SYNCOPE IN NEUROLOGICAL DISEASES

Transient loss of consciousness due to an acute decrease in cerebral b lood flow is the classical but not commonly accepted definition of syn cope. Besides cardiac or respiratory induced syncopes, various neurolo gical causes affecting the autonomic pathways, which are involved in m aintaining cerebral autoregulation, could lead to syncope. The most co mmon form is the simple fainting attack seen in young people (15 to 20 %). Special forms of vasovagal syncopes are the micturition and swallo wing syncopes. Usually there is some warning, including weakness, swea ting, pallor, nausea, yawning, sighing, hyperventilation, blurred visi on, impaired external awareness, and dilation of pupils, followed by u nconsciousness with pallor, coldness of the skin and sweating. At the onset of unconsciousness, the pulse is usually imperceptible; when it returns it is slow. Like most non-cardiac syncopes, vasovagal syncopes are often associated with a specific trigger mechanism such as pain, fear, emotional reactions, injury, surgical manipulation, and an uprig ht position. Orthostasis is the main trigger for syncope, and nearly e very syncope appears while the patient is standing or at least sitting . While the autonomic nervous system in vasovagal syncopes is physiolo gically intact, areflexic syncope results from either functional or st ructural lesions of the autonomic nervous system. Pathophysiologically , an insufficient compensatory increase in heart rate, cardiac output, and arteriolar vasoconstriction are due to a disfunction of the ortho static cerebrovascular autoregulation. Impairment of autonomic functio n due to a variety of lesions involving the autonomic reticular system , including syringobulbia, posterior fossa tumors, ischemia, and infla mmatory diseases, leads to blood pressure dysregulation. In general, s pinal cord transsection produces postural hypotension if the lesion is above the T6 level. Intramedullary and extramedullary tumors, transve rse myelitis and syringomyelia involving the cord above T6 level may a lso produce autonomic failure and syncope. In patients with polyneurop athy, autonomic involvement is not uncommon. It is particularly conspi cuous in diabetic neuropathy, and insulin treatment may further contri bute to the severity of postural hypotension. Autonomic involvement in Guillain-Barre syndrome leads to orthostatic hypotension and may be f atal. sometimes due to cardiac arrhythmia or asystole. Other neuropath ies leading to orthostatic hypotension and syncope include metabolic, autoimmune, hereditary, toxic. and inflammatory neuropathies. Impairme nt in Wernicke's encephalopathy may be related to central or periphera l involvement. The extent, to which autonomic function, and particular ly cardiovascular regulation is impaired in Parkinson's disease, is di sputed. but clinical data evidenced a higher probability for syncope. In other neurological diseases like the Shy-Drager syndrome, patients with multiple system atrophy, pandysautonomia, and idiopathic orthosta tic hypotension, syncopes are the leading symptom. The primary differe ntial diagnosis of syncope must be made to epilepsy. In many cases the distinction between syncope and epilepsy is an easy one when a detail ed history is available. Limpness, pallor, and sweating during unconsc iousness are much more characteristic of syncope than epilepsy. The du ration of a syncopal attack is relatively short, and a patient is usua lly mentally clear on regaining consciousness. Incontinence of urine s ometimes occurs in syncopal attacks, but fecal incontinence is exceedi ngly rare, if it occurs at all. Difficulty in diagnosis may arise if t he onset of the attack is sudden and if there are convulsive movements during the period of unconsciousness. In the absence of a detailed re port of clinical signs, the instrumental work-up may often be rather e xtensive including EEG monitoring studies during wakefullness and slee p. In the case of specific epileptic alterations an epileptic attack i s very probable while a normal or unspecific abnormal EEG cannot be us ed for differential diagnosis. A single orthostatic testing (Schellong 's test) can uncover orthostatic hypotension suggesting syncope. Howev er, the recently introduced combined registration of heart rate and bl ood pressure with measurement of the cerebral blood flow by transcrani al Doppler is particularly prognostic for the detection of cerebrovasc ular dysregulation in the presence of normal systemic blood pressure a nd heart rate. Nevertheless. some attacks of unconsciousness with conv ulsive movements remain unclear: Some of them have recently been class ified as convulsive syncopes. Physiologically, it can be assumed that either cerebral hypoxia (e.g. during a syncope) could induce epileptic alterations or the other way around, that epilepsy with consecutive c erebral hypoxia could lead to this syncope syndrome. In these cases, a clear differentiation between syncope and epilepsy may not be possibl e, but treatment in both directions may be worth a trial.