Chapter 27: Nasal Function and the Evaluation of Taste/Smell

Nasal Function

1. Warming and humidification

   1. Air is humidified to high relative humidity

   2. Air is warmed through heat diffusion and convection

   3. Optimum mucociliary clearance is facilitating by temperature of 37°C

2. Regulation of nasal secretions and airway resistance

   1. Proteins in nasal secretions

      1. Plasma proteins

         1. Albumin

         2. Immunoglobulins (IgG, IgM, IgA)

      2. Serous cell products

         1. Antibacterial defense molecules

            • Lysozymes

            • Lactoferrin

            • Secretory component

      3. Mucous cell products

         1. Mucoglycoproteins

         2. Mucins

      4. Epithelial goblet cell products

         1. Macromolecules and ions

   2. Proteins are dissolved or suspended in epithelial lining fluid

      1. Modified by epithelial ion pumps

      2. Plasma extravasation controlled by interepithelial tight junctions

         1. Dependent on rate of blood flow and plasma transudation

            • Arterial vasodilation and filling of venous sinusoidal vessels results in plasma extravasation, thickened nasal mucosa, increased airway resistance.

            • Vasoconstriction reduces mucosal blood flow, reduces plasma extravasation, relieves venous sinusoidal congestion.

   3. Autonomic innervation

      1. Vidian nerve

         1. Formed at junction of greater superficial petrosal (preganglionic parasympathetic fibers) and deep petrosal nerves (postganglionic sympathetic fibers)

         2. Postganglionic sympathetic and parasympathetic fibers carried in the nerve of the vidian canal and join branches of sphenopalatine nerve

            • Parasympathetic neurons

              1. Cotransmitter to acetylcholine (vasointestinal peptide [VIP])

                 • Vasodilation at arterial and sinusoidal vessels

                 • Enhanced secretory activity

                 • Mast cell degranulation

                   • – Release of histamine, bradykinin, arachidonic acid metabolites, ions

            • Sympathetic fibers join branches of sphenopalatine nerve and artery

              1. Cotransmitter to noradrenaline (neuropeptide Y)

                 • Vasoconstriction of venous sinusoidal vessels

         3. Nonadrenergic, noncholinergic responses

            1. Trigeminal sensory neurons use peptides as neurotransmitters

               1. Secreted by macrophages, eosinophils, lymphocytes, dendritic cells

               2. Substance P, neurokinin A, calcitonin gene-related peptide (CGRP)

                  • Induce vasodilation

                  • Increase vascular permeability

                  • Increase nasal resistance

                  • Stimulate glandular secretion

                  • Leukocyte chemotaxis

                  • Mast cell degranulation

         4. Nasal cycle

            1. Cyclical vascular phenomenon that occurs in 80% of normal individuals

            2. Alternating congestion/decongestion every 3 to 7 hours

            3. Centrally mediated autonomic tone of capacitance vessels of erectile mucosa

         5. Other influences on nasal resistance

            1. Exercise decreases nasal resistance

            2. Irritants (dust, smoke), cold/dry air, alcohol, pregnancy, hypothyroidism, and medications can cause congestion of capacitance vessels

               1. Medications that can cause nasal congestion

                  • Antihypertensives (alpha and beta blockers)

                  • Oral contraceptives

                  • Antidepressants

                  • Nonsteroidal anti-inflammatory medications

                  • Decongestants in excess may result in rhinitis medicamentosa

            3. Atrophic rhinitis

               1. Chronic, degenerative disorder characterized by nasal crusting, malodorous discharge, and nasal obstruction

                  • Atrophy of serous and mucinous glands

                  • Loss of cilia and goblet cells

                  • Inflammatory cell infiltrates

               2. Possible causes

                  • Underlying chronic inflammatory disease

                    1. Granulomatous disorders

                  • Irradiation

                  • Bacterial and viral infection

                  • Excessive nasal surgery

               3. Nasal airflow

                  1. At low flow rates, airflow is laminar.

                  2. Maximum velocity occurs in the nasal valve region.

                     1. Bernoulli principle

                        1. Airflow velocity is greatest at the narrowest segment.

                        2. Increased airflow velocity leads to negative pressure and nasal valve collapse.

                     2. Poiseuille law

                        1. Airflow resistance is inversely proportional to fourth power of the radius.

                        2. Small decrease in cross-sectional area produces large increase in airway resistance.

                  3. Relatively slow flow rates found in olfactory region with quiet breathing

                  4. During inspiration, main flow stream occurs in the lower and middle airway (space between middle meatus and nasal septum)

                  5. During expiration, maximum velocity is lower but expiratory air is evenly distributed across inferior, middle, and olfactory regions

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