Chapter 021. Syncope (Part 3)

Situational SyncopeA variety of activities, including cough, deglutition, micturition, and defecation, are associated with syncope in susceptible individuals. Like neurocardiogenic syncope, these syndromes may involve a cardioinhibitory response, a vasodepressor response, or both. Cough, micturition, and defecation are associated with maneuvers (such as Valsalvas, straining, and coughing) that may contribute to hypotension and syncope by decreasing venous return. Increased intracranial pressure secondary to the increased intrathoracic pressure may also contribute by decreasing cerebral blood flow.Cough syncope typically occurs in men with chronic bronchitis or chronic obstructive lung disease during or after prolonged coughing fits. ...
Nội dung trích xuất từ tài liệu:
Chapter 021. Syncope (Part 3) Chapter 021. Syncope (Part 3) Situational Syncope A variety of activities, including cough, deglutition, micturition, anddefecation, are associated with syncope in susceptible individuals. Likeneurocardiogenic syncope, these syndromes may involve a cardioinhibitoryresponse, a vasodepressor response, or both. Cough, micturition, and defecationare associated with maneuvers (such as Valsalvas, straining, and coughing) thatmay contribute to hypotension and syncope by decreasing venous return.Increased intracranial pressure secondary to the increased intrathoracic pressuremay also contribute by decreasing cerebral blood flow.Cough syncope typicallyoccurs in men with chronic bronchitis or chronic obstructive lung disease duringor after prolonged coughing fits. Micturition syncope occurs predominantly inmiddle-aged and older men, particularly those with prostatic hypertrophy andobstruction of the bladder neck; loss of consciousness usually occurs at nightduring or immediately after voiding. Deglutition syncope and defecation syncopeoccur in men and women. Deglutition syncope may be associated with esophagealdisorders, particularly esophageal spasm. In some individuals, particular foods andcarbonated or cold beverages initiate episodes by activating esophageal sensoryreceptors that trigger reflex sinus bradycardia or atrioventricular (AV) block.Defecation syncope is probably secondary to Valsalvas maneuver in olderindividuals with constipation. Carotid Sinus Hypersensitivity Syncope due to carotid sinus hypersensitivity is precipitated by pressure onthe carotid sinus baroreceptors, which are located just cephalad to the bifurcationof the common carotid artery. This typically occurs in the setting of shaving, atight collar, or turning the head to one side. Carotid sinus hypersensitivity occurspredominantly in men ≥50 years old. Activation of carotid sinus baroreceptorsgives rise to impulses carried via the nerve of Hering, a branch of theglossopharyngeal nerve, to the medulla in the brainstem. These afferent impulsesactivate efferent sympathetic nerve fibers to the heart and blood vessels, cardiacvagal efferent nerve fibers, or both. In patients with carotid sinus hypersensitivity,these responses may cause sinus arrest or AV block (a cardioinhibitory response),vasodilatation (a vasodepressor response), or both (a mixed response). Theunderlying mechanisms responsible for the carotid sinus hypersensitivity are notclear, and validated diagnostic criteria do not exist. Postural (Orthostatic) Hypotension Orthostatic intolerance can result from hypovolemia or from disturbancesin vascular control. The latter may occur due to agents that affect the vasculatureor due to primary or secondary abnormalities of autonomic control. Sudden risingfrom a recumbent position or standing quietly are precipitating circumstances. Orthostatic hypotension may be the cause of syncope in up to 30% of theelderly; polypharmacy with antihypertensive or antidepressant drugs is often acontributor in these patients .Postural syncope may occur in otherwise normal persons with defectivepostural reflexes. Pure autonomic failure (formerly called idiopathic posturalhypotension) is characterized by orthostatic hypotension, syncope and nearsyncope, neurocardiogenic bladder, constipation, heat intolerance, inability tosweat, and erectile dysfunction (Chap. 370). The disorder is more common in menthan women and typically begins between the ages of 50 and 75 years. Orthostatic hypotension, often accompanied by disturbances in sweating,impotence, and sphincter difficulties, is also a primary feature of a variety or otherautonomic nervous system disorders (Chap. 370). Among the most commoncauses of neurogenic orthostatic hypotension are chronic diseases of the peripheralnervous system that involve postganglionic unmyelinated fibers (e.g., diabetic,nutritional, and amyloid polyneuropathy). Much less common are the multiple system atrophies; these are CNSdisorders in which orthostatic hypotension is associated with (1) parkinsonism(Shy-Drager syndrome), (2) progressive cerebellar degeneration, or (3) a morevariable parkinsonian and cerebellar syndrome (Chap. 366). A rare, acutepostganglionic dysautonomia may represent a variant of Guillain-Barré syndrome(Chap. 380); a related disorder, autoimmune autonomic neuropathy, is associatedwith autoantibodies to the ganglionic acetylcholine receptor. There are several additional causes of postural syncope: (1) after physicaldeconditioning (such as after prolonged illness with recumbency, especially inelderly individuals with reduced muscle tone) or after prolonged weightlessness,as in space fli ...