The CN I runs from the top of the nose through the cribriform plate of the ethmoid bone to the olfactory bulb. From there, the olfactory tract runs across the base of the skull past the optic chiasm to the cortex, amongst other sites [Figures 2a, 2b]. Some of the fibres cross near the optic chiasm.
The sense of smell should be investigated if the patient complains of loss of smell (anosmia) or taste or if, based on your history-taking or neurological examination, you consider (unilateral) anosmia to be likely.
Procedure
- Ask the patient to close their eyes and hold one nostril shut.
- Hold a number of items under the open nostril, one after the other, such as coffee, peppermint, petrol, soap etc.
- Keep asking the patient whether he / she can smell something and if so, what. It may be necessary to pose multiple-choice questions, as the patient may smell something but be unable to spontaneously recognise the presented smell, i.e. ‘Is this coffee or peppermint?’
- Then examine the other nerve.
Interpretation
As suggestion may play a role in anosmia, the patient should also be presented with ammonia as well as the above-mentioned substances. A patient who does not respond or claims not to notice anything, is probably simulating the condition as ammonia also stimulates the trigeminal nerve. Thus, with loss of function of the olfactory nerve the patient will not smell ammonia, but should feel it.
The most common cause of anosmia is rhinitis. Anosmia can persist for some time after the rhinitis has cleared up. Congenital anosmia is also seen. If you have ruled the above-mentioned causes of anosmia, a lesion in the anterior fossa is likely (tumour, fracture, inflammation).
Figure 2a
Figure 2b