Possibly owing to the fact that, historically, most disorders of smell function have been difficult to diagnose and treat, physicians often downplay this sense in the routine neurological examination. This is unfortunate when one considers that olfactory disorders are relatively common and profoundly effect a patient's quality of life. Along with its sister sense of taste (see Chapter 42), olfaction determines, among other things, the flavour of foods and beverages, and provides an early warning system for detecting leaking natural gas, spoiled food, fire and other adverse environmental situations. Importantly, olfactory disturbances can be an early sign of such serious diseases or anomalies as Alzheimer's disease, idiopathic Parkinson's disease, epilepsy, multiple sclerosis, and schizophrenia. Although some patients initially present with a frank complaint of a smell disturbance, others are unaware of their dysfunction, pointing out the need for routine quantitative olfactory assessment, which is now easily performed in the office.

In this chapter, we (a) summarize key aspects of olfactory anatomy and physiology, (b) present up-to-date practical techniques for themanagementandquantitative evaluation of the olfactory system, and (c) describe basic olfactory disorders commonly encountered in the neurological setting.

Anatomy and physiology

Olfactory neuroepithelium: a portal to the central nervous system

The olfactory receptors are located within a ∼ 2 cm2 neuroepithelium lining the cribriform plate and regions of the superior turbinate, middle turbinate, and septum. The neurologist should be aware of the fact that, in addition to the main olfactory system (CN I), other specialized neural systems are present in the nose. These include (a) trigeminal (CN V) afferents responsible, for example, for the coolness of menthol vapours (Doty, 1995a), (b) a rudimentary and non-functional vomeronasal organ (VNO) near the base of the septum (Bhatnagar & Meisami, 1998; Smith & Bhatnagar, 2000), and (c) the poorly understood nervus terminalis or terminal nerve (CN O). CN O, a highly conserved neural plexus that ramifies throughout the nasal epithelium, is distinguished by ganglia at nodal points and a high gonadotropin content, and presumably plays no role in human odour perception (Schwanzel-Fukuda & Pfaff, 1995).