Olfactory nerve: from ugly duckling to swan

The olfactory nerve has never been the shining star of neurological examination. Quite the contrary, examining the first cranial nerve is often an overlooked step. As cases of anosmia secondary to COVID-19 infection continue to rise, the 2020 pandemic has shed new light on this much-forgotten nerve, its value as an aid to diagnosis of several diseases and its central role in our daily lives.

Objective:

We aimed to emphasize how essential and simple clinical examination of the olfactory system can be by highlighting practical techniques and clinical tips for its assessment. We also share pearls and pitfalls in localization and differential diagnosis, which may prove valuable to busy clinicians.

Methods:

A broad review of the literature was conducted by searching PubMed, Cochrane and Google Scholar for articles and books containing topics regarding examination of the olfactory nerve and its anatomy, physiology and pathology. No particular inclusion or exclusion criteria were used.

Results:

Forty different works were found, between books and articles, from which 20 were selected after careful analysis.

Conclusions:

Despite the tragedy and adversity that followed the COVID-19 pandemic, its legacy has taught us a crystal-clear lesson: olfaction should no longer be neglected in clinical practice.

Keywords:
Clinical Neurology; Smell; Infectious Disease; Olfactory Nerve

Loss or reduction of smelling capacity	Increase of smelling capacity
Hyposmia: increased threshold for detecting odors.	Hyperosmia: diminished threshold for detecting odors.
Anosmia: complete loss of odor perception.
Microsmia: decreased olfactory spectrum (differentiation of distinct odors).
e.g. allergic rhinitis, neuronal damage after upper respiratory tract infection and olfactory groove meningioma.	e.g. migraine, anxiety and pregnancy.

Distortion of illusion of smell	Olfactory hallucinations	Olfactory agnosia
Parosmia or dysosmia: distortion of the perception of an odor. Cacosmia: perception of unpleasant odors (coprosmia, when it is the odor of feces).	Phantosmia: smelling odors that are not there.	Incapacity to discriminate odors, with preservation of perceptual aspects of primary smell.
e.g. temporal lobe epilepsy and smell recovery after neuronal damage of any kind.	e.g. temporal lobe epilepsy, psychiatric disorders and during smell recovery after viral etiology.	e.g. cortical lesions and Korsakoff syndrome.

Key points during neurological examinations
1) Check that nostrils are unobstructed and that the airflow is normal.
2) Present different odors to the patient, while closing one nostril at a time with your finger on the ipsilateral upper lateral cartilage. Ask for a quick and deep inhalation and then for identification of the odors.
3) Preferably, formally validated psychophysical tests should be performed (for example: UPSIT test).
4) Avoid using irritating substances like alcohol and ammonia.
5) Do not forget to examine all cranial nerves, with particular attention to the facial nerve (especially the gustatory portion), trigeminal nerve and optic nerve (fundoscopy should always be performed).
6) During the examination, search for signs of neurological or systemic diseases that may be commonly associated with olfactory loss (e.g. cleft lip in Kallmann syndrome, nystagmus due to phenytoin use, rigidity and tremor from Parkinson’s disease, muscular atrophy from amyotrophic lateral sclerosis or motor impersistence from Huntington’s disease).

Clinical pearls for olfactory investigation
1) Lesions in any segment of the olfactory nerve usually affect the perception of more than a single stimulus, because of the proximity of the olfactory pathways.
2) The most reliable tests are those consisting of stimulus presentation that involves odor identification. These tests are therefore preferrable for clinical practice.
3) It is indispensable to test each nostril individually, in order to detect potential unilateral problems. It is essential to point out that patients with unilateral problems usually do not complain about olfactory loss.
4) Complaints of hyposmia or anosmia indicate a bilateral lesion.

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E-mail: revista.arquivos@abneuro.org

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[1] Correspondence:
Abelardo Queiroz Campos Araujo; Email: abelardo@ufrj.br.