4.5 Anosmia and Other Olfactory Disorders
Key Features
Olfactory sensory neurons situated in the nasal cavity constitute the first cranial nerve (CN I) and relay olfactory stimuli to the olfactory bulbs of the brain.
Anosmia, or olfactory loss, can be considered to be conductive (secondary to a process causing nasal obstruction), sensorineural (secondary to a process affecting olfactory neurons or central pathways), or mixed.
The most common causes of olfactory sensory loss are rhinosinusitis, head trauma, or postviral anosmia. Presbyosmia, or age-related decline in olfactory function, is well documented in patients over age 65. Less commonly, effects of drugs or systemic illness have been reported to impact olfactory function adversely.
Anosmia is an absence of olfactory function. Hyposmia (or microsmia) is a decrease in olfactory function. Parosmia is an altered olfactory perception in the presence of stimulus, usually considered foul. Phantosmia is an olfactory perception in the absence of stimulus.
Epidemiology
Approximately 2 to 3 million Americans suffer from chemosensory dysfunction. Olfactory loss is present in 1% of those under age 60 but in > 50% of those over 60 years of age. Presbyosmia is present in most of those > 80 years of age.
Clinical
Signs
In cases of conductive anosmia, nasal endoscopy will usually reveal evidence of obstructive disease, such as mucosal edema, inflammation, mucopus, nasal polyps, or other intranasal mass. In sensorineural anosmia, there may be no obvious exam findings. However, intracranial lesions often cause additional neurologic defects such as altered mental status, urinary incontinence, or seizures. Neoplasms such as an olfactory groove meningioma may cause Foster-Kennedy′s syndrome (ipsilateral anosmia, optic atrophy and central scotoma, contralateral papilledema). Objective olfactory testing will reveal the sensory loss.
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