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.

Intensive Care Idiosyncrasies

  • Testing requires a conscious cooperative patient
  • Bilaterally unobstructed nostrils and a patent airway are required (to sniff with)
  • Higher cognitive functions need to be intact (to report on the character of the smell)


  • The patient's eyes must be closed
  • Each nostril is tested individually; the other nostril is blocked
  • Cheap coffee is presented to the open nostril, and the patient is instructed to sniff it
  • The patient needs to report whether they recognise the smell.

Essential points:

Each hemisphere receives input from both the olfactory bulbs.

Thus, hemispheric damage cannot cause unilateral anosmia.

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.

Olfactory nerve pathways

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

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

Causes of Dysosmia

Characteristic associated conditions

Causes of Olfactory Hallucinations

Characteristic aetiologies


A splendid resource for digital artwork exists at Cranial Nerves Illustrated, courtesy of the University of Toronto. The authors of the book and images - L. Wilson-Pauwels, P. Stewart, E.J. Akesson and S.D. Spacey - have made the images available for free, provided they are exploited as a teaching resource.

The best textbook for an intimately detailed clinical examination of the cranial nerves is Walker, Hall and Hurst's "Clinical Methods: The History, Physical, and Laboratory Examinations" 3rd edition. It is all the more amazing for its free online availability.

The most detailed discussion for the causes of disorders of taste and smell comes from Chapter 20 (Pasquale and Mair), cited in Bradley's "Neurology in Clinical Practice: Principles of Diagnosis and Management" The 2012 2-volume 2nd edition will set you back $519.

Favre, J. J., et al. "Blood supply of the olfactory nerve." Surgical and Radiologic Anatomy 17.2 (1995): 133-138.

Cereda, C., et al. "Strokes restricted to the insular cortex." Neurology 59.12 (2002): 1950-1955.

Price, Joseph L. "Beyond the primary olfactory cortex: olfactory-related areas in the neocortex, thalamus and hypothalamus." Chemical Senses 10.2 (1985): 239-258.

Zusho, Hiroyuki. "Posttraumatic anosmia." Archives of Otolaryngology 108.2 (1982): 90-92.

Lieblich, Jeffrey M., et al. "Syndrome of anosmia with hypogonadotropic hypogonadism (Kallmann syndrome): clinical and laboratory studies in 23 cases." The American journal of medicine 73.4 (1982): 506-519.

Kuo, Yu-Heng, et al. "Sertraline alleviated osmophobia caused by partial hypopituitarism with isolated ACTH deficiency." General hospital psychiatry35.5 (2013): 574-e9.

Amoore, John E. "Olfactory genetics and anosmia." Olfaction. Springer Berlin Heidelberg, 1971. 245-256.

Schurr, P. H. "Aberrations of the sense of smell in head injury and cerebral tumours." Proceedings of the Royal Society of Medicine 68.8 (1975): 470.

Sherman, A. H., J. E. Amoore, and V. Weigel. "The pyridine scale for clinical measurement of olfactory threshold: a quantitative reevaluation."Otolaryngology and head and neck surgery 87.6 (1978): 717-733.

Royall, Donald R., et al. "Severe dysosmia is specifically associated with Alzheimer-like memory deficits in nondemented elderly retirees."Neuroepidemiology 21.2 (2002): 68-73.

Jones, Barbara P., Howard R. Moskowitz, and Nelson Butters. "Olfactory discrimination in alcoholic Korsakoff patients." Neuropsychologia 13.2 (1975): 173-179.

Moberg, Paul J., et al. "Olfactory recognition: Differential impairments in early and late Huntington's and Alzheimer's diseases." Journal of clinical and experimental neuropsychology 9.6 (1987): 650-664.

PENG, Rong, Jin-hong ZHANG, and Yan WU. "RESEARCH ON DYSOSMIA IN PATIENTS WITH PARKINSON'S DISEASE." Modern Preventive Medicine 1 (2007): 087.

Wolberg, Faith L., and Dewey K. Ziegler. "Olfactory hallucination in migraine."Archives of neurology 39.6 (1982): 382-382.

Kopala, L. C., K. P. Good, and W. G. Honer. "Olfactory hallucinations and olfactory identification ability in patients with schizophrenia and other psychiatric disorders." Schizophrenia research 12.3 (1994): 205-211.

Gloor, Pierre, et al. "The role of the limbic system in experiential phenomena of temporal lobe epilepsy." Annals of neurology 12.2 (1982): 129-144.