Examination and lesions of the olfactory nerve (CN I)

Even the ever-thorough Talley and O'Connor remark that "the first cranial nerve is not tested routinely". This is even more correct in the ICU setting. A brief informal survey of senior ICU staff revealed none who have ever performed olfactory nerve testing in the course of their practice.

Anatomy of the Olfactory Pathways

The olfactory nerve, around the area of the optic chiasm, divides into three striae (and thus this division is called the "trigone"). These are the anterior, medial and lateral striae. The lateral striae travels to the ipsilateral olfactory cortex in the uncus, and the anterior stria cross the anterior commissure to communicate with the contralateral olfactory cortex. Additionally, it appears the olfactory bulbs talk to each other via the medial striae, and a fair amount of neural processing seems to occur in the bulbs themselves.

Better images of the olfactory nerve fiber pathways are available from Cranial Nerves Illustrated.

The primary olfactory cortex occupies a portion of the insula, uncus and the parahipocampal gyrus.

 

Interruption of blood supply

The olfactory bulb is said to receive its blood supply from the branches of the anterior cerebral artery, whereas the primary olfactory cortices are irrigated by the lenticulostriate arteries arising from the first segment of the MCA, by the posterior temporal branches of the MCA, and possibly others. The areas of the brain involved in olfaction are actually quite widespread, which makes it difficult to use it in infarct localisation.

Generally speaking, an isolated unilateral stroke in the primary olfactory area will not yield any clinically significant olfactory symptoms - the contralateral area should still receive bilateral olfactory information. However, case reports demonstrate that the olfactory cortex shares some vascular real estate with the gustatory cortex, and abnormalities of taste perception may develop with isolated strokes of the insula.

Assessment of the Olfactory Pathways

The olfactory epithelium responds to subtle smells. The trigeminal nerve fibers which also traffic through this area will be irritated by noxious odours, but will not respond to the scent of a rose, or a nice hot chocolate.

Roses and hot chocolate are frequently a scarce commodity in the ICU setting. Cheap coffee, however, is abundant. The smell of coffee powder should be easy enough to identify, and it would make a sensible scent to test with, if one is ever in the situation when testing olfactory perception is for some reason crucial.

One assess olfaction by asking the patient to close their eyes, and then blocking one nostril and confronting the other nostril with the smelly substance. To merely waft it at the patient is not enough: the olfactory mucosa is a small (2.5cm²) area at the very apex of each nostril, and the patient actually needs to sniff the presented material in order to get the correct amount of stimulus.

If for whatever reason you develop the impression that your patient has some sort of functional (i.e. hysterical) anosmia, one can follow the cheap coffee test with the god-help-us-reeking ammonia test. The patient who also fails to smell the ammonia is probably in the grip of some sort of anxiety-associated conversion disorder; whereas the genuine anosmic will recoil from the stink.

Meaning of intact function

So, you can smell with both nostrils. What does this mean?

• The nose and cribriform plate are probably bilaterally intact
• The olfactory pathways are intact
• At least one hemisphere of the brain is intact.
• Higher function is intact (as the patient needs to actually recall the scent to identify it correctly)

Interpretation of lesions

Generally speaking, bilateral anosmia can be caused by any damn thing (usually something destructive and global, like meningitis or base of skull fracture), whereas unilateral anosmia must be localised above the olfactory trigone, and is therefore a problem of the single olfactory tract olfactory bulb or cribriform plate.

The sort of findings one might generate when examining the olfactory nerve are as follows:

• Bilateral anosmia: neither nostril can smell anything.
• Unilateral anosmia: one nostril is affected.
• Hypoosmia: the sense of smell is present but its sensitivity is decreased.
• Hyperosmia, "osmophobia": the perception of smells is pathologically exaggerated.
• Dysosmia: the sense of smell works, but is distorted.
• Olfactory hallucinations: "Eew! Can anybody else smell that? ... ...no?..."

Causes of Bilateral Anosmia

Both the nostrils are damaged or blocked Trauma Tumour Sinusitis/rhinitis Both the cribriform plates are destroyed Trauma Tumour Meningitis Both the olfactory bubs are destroyed Trauma Tumour Meningitis Neither of the olfactory bulbs had ever worked in the first place Congenital anosmia eg. Kallmann's syndrome The cellular sensory apparatus has been rendered insensitive Smoking Chemotherapy or radiotherapy

Causes of Unilateral Anosmia

One of the nostrils is damaged or blocked Trauma Tumour Sinusitis/rhinitis One of the cribriform plates are destroyed Trauma Tumour One of the olfactory bubs are destroyed Trauma Tumour - particularly, olfactory groove meningioma

Causes of Hypoosmia

Local processes Sinusitis/rhinitis Nasal polyps Global processes Smoking Chemotherapy Chronic industrial solvent exposure Characteristic associated conditions Vitamin A deficiency Pernicious anaemia NIDDM (where it is a part of the spectrum of neuropathy)

Causes of Hyperosmia

Characteristic associated conditions Addison's disease Cystic fibrosis Pituitary tumours

Causes of Dysosmia

Characteristic associated conditions Alzheimer's disease Korsakoff's psychosis Huntington's chorea Parkinson's disease.

Causes of Olfactory Hallucinations

Characteristic aetiologies Migraine Schizophrenia Temporal lobe epilepsy