© 2004 by European Society of Cardiology
EDITORIAL
Defaecation syncope: new light on an old problem
Richard D. Ruppert Health Center, Division of Cardiology, The Medical College of Ohio 3120 Glendale Ave., Toledo, OH 43614-5809, USA
Manuscript submitted 7 January 2004. Fax: +1-419-383-3041. E-mail address: bgrubb{at}mco.edu.
Syncope, the sudden loss of consciousness with spontaneous recovery, is a frequent clinical problem for which patients are commonly referred for evaluation [1]
. While a number of conditions may result in syncope, one particular group of causes has been the subject of intense study over the last two decades. For years prior to this it had been postulated that many episodes of unexplained syncope occurred due to periods of decompensation in the autonomic nervous system that would lead to hypotension (with or without bradycardia) sufficiently profound so as to cause cerebral hypoperfusion and loss of consciousness, an entity known as vasovagal (or more recently as neurocardiogenic or reflex) syncope. In a landmark paper by Kenny and colleagues, head upright tilt table testing was introduced as a method for provoking autonomic nervous system decompensation in a laboratory setting [2]. This not only provided a long sought after diagnostic modality for unmasking vasovagal syncope, but also introduced a controlled setting in which detailed measurements and observations could be made during the syncope itself. Consequently, there has been a sudden and tremendous increase in our knowledge concerning these disorders. At the same time, it has become evident that classic vasovagal (neurocardiogenic) syncope is but one aspect of a broad, heterogenous collection of disturbances in the body's autonomic nervous system, which lead to alterations in normal homeostatic mechanisms to a sufficient extent so that loss of consciousness occurs.
One group of these disorders is presently referred to as "reflex syncopes", a classification which has included not only neurocardiogenic (vasovagal) syncope, but defaecation, micturition, swallow and cough syncope, as well as carotid sinus hypersensitivity [3]. These disorders were linked together not only because of the observed similarities in the nature of the syncopal episode (sudden provoked hypotension with or without bradycardia), but also because the disorders frequently would coexist in the same individual [4]. In each of these conditions sudden activation of unmyelinated C fibres (or mechanoreceptors) were thought to send a flood of neural activity to the medulla (in particular the area known as the Nucleus Tractus Solitarius) that would mimic the conditions normally seen in hypertension, thus provoking sympathetic withdrawal with subsequent vasodilation and hypotension [1]. However, the association of these disorders has been mainly one of intuition and less one of observation due to the inherent difficulties in provoking many of these conditions in the same controlled manner as tilt table testing provides in neurocardiogenic syncope.
Despite these limitations, Allan and associates have once more come forward to shed light on the poorly studied disorder of defaecation syncope. By performing a careful series of detailed evaluations of patients with defaecation syncope, she and her colleagues have made a series of fascinating observations [5].
The process of normal bowel motility and transport through the gastrointestinal tract occurs due to a delicate balance of several control mechanisms [6]. These include the contractile and electrical functions of smooth muscle, control by the intrinsic nervous system via neurotransmitters such as acetylcholine as well as a variety of gastrointestinal peptides, and through extrinsic neural pathways (both parasympathetic and sympathetic). The system that initiates the aforementioned components is referred to as the intrinsic or enteric nervous system. This system is distinct from the sympathetic and parasympathetic segments of the autonomic nervous system and contains approximately 100 million neurons (roughly the same number present in the spinal cord). Enteric activity is modulated by a balance between parasympathetic excitatory input from the craniospinal nerves (vagus and S2, 3, 4) and the thoracolumbar sympathetic outflow, which is predominantly inhibitory to the gut but excitatory to the sphincters [7]. Defaecation results from a series of integrated motor responses. Normal continence occurs due to the acute angle between the rectum and the anal canal and by the activity of the anal sphincters. This puborectalis "sling" serves to maintain an acute angle. For defaecation to occur, this sling relaxes, permitting the rectoanal angle to open, producing a straighter rectal conduit. Faeces are expelled by a diffuse contraction of the distal bowel, supported by contraction of the diaphragm, abdominal and chest wall muscles as well as by Valsalva's manoeuver. Expulsion of faeces occurs when the anal sphincters are inhibited by parasympathetic (pudendal nerve, S2, 3, 4) input. Afterwards, continence is regained by contraction of the puborectalis (via the parasympathetic pudendal nerve), contraction of the internal sphincter (via the sympathetic lumbar colonic nerves) and lastly by contraction of the external sphincter (via the parasympathetic pudendal nerve) [8].
Allan's careful evaluation of a group of seven patients with recurrent defaecation syncope yielded several interesting findings. One was that head upright tilt table testing was normal in every patient evaluated. This would suggest that the exact triggers for neurocardiogenic syncope differ from those of defaecation syncope. A further battery of autonomic tests revealed that, compared with normals, Valsalva's manoeuver showed distinct evidence that patients with defaecation syncope had evidence of mild to moderate sympathetic failure. This was confirmed by calculation of adrenergic and cardiovascular heart rate index. These observations suggest that the patients studied may suffer from an early form of autonomic failure. Normally, the sympathetic system of the gut inactivates neural circuits that generate motor activity. The loss of inhibitory sympathetic input can result in excessive and uncoordinated phasic pressure activity in the gut [7], allowing for greater bowel wall tension with subsequent mechanoreceptor activation, and greater than normal augmentation of abdominal pressure during Valsalva's manoeuver.
Allan further goes on to speculate that this process may represent either a central abnormality of autonomic nervous system modulation or alternatively, local abnormalities of autonomic innervation of the capacitance vessels in either the lower limbs or splanchnic vasculature or both. With regard to the latter, recent neuroimaging studies applied to the heart have been used to determine the integrity of sympathetic innervation in various forms of autonomic failure. Single photon emission computed tomography after injection of 123I-MIBG and positron emission tomography (PET) after injection of 6-[18F] fluorodopamine are employed to determine the pre-syncopal concentrations of noradrenaline [9]. These studies have shown denervation of sympathetic nerve terminals in pure autonomic failure (PAF) and Parkinson's disease (PD), while patients with multiple system atrophy (MSA) have intact innervation (thus suggesting that in PAF and PD there are abnormalities in autonomic innervation while in MSA the lesion appears to be central in nature [10]). Such studies may help to clarify better which situation is present not only in defaecation syncope but also in related conditions. A central defect in neurotransmitter production could also be implicated to explain these findings. Chronically low central serotonin levels have been implicated in the pathogenesis of neurocardiogenic syncope, and treatment with serotonin enhancing agents have suggested value in their treatment [11,12]. Lastly, the authors' speculation that specific blood supply to the liver and spleen certainly warrants further investigation. A good paper often raises as many questions as it answers, and Allan et al.'s contribution has shed a bright new light on an old problem.
References
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[5] Allan L, Johns E, Doshi M, Kenny RA, Newton JL. Abnormalities of sympathetic and parasympathetic autonomic function in subjects with defaecation syncope. Europace 2004; 6: (in press).
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[9] Eisenhofer G, Hovevey-Sion D, Kopin IJ, Miletich R, Kirk KL, Finn R. Neuronal uptake and metabolism of 2- and 6-fluorodopamine: false neurotransmitters for positron emission tomographic imagery of sympathetically innervated tissues. J Pharmacol Ther 1989; 248: 419–427.
[10] Goldstein DS, Robertson D, Esler M, Straus S, Eisenhofer G. Dysautonomics: clinical disorders of the autonomic nervous system. Ann Intern Med 2002; 137: 753–763.
[11] Grubb BP and Karas BJ. The potential role of serotonin in the pathogenesis of neurocardiogenic syncope and related autonomic disorders. J Interv Card Electrophysiol 1998; 2: 325–332.[CrossRef][Web of Science][Medline]
[12] Girolamo ED, Iorio CD, Sabatini P, Leonzio L, Barbone C, Barsotti A. Effects of paroxetine hydrochloride, a selective serotonin reuptake inhibitor on refractory vasovagal syncope: a randomized placebo controlled study. J Am Coll Cardiol 1999; 33: 1227–1230.
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