Page 24 - Feline Cardiology

Chapter 2: Exercise Intolerance and Syncope 15

Subtle, nonspecific clinical signs (e.g.,
Overt signs with consciousness preserved (e.g.,
Sudden cardiac death
True symptoms (abnormal sensations perceived
intermittent weakness, lethargy)
stumbling, ataxia, expression or demeanor
by the patient but not apparent externally)
suggesting discomfort or confusion)
SEVERITY Syncope Cardiogenic seizures Clinical Entities
Figure 2.2. Scale of severity of clinical signs caused by poor cerebral perfusion (forward heart failure), from least to most severe.
cat in whom a persistent cause for syncope only partially Fox et al. 1995; Stamoulis et al. 1992; Harpster 1977;
resolves such that the end of one episode of syncope is Johnson and Sisson 1993), heartworm disease (HWD;
not clearly distinguished from the onset of the next (Fox n = 6) (Malik et al. 1998; Atkins et al. 1985), atrial septal
et al. 1991; Penning et al. 2009). Such patients, who may defect (ASD) (n = 2) (Chetboul et al. 2006), arrhythmo-
experience dozens of syncopal episodes daily, typically genic right ventricular dysplasia/cardiomyopathy
have a grave prognosis if the cause of the syncope is not (ARVD/C; n = 2) (Fox et al. 2000; Harvey et al. 2005),
identified and treated immediately. cardiac lymphoma (Meurs et al. 1994), hyperthyroidism
While neurologic disease (Darke et al. 1989), and (Forterre et al. 2001), hyperthyroidism in which syncope
autonomic disturbances (Kapoor 2002) may cause and 3rd-degree AV block occurred during methimazole
syncope, these etiologies are rare in feline syncope. treatment but syncope resolved after radioiodine
Rather, syncope in cats is generally associated with struc- therapy despite the persistence of 3rd-degree AV block
tural cardiac lesions and a concurrent arrhythmia: pro- for an additional 32 months (Johnson and Sisson
found tachycardia or profound bradycardia, with the 1993), defecation in a cat with multiple systemic and
latter appearing to be more common. Bradycardia cardiac problems (Whitley and Stepien 2001), and a
decreases cardiac output (stroke volume × heart rate), current diagnosis (Darke et al. 1989) or distant history
causing cerebral hypoperfusion. This is especially likely (Fox et al. 1991) of dysautonomia. In 5 cats with syncope,
to cause overt signs like syncope when bradycardia echocardiographic results were within normal limits
is very severe (e.g., feline heart rate = 100 beats/minute (Fox et al. 1991; Stamoulis et al. 1992; Ferasin et al. 2002;
or less) and peracute in onset, before vasoconstrictive Willis et al. 2003; Côté et al. 1999); in 2 of these, nec-
and other adaptive mechanisms have had sufficient ropsy revealed excessive left ventricular moderator
time to optimize cerebral perfusion. Severe tachycardias bands and myocardial and conductive tissue fibrosis
also can produce cerebral hypoperfusion; at very (Liu 1994) and normal gross and histologic results
rapid heart rates, diastolic filling time is compromised, (Willis et al. 2003). The latter results are mirrored
and stroke volume can fall dramatically (see Chapter in human medicine, where in one-third of patients
18). For example, anesthetized cats experimentally with syncope, an inciting cause is never found (Kapoor
induced to have ventricular tachycardia at 300 beats/ 2002).
minute for 1 minute experience a 30% decrease in cere-
bral blood flow (Kobari et al. 1992). Therefore, a patient SIGNALMENT
suspected of having syncope presents 3 levels of diag-
nostic challenge: Is it truly syncope? If so, is it due to an There is no distinguishing feature about the signalment
arrhythmia? And if it is, is the arrhythmia a tachycardia of cats with syncope. It appears to mirror the expected
or a bradycardia? signalment of cats with the associated structural heart
In feline general practice, syncope is an uncommon disorders described above.
clinical sign. When it does occur, an underlying struc-
tural heart lesion is generally present: syncope in cats has HISTORY AND CHIEF COMPLAINT
been associated with hypertrophic cardiomyopathy Classic syncope is a sudden-onset, sudden-termination
(HCM; n = 23) (Rush et al. 2002; Kaneshige et al. 2006; loss of consciousness and ambulation. Owners observing