Diagnosing Autonomic Nervous System Disorders – Existing Guidelines and Future Perspectives
Before ordering any kind of autonomic testing, a clear hypothesis should be formulated. For example, head-up tilt testing (HUT) is only helpful for diagnosing reflex syncope or orthostatic hypotension. Rarely, HUT may provoke psychogenic seizures, but other causes of TLOC – such as cardiac syncope and epileptic seizures – will not. Thus, in the evaluation of a patient with syncope while jogging HUT is severely contraindicated, since the circumstances favour a cardiac cause, and ordering an autonomic test may result in a substantial delay in diagnosis. It also should be noted that additional investigations may confuse the diagnosis. For example, conducting HUT or the Ewing battery in elderly patients will very likely detect orthostatic hypotension (OH), since OH affects up to 30% of all elderly people.3,5
Using these simple means, autonomic screening can be available on every ward, or in the outpatient department, allowing diagnoses to be made without the need to wait for additional tests.
However, the majority of cases are asymptomatic, therefore it is crucial to obtain more evidence before assuming causality. In patients with TLOC and OH it is necessary to establish whether clinical events were typically provoked by prolonged standing, cessation of exercise (post-exercise hypotension) and ingestion of a meal (post-prandial hypotension).2,3
Again, it is
important to detail as many events as possible in order to recognise a clear pattern. Additionally, during the autonomic examination the patient should be questioned whether he or she experiences his or her typical symptoms.
Bedside Testing
After taking a precise history, there are simple means of investigating ANS without a sophisticated autonomic laboratory. TLOC caused by cardiac syncope has a high risk of mortality, therefore it is of utmost importance to first rule out this diagnosis. A simple 12-lead ECG can confirm the presence of bifascicular block, inadequate sinus bradycardia, pre-excited QRS complex, QT interval abnormities, negative T waves in right precordial leads, epsilon waves and ventricular late potentials.2 cardiologist immediately.
These patients should be referred to a
Standard somatic and neurologic examinations are required, particularly when orthostatic hypotension is suspected. Dehydration might add to the development of orthostatic hypotension or reflex syncope. Polyneuropathy of the somatic nerves might accompany autonomic neuropathy. Parkinsonian signs might hint to the diagnosis of an alpha-synucleinopathy.
Finally, an inexpensive standing test can add valuable information. The Schellong test is commonly used but, to save time, the protocol can be changed. The patient rests in the supine position for five to 10 minutes. BP and pulse rate measurements are taken regularly until a steady state is reached. These data are important as the starting point for further analysis. The patient moves to an upright position and another measurement is taken immediately and then at at least every minute for up to 10 minutes. The patient is asked to report symptoms like dizziness, fatigue, headache and nausea. A decrease in BP of 20mmHg systolic and/or 10mmHg diastolic within three minutes of becoming upright gives a diagnosis of orthostatic hypotension.6
A
pulse rate increase from supine to standing of more than 30 beats per minute, or to greater than 120 beats per minute, suggests postural tachycardia syndrome (PoTS). In only few patients, the standing test provokes reflex syncope, and in these patients syncope typically occurs with prolonged standing. A study in 67 Austrian army recruits found the standing test to have a sensitivity of 61% and a specificity of 100% for PoTS.7
A sensitivity of 31% and a specificity of 100% for reflex syncope were noted in the same study. EUROPEAN NEUROLOGICAL REVIEW
Cardiovascular Autonomic Testing Autonomic Challenge Manoeuvres (Ewing Battery) The cardiovascular and the respiratory systems play key roles in the maintenance of homeodynamics and there exist close similarities in the central organisation of both. Reflex regulation of the cardiovascular system involves the activation of different types of receptors located mainly within the heart and blood vessels. Information collected is integrated in the brain stem area and, through negative feedback mechanisms, an appropriate response is generated and is conveyed to the target organs by the peripheral autonomic nervous system.
Autonomic testing has a clear focus on the cardiovascular system, and its interactions with the respiratory system, due to the ease of non-invasive recording of cardiovascular variables. Based on this premise a standard battery of autonomic challenge manoeuvres was suggested by Ewing and Clarke.8
The Valsalva manoeuvre and
It should be stressed that no single test can provide a global assessment of autonomic function. The normative values of these tests depend upon a large number of factors, including: the specific laboratory conditions (e.g. room temperature, instrumentation); the protocol followed (e.g. duration of the stimulus, body position during and prior to testing); and patient-related factors (e.g. age, medication, consumption of caffeine, etc.) Care must therefore be taken when interpreting individual results on the basis of published normative values. From a clinical point of view it should be stressed that autonomic challenge manoeuvre tests are directed towards the determination of autonomic failure, as may be seen in the context of orthostatic intolerance. The Ewing battery is usually not helpful for detecting autonomic overactivity as seen in reflex syncope. For these patients HUT may be indicated.
active standing and deep breathing are the most valuable tests for clinical evaluations. Many other tests activate sympathetic or parasympathetic responses, including the cold pressor test, the cold face test, the sustained hand grip test, squatting, coughing and mental arithmetic.9
Valsalva Manoeuvre
This assesses the sympathetic and the parasympathetic reaction to baroreflex activation. The subject is asked to exhale into an almost occluded mouthpiece and to maintain an expiratory pressure of 40mmHg for 15 seconds. A very small hole in the mouthpiece keeps the glottis open. The test is divided into four phases. Phase 1 occurs during the first two to three seconds of the forced expiration and is associated with a brief decrease in HR and increase in BP due to mechanical compression of the aorta. In phase 2, BP first decreases and then increases in the latter part of the phase. Phase 3 describes the first one to two seconds after release of the expiratory strain with a consecutive decline in BP and increase in HR. Finally, phase 4 is associated with a BP overshoot due to persistent sympathetic activity together with normalisation of venous return. The increased BP mediates baroreflex-induced bradycardia and is quantified using the Valsalva ratio (i.e. the ratio of the highest HR during expiration to the lowest HR during the first 20s after the release of strain). Results depend on the position, age and gender of the subject, as well as the duration and intensity of the expiratory pressure. In patients with autonomic dysfunction, typically there is a loss of both the BP overshoot and the reflex bradycardia.
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