Chapter 93

Smell and Taste Disturbances

Margaret Mahoney, Catherine M. Franklin, Patricia A. Reidy, Emily Kawacki Sheff, Daniel E. Kane, Elissa Ladd, Patrice K. Nicholas, Jason R. Lucey

Definition and Epidemiology

Disorders of smell and taste can be seriously debilitating to patients and are often diagnostic dilemmas for health care providers. Olfactory dysfunction can be described as loss of the sense of smell (anosmia), smell distortion (parosmia), or diminished sense of smell (hyposmia).¹^,² These dysfunctions can result from aging, tobacco, toxins, medications, malignant neoplasms, nasal inflammation, infection, malnutrition, head or facial trauma, Parkinson disease, Alzheimer disease, multiple sclerosis, diabetes, or inflammatory autoimmune conditions.¹^–¹⁰

Taste disorders include diminished taste (hypogeusia), unpleasant taste (aliageusia), and any persistent taste (dysgeusia). Ageusia, or absent taste, does occur but is less common.¹ Taste disorders are often caused by conditions similar to those causing smell disorders but can also be associated with endocrinologic dysfunction, anesthesia, malignant neoplasms, head and neck irradiation, surgical procedures, iatrogenic causes, kidney or gastric dysfunction, metabolic or hepatic disorders, environmental exposure, substance abuse, or psychiatric disorders.⁸^,¹⁰^–¹²

Pathophysiology

During the process of smelling, odorant molecules are taken in through the nose; these molecules must pass through the nasal cavity to reach the cribriform area and become soluble in the mucus that lies over the dendrites of the olfactory receptor cells.¹³^,¹⁴ The inability of odorant molecules to reach the receptor cells of the olfactory nerve (cranial nerve [CN] I) is the most common cause of olfactory dysfunction. Therefore, anosmia or hyposmia can be caused by any disease process that prevents the odorant molecules from reaching these receptor cells. The dysfunction may occur in reception or transmission. Barriers to reception may include physical barriers—polyps, septal deformities, rhinitis, and nasal tumors—or epithelial cell changes that occur with aging, trauma, and chemical or iatrogenic exposure.³ Transmission may be affected by neurodegenerative diseases.²^,⁶^,⁸

Similar to the impact on smell of damaged nasal mucosa receptors, damaged taste receptors (buds) can impair taste. This is especially prevalent with aging, which results in the loss of taste buds.¹^,¹⁰ Other conditions that can impair taste include heavy smoking, viruses, chemical exposure, environmental exposure, iatrogenic exposure, and radiotherapy of the head and neck.¹^,¹⁵ Ageusia also may result from disease of the chorda tympani or the gustatory fibers but is rare. Lesions involving sensory pathways to the taste centers of the brain, or diseases of the taste centers of the brain itself, can also interfere with the sense of taste.⁸^,¹¹^,¹⁴

Clinical Presentation

Problems with taste and smell may or may not be associated with symptoms related to disorders that cause ageusia and anosmia. Most often, the presenting complaint is loss of taste or smell after an upper respiratory tract infection. In young adults, the loss of smell often results from head trauma. If a patient has lost or experienced a decreased sense of smell, a thorough evaluation for intranasal and intracranial disease is required. A complete medical, social (including substance abuse and occupation), and medication history is essential to diagnosis. Onset of symptoms (gradual versus acute) and associated symptoms should also be determined.⁵

Physical Examination

The examination should confirm the patient’s subjective complaint. Assessment for the loss of taste and smell focuses on the CNs that provide information about taste and smell. The olfactory nerve (CN I) is a sensory nerve. Testing of this nerve begins with asking the patient to identify odors that are nonirritating and aromatic, such as coffee, isopropyl alcohol, and toothpaste. After testing of CN I, the provider should inspect the nasopharynx for abnormalities (e.g., polyps), crusting, amount of mucus present, and any signs of upper respiratory tract problems. The pharyngeal examination should determine the presence of lesions, inflammation, or exudate.⁵

The glossopharyngeal nerve (CN IX) is a mixed sensory-motor nerve. The sensory portion controls the taste sensation for the posterior third of the tongue. CN IX is tested along with the facial nerve (CN VII), which is also a mixed sensory-motor nerve. The sensory part of CN VII controls taste sensation for the anterior two thirds of the tongue. Each side of the tongue should be tested with sweet, salty, sour, and bitter flavors. The patient should protrude the tongue while identifying the taste and rinse the mouth before testing the other side. This process should be repeated with the posterior portion of the tongue.⁸^,¹²^,¹⁵

After determining whether the complaint is related to olfactory or taste dysfunction, the provider should perform a more comprehensive examination, including weight, vital signs, and a conscientious ear, nose, throat, and neurologic evaluation.

Diagnostics

Assessment of odor and taste identification is an essential component of diagnostic testing for the loss of taste and smell. Two tests that are appropriate for the primary care office are the University of Pennsylvania Smell Identification Test and the Sniffin’ Sticks.⁵^,¹⁶ If these tests are unavailable, the patient should be referred to a specialist for specific assessment of smell and taste dysfunction. Laboratory testing should include a complete blood count (CBC), electrolytes, blood urea nitrogen (BUN), creatinine, liver function tests (LFTs), thyroid-stimulating hormone (TSH), antinuclear antibodies, and erythrocyte sedimentation rate (ESR). If Sjögren syndrome is suspected, antibodies to Ro/SSA and La/SSB should be assessed. Levels of vitamin and metal concentrations may be indicated, depending on social history. Magnetic resonance imaging (MRI) is used to evaluate soft tissues and mucosal edema; computed tomography (CT) is useful to assess the skull base and sinuses.⁵ Further testing should be based on clinical presentation and physical findings.

Diagnostics

Smell and Taste Disturbances

Laboratory