16 Abnormalities of smell and taste

Since both the sensation of smell, olfaction, and taste, gustation, rely on chemical stimuli to excite their receptors, they are known as the chemosensory system (Smith and Shepherd 1999). Both of these senses are interdependent together providing the sensation of flavour of food and drink, but dysfunction of one may be misinterpreted as an abnormality of the other. Although loss of either sensation is rarely a major handicap, they are essential to detect noxious odours, such as smoke or gas, and to avoid spoiled food or potential poisons. Their loss could, therefore, have serious consequences. In addition, loss of smell or taste may indicate serious intracranial or systemic disease.

Odours, which must be volatile and soluble in water, are detected by specialized olfactory receptor cells in the olfactory epithelium, located in the mucous membrane of the upper and posterior parts of the nasal cavity, the superior turbinates and nasal septum, which measures 2–5 cm², and by the free nerve endings of the trigeminal nerve. The olfactory epithelium contains three cell types, the olfactory sensory neurons, approximately 6–10 million in each nasal cavity, the sustentacular or supporting cells which maintain the electrolyte concentration in the extracellular milieu, especially K⁺, and basal cells which are the source of new receptor and sustentacular cells, since the former have a life span of only 4–8 weeks (Fig. 16.1).

The olfactory receptor cell is a bipolar sensory neuron with a thin, single dendritic knob which extends into the mucous layer of the nasal cavity. The mucous layer contains immunoglobulins A and M, lactoferrin, lysoenzyme, and odorant-binding proteins. These molecules are thought to prevent the passage of noxious pathogens into the intracranial cavity via the olfactory nerve. From the knob protrude 10–30 non-motile cilia which bear the specific membrane receptor proteins and where signal transduction is initiated. When an odour binds to a receptor, there is activation of a membrane-bound GTP-dependent adenyl cyclase, a G protein, which then activates a second messenger leading to conformational changes in the transmembrane receptor and a series of intracellular events leading to the generation of axon potentials (Shepherd 1994; Smith and Shepherd 1999). Each olfactory sensory neuron expresses a single functional odorant receptor gene and those neurons with the same odour receptor are randomly dispersed within one olfactory epithelial area. Consistent with their ability to detect and discriminate diverse odorants, mammals have as many as 1000 different odour receptors that vary in protein sequence and are used combinatorially to detect different odours and encode their unique identities (Mombaerts 2004). Most mammals possess a second olfactory system—the accessory olfactory system or vomeronasal system. The mammalian vomeronasal organ is generally considered to specialize in pheromone detection. Very thin unmyelinated nerve axons leave the receptor cells and converge into small fascicles, enwrapped by Schwann cells, which pass through the cribriform plate of the ethmoid bone to the olfactory bulb. These axons collectively constitute the olfactory or first cranial nerve, and terminate within the olfactory glomeruli of the olfactory bulb. Here they form synaptic contacts with interneurons that have processes restricted to the bulb and with output neurons, the mitral and internal tufted cells, that contribute axons to the lateral olfactory tract. From the olfactory tract axons project to terminate in primitive cortical areas, known as the primary olfactory cortex. In humans, this probably includes small portions of the uncus, hippocampal gyrus, amygdaloid complex, and entorhinal cortex (Fig. 16.1).

Disturbances of olfaction can be grouped into four main subtypes:

In the clinical examination of olfactory function it is necessary to discriminate between deficits due to nasal obstruction, which prevent the access of volatile substances from reaching the olfactory epithelium, transport olfactory loss, and neurogenic loss which may be due to abnormalities of the receptors or their axons, sensory olfactory loss, or to pathological processes affecting the central pathways. Transport olfactory loss can result from a variety of causes, which include rhinitis, upper respiratory infection, polyps, sinusitis, and neoplasms. The symptoms of impaired olfactory detection, discrimination, or distortion of normal smells are no

different to those accompanying sensory olfactory loss, which may be due to impaired receptor cell turnover resulting from radiation or chemotherapeutic drugs, or damage to the olfactory axons due to closed head injury, toxic substances, and viral infection.

It is therefore essential that in addition to taking a full history and testing smell and taste, careful examination of the nose, mouth, and nasopharynx is undertaken. Examination of smell is usually carried out using a variety of familiar odiferous substances such as coffee, oil of peppermint, tobacco, oil of cloves, and vanilla. A bottle of each is held under each nostril with the other being occluded by a finger. The patient is asked whether or not he can detect an odour and if so whether or not it can be identified. If the odour can be detected even if it cannot be described, it may be assumed that the olfactory nerves are relatively intact. Malingering can be detected by using ammonia, which stimulates the trigeminal nerve. If the patient denies noticing the stimulus, the anosmia is likely to be bogus.

A more refined assessment of olfaction can be performed using the 40-item ‘scratch ’n sniff’ test developed and standardized by Doty and colleagues (1984), the University of Pennsylvania Smell Identification Test, or UPSIT. This test is highly reliable and allows the classification of patients into discrete categories of dysfunction.

These are reviewed by Doty (2003). The commonest cause of permanent olfactory loss appears to be a severe upper respiratory infection, usually viral in origin, in which the neuroepithelium is damaged (Deems et al. 1991). The second most common cause of olfactory dysfunction is head injury. The third commonest cause is hypertrophy and hyperaemia of the nasal passages, from whatever cause, which leads to hyposmia or anosmia, due to the inability of odours to reach the olfactory epithelium. Chronic rhinitis and sinusitis of allergic, infective, or vasomotor origin are frequent causes. Nutritional and endocrinological disorders, such as thiamine deficiency, adrenal insufficiency, vitamin A deficiency, cirrhosis, renal failure, hypothyroidism and Cushing’s syndrome may also have similar effects, due to sensorineural dysfunction (Doty et al. 1992). A frequent cause of hyposmia is heavy smoking. Infections due to influenza, herpes simplex, and hepatitis viruses can lead to hyposmia or anosmia due to destruction of the receptor cells, and recovery may not occur if the basal cells are also destroyed. There are several congenital diseases in which the receptor cells are absent or hypoplastic, including Kallman’s syndrome of anosmia and hypogonadotrophic hypogonadism, Turner’s syndrome, and albinism.

Loss of smell in head trauma is usually due to the severing of the delicate axons of the receptor cells. The incidence of smell dysfunction following head trauma is 7–15 per cent and is proportional to the severity of the injury. Anosmia or microsmia may be unilateral or bilateral. The loss is due to shearing of the olfactory filaments as they pass through the cribriform plate, although contusions to the frontal and temporal poles can also be present (Doty et al. 1997). Recovery of smell occurs in about a third of cases but is unlikely to occur if the loss of smell has been present for more than one year after injury (Sumner 1967).

The olfactory epithelium can be damaged by a variety of toxic agents including organic solvents such as benzene, and drugs such as antimicrobial agents (ampicillin, griseofulvin, streptomycin, tetracyclines), anti-inflammatory agents (allopurinol colchicine, gold, D-penicillamine, phenylbutazone), antiproliferative agents (methotrexate, vincristine, doxorubicin), and other drugs including phenindione, amphetamines, cocaine, and corticosteroids.

Olfaction shows a gradual deterioration with age with greater and earlier loss occurring in men than in women. Approximately half of the population between the ages of 8 to 85-years have meaningful olfactory loss (Hoffman et al. 1998). Impaired odour detection and/or discrimination may be one of the first signs of a number of neurological disorders including Alzheimer’s disease, idiopathic Parkinson’s disease, and schizophrenia (Mesholam et al. 1998; Doty 2003). In Parkinson’s disease the prevalence of olfactory impairment is higher than some of the cardinal neurological signs. Several investigators have shown that scores on a simple three-item microencapsulated odour test differentiate better between Alzheimer’s disease and depression than scores on the Mini-Mental State Examination (McCaffrey et al. 2000). Alcoholics with Korsakoff’s psychosis have a defect of odour discrimination, as have some patients with temporal lobe epilepsy. A similar deficit is found in patients in whom anterior temporal lobe or orbitofrontal cortical excision has been performed. Anosmia may be the first symptom of an olfactory groove meningioma which may involve the olfactory bulb and tract, and extend posteriorly to involve the optic nerve leading to atrophy, the Foster Kennedy syndrome.

There is no specific treatment for patients with hyposmia or anosmia, unless there is a local or systemic remediable cause. In some patients a brief course of systemic steroid therapy—definitely not longer-term—can help to distinguish between conductive and sensorineural olfactory loss, with the former responding to treatment. It is important to realize that many patients with taste and smell disorders experience considerable depression which requires treatment. However, these patients are at potential risk from inhaling noxious fumes and failing to detect burning, so it is important to advise them of the necessary precautions. These should include the use of domestic smoke and gas detectors, and the provision of adequate ventilation in enclosed areas in which toxic solvents are being used.

The commonest cause of parosmia, or dysosmia, the distortion of normal smell, is a local nasopharyngeal condition such as sinusitis. Other causes include temporal lobe seizure, partial injuries of the olfactory bulb, and depression. The majority of patients have associated hyposmia or anosmia.

Olfactory hallucinations are always of central origin, and are most often due to temporal lobe seizures known as uncinate seizures. Other causes include Alzheimer’s disease, endogenous depression, schizophrenia, and alcohol withdrawal (Pryse-Phillips 1975).

The tongue can identify a wide array of tastes that can be classified as sweet, bitter, salty, or sour, with umami being increasingly recognized as a fifth taste modality. Disturbances of taste are far less frequent than disorders of smell, and frequently patients with lack or loss of taste turn out to have impaired olfaction with normal taste sensation. Several disorders of taste are recognized: ageusia or loss of taste, hypogeusia or diminished sensitivity, dysgeusia or parageusia which are distortions of normal taste, and finally gustatory hallucinations.

The peripheral receptors for taste—the taste buds—are mainly located on the surface of the tongue and in smaller numbers over the soft palate, the pharynx, larynx, and oesophagus (Fig. 16.2). Each taste bud consists of about 200 vertically orientated receptor cells, such that the superficial portion of the bud is marked by an excavation, the taste pit or pore, into which the microvilli of the receptor cells project. Receptor cells have a limited lifespan of about 10 days and undergo constant replacement from adjacent basal epithelial cells. Fine unmyelinated sensory fibres pass up through the base of the bud to innervate the receptor cells, which have no axons. Taste afferent fibres from the anterior two-thirds of the tongue course through the lingual nerve, a branch of the trigeminal nerve, which they leave via the corda tympani to join the facial nerve, the nervus intermedia portion, with their cell bodies in the geniculate ganglion. The posterior third of the tongue and pharynx, are supplied by the glossopharyngeal nerve, with cell bodies in the nodose ganglion. Afferents from taste buds on the palate travel with the superficial petrosal nerve, and from the larynx and oesophagus via the vagal afferents. Gustatory fibres from these nerves project to the ipsilateral solitary tract, from which they project via the gustatory lemniscus to the thalamus and then to the postrolandic sensory cortex.

The tongue taste receptors respond to chemical substances in solution, the four primary taste sensations being salty, bitter, sweet, and sour. More complex taste sensations are derived from combinations of these four basic tastes and from olfaction (Smith and Shepherd 1999). For a full description of the molecular basis for taste see reviews (Breslin and Huang 2006; Small 2006).

Patients presenting with a disturbance of taste should be asked about any associated disorder of smell, and any associated medical conditions or medications (Doty et al. 1992). In addition to testing smell and taste it is essential to carefully examine the oral cavity, noting any evidence of infection, masses, and atrophy and dryness of the tongue, gums, and dentition.

Taste can then be assessed using standard solutions of sugar, sodium chloride, acetic acid, and quinine. Electrical stimulation of the tongue, electrogustometry, can also be used as a sour stimulus by simply applying a low-voltage direct current (Stillman et al. 2003). If the taste loss is bilateral the solutions can be swished around the mouth and the patient asked to identify the taste. The solution is then spat out and the mouth rinsed with water before the next solution is tried. If the taste loss is unilateral or focal the tongue is protruded and gently held with a piece of gauze. Crystals of salt or sugar are then placed on the tongue and the patient asked to identify the taste.

Disturbances of taste are due either to local causes involving the tongue and/or taste buds or to damage to the peripheral or central neural pathways. The commonest associated cause of hypogeusia is an upper respiratory tract infection and smoking (Henkin et al. 1975). Deficiency of saliva as in Sjogren’s syndrome, or hyperviscosity as in cystic fibrosis, pandysautonomia or post irradiation of the head and neck results in dryness of the mouth, xerostomia, this leads to disturbed taste, because taste stimuli are only effective in a fluid medium. There may also be an accompanying reduction in the number of papillae and taste buds, which may possibly be due either to the loss of the lubricating effect of saliva or of trophic factors contained within it. Unfortunately, artificial saliva or regular water mouthwashes do not appear to restore normal taste in patients with xerostomia. Other causes of ageusia or hypogeusia include scleroderma, hypothyroidism, adrenocortical insufficiency, Cushing’s syndrome, diabetes mellitus, chronic renal failure, liver cirrhosis, niacin or vitamin B₆ deficiency, zinc deficiency, and neoplasia of the oral cavity and base of skull. Reduced or distorted function of taste not infrequently occurs following influenza-like infections. A unilateral loss of taste is often found in cases of Bell’s palsy.

Post-traumatic ageusia is far less common than post-traumatic anosmia, occurring in less than 1 per cent of serious head injuries (Sumner 1967). However, it always occurs in association with anosmia, and often the ageusia resolves within a few weeks. The cause for such ageusia is unclear. Although bilateral lesions near the frontal operculum and paralimbic areas would result in both ageusia and anosmia, this would not explain the frequent recovery of ageusia in advance of the anosmia. It is likely that many cases of ageusia are in fact mislabelled cases of anosmia.

Gustatory hallucinations occur much less frequently than olfactory ones in association with epileptic seizures. Such an aura, which may represent a primary taste such as sweet, bitter, or a peculiar and rotten one, usually occurs in a seizure originating from the frontoparietal or suprasylvian cortex or the uncal region (Hausser-Hauw and Bancaud 1987).

Management of taste disorders includes the use of various salivary substitutes for xerostomia, the correction of any nutritional deficiency, and in some cases flavour enhancers.