Chapter 148 Syncope

OVERVIEW

Syncope, or fainting, is a sudden and unexplained loss of consciousness and postural tone usually related to inadequate delivery of oxygen to the brain. This chapter outlines a general approach to the diagnosis of the syncopal patient. Effective therapy of syncope is predicated on identifying and managing the underlying disorder.

Characteristics of Syncope

• The event is often precipitated by exercise, sudden activity, or excitement (sympathetic stimulation). However, syncope can occur during rest or even sleep, especially in cats.

• Along with a loss of consciousness, there is a failure to maintain postural tone, causing falling. If the dog or cat is recumbent at the onset of the event, it is common for the patient to fall backwards or to roll to lateral recumbency.

• Brief stiffening is common along with forelimb rigidity and opisthotonus.

• Minor convulsive activity may be observed, such as focal head twitching. This is more common in cats wherein syncope often resembles a true seizure event.

• Urination during the event is a frequent observation. Defecation may occur but is far less common.

• Mucous membrane color is initially pale pink to white when the cause is low blood pressure; subsequently, cyanotic, normal, or flushed membranes may be observed. If hypoxemia from respiratory disease or a right-to-left shunt is the cause of fainting, cyanosis may precede the spell.

• Following recovery of blood pressure and cerebral function, mentation and behavior are typically normal. However, if the syncope is related to a protracted cardiac arrhythmia or ongoing or progressive pathology, as with a stroke, there may be a longer period of listlessness, stupor, or profound weakness.

• Some patients with syncope also demonstrate pre- or near-syncope. This is characterized by a sudden jerk of the head and body. This apparent “lightheadedness” abates just in time to prevent falling.

Differential Diagnosis

• The differential diagnosis of syncope includes seizure disorder or epilepsy; sleep disorder, including narcolepsy; stroke or transient ischemic attack of the CNS; metabolic disease affecting the CNS; and drug intoxication.

• Syncope is readily confused with seizure disorders or epilepsy (see Chapter 127). Facial fits, generalized tonic-clonic movements, and postictal behavioral abnormalities are more typical of true seizures. However, clients can readily confuse the two, and even after obtaining a careful history, an experienced clinician may remain uncertain about the nature of the event.

• Sleep disorders such as narcolepsy are rare (see Chapter 127) but may be precipitated in dogs by eating. The event appears similar to actual sleep, and in some cases, dogs may chew their food and literally fall asleep on the food bowl. Cataplexy, a loss of postural tone with maintenance of consciousness, is rare in dogs and cats.

• Strokes or transient ischemic attacks can cause true syncope or, more commonly, lead to progressive or episodic neurological dysfunction (see Chapter 126).

• Central nervous system disorders, such as primary brain lesions, may lead to altered consciousness and episodic falling, which might be misinterpreted as a syncopal attack (see Chapter 126). Acute vestibular syndromes may lead to falling, but mentation is normal, and the signs are generally obvious during neurological examination (see Chapters 61 and 126).

• Metabolic diseases may alter cerebral function and be associated with staggering or falling; these include hypoglycemia, hepatic encephalopathy, and hypoadrenocorticism when it causes profound hypotension.

• Overdose of prescribed drugs or ingestion of human drugs or toxins in the household can at times lead to altered cerebral function as well as hypotension, ataxia, or weakness. The resulting fall may be interpreted as syncope. In some cases, drugs cause profound hypotension leading to true fainting.