Injury to any part of the optic pathway produces visual defects whose nature depends on the location and extent of the injury. Visual defects are said to be homonymous when restricted to single visual field, right or left and heteronymous when parts of both visual fields are involved. The lesions can be localized because of the retinotopic organization of each nucleus area or tract in the visual system.

Examples of the lesions include:

• Destruction of one eye or cutting of one optic nerve produces blindness in that eye. Because of the extensive overlap of visual field representation in the two retinae, loss of one eye eliminates only part of the temporal visual field on the side of the lesion.
  
  
• Defects at the optic chiasma can produce opposite effects.
  
  
  • Cutting of the crossing fibres will produce a loss of both the temporal visual fields (nasal retinal fields) in both eyes. This is called bitemporal hemianopia. Such a condition is relatively common and arises from tumors of the pituitary pressing upon, the chiasma.
    
  • If the lateral surface of the chiasma is pressed upon, uncrossed axons will be damaged and loss of the nasal visual field in one eye will occur. This relatively rare condition may be due to aneurysms of one common carotid artery.

• Fractures of the cribriform plate of the ethmoid bone or haemorrhage at the base of the frontal lobes may cause tearing of the olfactory filaments, impairing olfactory sense.
  
  
• The olfactory nerve may be involved as a consequence of meningitis or abscess of the frontal lobe.
  
  
• Unilateral anosmia may be of diagnostic significance in localizing intra-cranial neoplasms, especially meningiomas of the sphenoidal ridge or olfactory groove.
  
  
• Olfactory "hallucinations" frequently are a consequence of lesions involving or irritating the parahippocampal gyrus, the uncus, or adjoining areas. The olfactory sensations, which these patients experience usually, are described as disagreeable in character and may precede a generalized convulsion. Such seizures are referred to as uncinate fits.
  
  
• Changes in olfactory epithelium may herald Alzheimer's disease.

Clinical considerations: Vestibular system.

Vertigo

Refers specifically to the sensation of turning or rotating, as if the external word were rotating around the individual, or as if the individual were in space.

Vestibular neuritis

This is an acute inflammation of the vestibular labyrinth.

Meniere's disease

This is a disorder of unknown aetiology, characterized by a progressive loss of or fluctuation in hearing, and tinnitus, and, later, by nystagmus, vertigo and nausea.

Applied anatomy: Cochlear nerve

Sensorineural deafness may result from pathology of

a)  the receptor organ – the cochlea
b)  the cochlea nerve or
c)  less often, central lesions involving the auditory pathways.

Involvement of these structures produces an ipslateral hearing loss.  If cochlear damage is restricted, the hearing loss may involve only the specific frequencies associated with the affected part.