For detailed description of cranial nerves see the author’s TEXTBOOK OF ANATOMY. Here we will consider how these nerves are tested during clinical examination, and some important features arising from damage to these nerves.
The olfactory nerve is tested by asking the patient to recognize various odours. The right and left nerves can be tested separately by closing one nostril and putting the substance near the open nostril.
To test the optic nerve first ask the patient if his vision is normal. Acuity (sharpness) of vision can be tested by making the patient read letters of various sizes printed on a chart from a fixed distance. It must of course be remembered that loss of acuity of vision can be caused by errors of refraction, or by the presence of opacities in the cornea or the lens (cataract).
As part of a normal clinical examination the field of vision can be tested as follows. Ask the patient to sit opposite you (about half a meter away) and look straight forwards at you. As one eye to be tested at a time ask the patient to place a hand on one eye so that he can see only with the other eye. Stretch out one of your arms laterally so that your hand is about equal distance from your face and that of the patient. In this position you will probably not be able to see your hand. However, you may notice it if you move a finger. Keep moving your index finger and gradually bring the hand towards yourself until you can just see the movements of the finger. This gives you an idea of the extent of your own visual field in that direction. By asking the patient to tell you as soon as he can see the moving finger (making sure that he does not turn his head) you can get an idea of the patients field of vision in the direction of your hand. By repeating the test placing your hand in different directions a good idea of the field of vision of the patient can be obtained. If an abnormality is suspected detailed testing can be done using a procedure called perimetry.
If there is any doubt about the integrity of optic nerve the retina is examined using an ophthalmoscope. With this instrument we can see the interior of the eye through the pupil of the eye. The optic disc and blood vessels radiating from it can be seen.
Injuries to different parts of the visual pathway can produce various kinds of defects. Loss of vision in one half (right or left) of the visual field is called hemianopia. If the same half of the visual field is lost in both eyes the defect is said to be homonymous and if different halves are lost the defect is said to be heteronymous. Note that the hemianopia is named in relation to the visual field and not to the retina.
Injury to the optic nerve will obviously produce total blindness in the eye concerned. Damage to the central part of the optic chiasma (e.g., by pressure from an enlarged hypophysis) interrupts the crossing fibres derived from the nasal halves of the two retinae resulting in bitemporal heteronymous hemianopia. It has been claimed that macular fibres are more susceptible to damage by pressure than peripheral fibres and are affected first. When the lateral part of the chiasma is affected a nasal hemianopia results. This may be unilateral or bilateral. Complete destruction of the optic tract, the lateral geniculate body, the optic radiation or the visual cortex of one side, results in loss of the opposite half of the field of vision. A lesion on the right side leads to left homonymous hemianopia. Lesions anterior to the lateral geniculate body also interrupt fibres responsible for the pupillary light reflex .
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