Porth's Essentials of Pathophysiology, 4e

542

Respiratory Function

U N I T 6

Etiology Influenza is caused by viruses belonging to the Ortho­ myxoviridae family, whose members are characterized by a segmented, single-stranded ribonucleic acid (RNA) genome. 13–15 There are three distinct types of influenza viruses, designated A, B, and C. Influenza A and B cause epidemics. Influenza C does not cause epidemics, but is responsible for mild upper respiratory infections in chil- dren and adults. Influenza A viruses are further categorized into sub- types based on two glycoproteins studding their lipid envelope: hemagglutinin (H) and neuraminidase (N) (Fig. 22-2). Hemagglutinin, for which there are 16 differ- ent variants (H1 thru H16), allows the virus to anchor to the surface of epithelial cells in the respiratory tract; and neuraminidase, of which there are 9 variants (N1 thru N9), allows for digestion of host secretion and, later, release of viral particles from host cells. 15 For example, an influenza virus circulating worldwide in 2013 was identified as H3N2. Immunity to the surface H and N antigens reduces the likelihood and severity of infec- tion with the influenza virus. Epidemics and pandemics result from the ability of the influenza virus to develop new subtypes against which the population is not protected. 13–15 The genetic diver- sity of influenza A is fostered by its segmented genomic structure and ability to infect and replicate in humans and many avian and animal species, including swine. Epidemics of influenza A occur when minor changes in the amino acids of the H and N glycoproteins, called antigenic drift, generate a new subtype to which the population is only partially protected by cross-reacting antibodies. Pandemics occur when a process called an antigenic shift causes both the H and N antigens to be replaced through recombination of the RNA segments with those of animal viruses, making all individuals sus- ceptible to the new influenza virus. As with many viral respiratory tract infections, influ- enza is more contagious than bacterial respiratory tract infections. In contrast to the rhinoviruses, transmission occurs by inhalation of droplet nuclei rather than touch- ing contaminated objects. Adults are usually considered infectious from the day before symptom onset to 5 to 10 days after the first symptoms appear. 15 Children can be infectious for greater than 10 days, and young chil- dren can shed virus for up to 6 days before their illness onset. Severely immunocompromised persons can shed virus for weeks or months. Pathogenesis The influenza viruses can cause three types of infections: an uncomplicated upper respiratory infection (rhinotra- cheitis), viral pneumonia, and a respiratory viral infec- tion followed by a bacterial infection. Influenza initially establishes upper airway infection. In doing this, the virus first targets and kills mucus-secreting, ciliated, and other epithelial cells, leaving gaping holes between the underlying basal cells and allowing extracellular fluid to escape. This is the reason for the rhinorrhea or “runny nose” that is characteristic of this phase of the infection.

person open his or her mouth, and observing the hard palate for light transmission. Sinus radiographs and com- puted tomography (CT) scans may be used. CT scans usually are reserved for diagnosis of chronic rhinosinus- itis or to exclude complications. 7,8 Magnetic resonance imaging (MRI) is expensive and usually reserved for cases of suspected neoplasms or fungal sinusitis. Treatment. Treatment of rhinosinusitis depends on the cause and includes appropriate use of antibiotics, intra- nasal corticosteroids, mucolytic agents, and symptom relief measures. 7–12 Antibiotics are usually reserved for persons with severe or persistent symptoms and specific findings of bacterial infection. The treatment of acute rhinosinusitis includes measures to promote adequate drainage by reducing nasal congestion. The topical α -adrenergic decongestants may be used on a short-term (3 days) basis in older children and adults for this pur- pose. The use of antihistamines is controversial, par- ticularly for acute rhinosinusitis, because they can dry up secretions and thereby decrease drainage. Mucolytic agents such as guaifenesin may be used to thin secre- tions. Intranasal corticosteroids reduce inflammation and edema of the nasal mucosa, nasal turbinates, and sinus ostia. They may be used as an initial treatment in persons with acute rhinosinusitis and in those with both allergies and rhinosinusitis. 7,8 Nonpharmacologic mea- sures include saline nasal sprays and steam inhalation. Surgical intervention directed at correcting obstruc- tion of the ostiomeatal openings may be indicated in persons with chronic rhinosinusitis that is resistant to other forms of therapy. Indications for surgical inter- vention include obstructive nasal polyps and obstructive nasal deformities. Complications. Because of the sinuses’ proximity to the brain and orbital wall, sinusitis can lead to intracra- nial and orbital wall complications. Intracranial compli- cations are seen most commonly with infection of the frontal and ethmoid sinuses because of their proximity to the dura and drainage of the veins from the frontal sinus into the dural sinus. Orbital complications can range from edema of the eyelids to orbital cellulitis and subperiosteal abscess formation. Facial swelling over the involved sinus, abnormal extraocular movements, protrusion of the eyeball, periorbital edema, or changes in mental status may indicate intracranial complications and require immediate medical attention. Influenza Influenza is one of the most important causes of acute upper respiratory tract infection in humans. Until the advent of acquired immunodeficiency syndrome (AIDS), influenza was the last uncontrolled, potentially fatal pan- demic. In the United States, epidemics of influenza typi- cally occur during the winter months, accounting for over 35,000 deaths annually. 13 Rates of infection are highest among children, but rates of serious illness and death are highest among persons who are 65 years of age or older.