25th June 2023

CN1 – Olfactory nerve

The olfactory nerve arises from the olfactory epithelium, passing through the cribriform plate of the ethmoid bone and running through the olfactory bulb to terminate in the primary olfactory cortex. It carries afferent impulses for the sense of smell and is commonly affected by trauma and infection.

Problems with olfaction can also be a sentinel sign of neurodegenerative disorders which affect the olfactory bulb. Hyposmia is recognised as an early ‘pre-clinical’ sign of Parkinson’s disease, present in 75% of patients with idiopathic Parkinson’s disease, higher than the prevalence of resting tremor. Other α-synuclein and tau-related disorders, e.g. Alzheimer’s dementia, also have associations with olfactory dysfunction.

Kallmann syndrome

Kallmann syndrome is a rare form of hypogonadotropic hypogonadism. It is associated with anosmia or hyposmia as a result of failure of production or activity of gonadotropin-releasing hormone (GnRH) by the hypothalamus. This defect in GnRH producing neurons is also associated with the loss of olfactory neurons, resulting in the syndrome of anosmia and hypogonadism, failure to achieve puberty and cranio-facial defects.