Describe olfaction and the disorders of smell.

Physiology of Olfaction

• Olfaction is mediated by the olfactory epithelium, a small patch of specialised neuroepithelium (2 to 5 cm² each side) in the roof of the nose, the upper septum and the superior turbinate.
• It is the only place in the body where neurones are directly exposed to the external environment.
• The olfactory neurones regenerate throughout life from basal cells, which is unique in the nervous system and is why post-viral anosmia may recover over months — and why patients should not be told the loss is necessarily permanent

Disorders of Smell

• Terminology — anosmia (complete loss); hyposmia (reduced); parosmia (a distorted perception of a real odour, unpleasant); phantosmia (perception without a stimulus, a hallucination); and cacosmia (perception of a foul smell, often from a real local source)
• The causes divide into three mechanisms.
• 1. Conductive (transport) loss — odorant cannot reach the olfactory cleft: nasal polyps, chronic rhinosinusitis, allergic rhinitis, severe septal deviation, tumour, and mucosal oedema. These are the treatable causes, and the loss is often fully reversible
• 2. Sensorineural loss — damage to the epithelium or the nerve: post-viral (a leading cause, classically after influenza and prominently after COVID-19); head injury, in which the fibres are sheared as they cross the cribriform plate, after an occipital blow, and which is permanent; toxins and drugs; and ageing
• 3. Central — parkinson and alzheimer disease, in both of which anosmia may precede the motor or cognitive features by years and is an early marker; frontal lobe or olfactory groove meningioma, which may present with foster kennedy syndrome; temporal lobe epilepsy, in which an olfactory aura (uncinate fit) occurs; and multiple sclerosis
• Kallmann syndrome — congenital anosmia with hypogonadotrophic hypogonadism, from failed migration of the olfactory and GnRH neurones.
• Assessment — establish onset (sudden or gradual), preceding illness or injury, and unilateral or bilateral; a full nasal examination with endoscopy to see the olfactory cleft specifically; and formal smell testing (UPSIT, Sniffin’ Sticks).
• Imaging — CT for sinonasal disease, and MRI where a central cause is suspected, particularly with unilateral loss, phantosmia or neurological signs
• Management — treat the conductive causes.
• Olfactory training — repeated sniffing of four distinct odours (classically rose, eucalyptus, lemon and clove) twice daily for months — has reasonable evidence in post-viral and post-traumatic loss and is cheap, safe and the only intervention for sensorineural loss