1st Cranial nerve – Olfactory nerve Lesions


An olfactory lesion is a pathological process negatively affecting the function of the first cranial nerve. Olfactory lesions can be sub-divided into the following categories: a) anosmia: inability to appreciate qualitative olfactory sensations, b) partial anosmia: the ability to appreciate some but not all odorants, c) microsmia or hyposmia: reduced sensitivity to odorants d) hyperosmia: pathologically acute sense of smell, e) dysosmia, cacosmia, or parosmia: altered/distorted sense of smell, f) phantosmia: an olfactory hallucination, and g) olfactory agnosia: the inability to interpret an odorant.


Head trauma, toxic exposure, smoking history, sinonasal diseases, rhinoplasty, and upper respiratory tract infections should be considered. Certain medications such as antifungal agents and angiotensin-converting enzyme inhibitors have been known to influence olfaction. Conditions such as epilepsy, multiple sclerosis, Parkinson’s disease, and Alzheimer’s disease are also associated with olfactory nerve lesions.


The physical examination should cover the neurologic system and intranasal anatomy. Nasal endoscopy is often warranted, and can reveal polyps, membranous inflammation etc. The neurological examination should aim to exclude any sinister pathology and assess cerebral, cerebellar and cranial nerve function.

Radiographic studies should be reserved for specific indications such as fractures and intracranial lesions.

The most widely used quantitative clinical test is the University of Pennsylvania Smell Identification Test (UPSIT), which consists of four booklets containing 10 microencapsulated odours in a “scratch-and-sniff” format.