ethambutol, methanol and ethylene glycol
- optic nerve injury secondary to orbital fracture or penetrating injury to the eye.
Lesions at the optic chiasm result in a bitemporal hemianopia. For example:
- pituitary tumour
- optic glioma
- aneurysm of the internal carotid artery.
Lesions central to the optic chiasma result in a homonymous hemianopia (or quadrantanopia). For example:
- cerebrovascular events
- brain abscess
- brain tumours.
CT head should be requested in trauma or to look for pituitary tumours, cerebrovascular events or space-occupying lesions. MRI scanning is preferable in optic neuritis as it is better able to demonstrate demyelination.
Visual Pathways and Visual Field Defects