on Monday 12 April 2010
by Robin author list
in article > Neurology
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The optic nerve

Origin:
The optic nerve is composed of retinal ganglion cell axons and Portort cells which then forms the optic chiasm where there is a partial decussation of fibres from the temporal visual fields of both eyes. Most of the axons of the optic nerve terminate in the lateral geniculate nucleus from where information is relayed to the visual cortex in the occipital lobe, while other axons terminate in the pretectal nucleus.

Function:
It carries the optic impulses from the retina to the optic chiasma and through the optic tract to the lateral geniculate body, also the afferent  pathway for the pupillary light reflex from fibers from the superior colliculus of midbrain.

Testing:
Visual acuity:
Snellen's chart for distant vision and Jaegar cards for near vision. Snellen's chart is used at 6m away from the patient and the patient is asked to read out the alphabets using one eye covered at a time.

Jaegar cards are  kept 30 cm away from the patient and is asked to perform the test as above.

Visual field:
The periphery of visual fields are charted using confrontation method or using the perimetry method.

The patient and the examiner sits face to face  and both cover the opposite side eye to each other and the patient is asked to fix his vision on the examiner's pupil. The examiner is then uses his hand moving to four corners of the patient's vision to sketch the vision fields.

Common visual field defects due to neurological conditions encountered by PT:

  • Total unilateral loss of vision/with or without partial contralateral upper temporal field defect due to optic neuritis, optic nerve compression/ with  involvement of nasal crossing fibers.
  • Homonymous hemianopia due to lesion anywhere from optic tract to occipital cortex.
  • Upper quadrantic  homonymous defect due to temporal lobe lesions involving optic radiations.
  • Lower quadrantic homonymus defect due to lesions of upper radiations/calcarine area.
  • Bitemporal hemianopia due to leisons at the optic chiasma.

Bibliography
Spillane, John A. Bickerstaff's Neurological Examination In Clinical Practice, 6th Ed. India: Wiley, 2008