Orthostatic intolerance (OI) covers a spectrum of symptoms including presyncope and syncope, weakness and fatigue, tachycardia or palpitations, nausea, and difficulty concentrating. Symptoms can be aggravated by prolonged standing, physical exertion, environmental warming, post-prandial states, and menses. Diagnosis is based on history and results of heads-up tilt table testing.
Orthostatic intolerance is a subset of dysautonomia. Common OI disorders include vasovagal response, cardioinhibitory syncope, and postural orthostatic tachycardia syndrome (POTS) (see Table 20.5).
A vasovagal response (VVR) occurs when the blood pressure and heart rate decrease to a threshold precipitating syncope. In general, the VVR slows the heart rate, decreases the blood pressure, contracts the pupils, and increases gastrointestinal activity. Factors provoking VVR include dehydration, sleep deprivation, stress and anxiety, and even pain. These triggers are usually added to an innate tendency towards VVR in susceptible individuals and in certain pathologic states.
Stimulation of vagal pathways causes slowing of the heart rate, which if sufficient, can cause fainting or even cardiac arrest. Usually when this happens, the ventricles start to beat on their own accord despite continued vagal stimulation.
Hypotension is generally associated with increased nervous system activation and reflex tachycardia, although one type of hypovolemic hypotension, occurring after hemorrhage or certain drugs, induces a decrease in heart rate. Both types of VVR result from abnormal excitation of the vagus nerve, and hence the term used to describe the resulting loss of consciousness that may result is vasovagal syncope.
Cardioinhibitory syncope is the response of the inhibition of sinus and atrioven-tricular node activity. It is associated with a vasodilatory response (arterial dilation), decreased blood pressure, nausea, salivation, and diaphoresis. The symptoms are common after an increase of parasympathetic output. This increased output can occur after direct stimulation of the vagal nerve or as a response to cessation of sympathetic activity.
Postural orthostatic tachycardia syndrome is the most common OI diagnosis for adults seeking referral. This syndrome not only causes daily symptoms, but it can also disrupt the patient's ability to work or do daily tasks. Postural orthostatic tachycardia syndrome is diagnosed when symptoms of OI are present, and the heart rate
Table 20.5 The orthostatic intolerance disorders
OI disorders Diagnosis characteristics
Vasovagal response (neurocardiogenic) Decreased BP and decreased HR/bradycardia
Cardioinhibitory syncope Inhibition of sinus and AV node activity
Vasodilatory response POTS Increased heart rate
1. Increased heart rate 30 bpm in first 10 min of tilting
2. Heart rate 120 bpm in first 10 min of tilting
3. Increased heart rate of 30 bpm when isoprenaline is infused at a rate of 1 mg/ml
Table 20.6 Diagnostic steps for establishing orthostatic intolerance, the basics
• Head-up tilt table testing (70°) - diagnostic testing to identify patterns of blood pressure and pulse fluctuation in relation to upright posture
• Quantitative Sudomotor Axon Reflex Testing (QSART) - a measure of the autonomic nerves that control sweating
• Blood volume studies, radionuclide hemodynamic studies - method to evaluate blood volume, velocity of systemic blood movement in the areas of blood pooling
• Autonomic reflex testing (valsalva and cold pressor tests) - a method to evaluate autonomic function while evaluating heart rate variability and response to various stimuli increases above 120 beats per minute within 10 min of head-up tilt, or increases by at least 30 beats per minute when transitioning from a supine position to an upright position. Most commonly, POTS affects female patients aged 12-50 years old and usually presents after a virus or inflammatory condition. The frequency of children and adolescents experiencing POTS is on the rise, but the pathophysiology of this disorder remains incompletely understood.
Postural orthostatic tachycardia syndrome is thought to be associated with abnormal venous pooling and fluid collection in the lower extremities. Symptoms that often accompany POTS are tachycardia, hypotension, dizziness, fatigue, palpitations, and nausea.
Diagnosis of OI requires a good history as well as a tilt table test and, often, additional workup (listed in Table 20.6).
Episodes of syncope and near-syncope are not uncommon in individuals with migraine, occurring more commonly in migraineurs than in the general population. Migrainous syncope and near-syncope can be ictal or interictal.
Migraineurs often have lower blood pressure than non-migraine individuals. The basis for this is not fully understood; however, problems in the neurocardiac axis, as manifested by these higher rates of syncope and near-syncope, may reflect certain genetic subforms of migraine.
Orthostatic intolerance can occur without migraine, but it is frequent to have both disorders. It is difficult to separate whether they are comorbid and separate or whether one disorder provokes the other. However, it is not uncommon to see a woman with an acute migraine around her period with blood loss and feeling faint without a true disorder of OI.
A case series by Stillman in 2003 reviewed patients with headache (all meeting IHS criteria for migraine) and symptoms of presyncope or frank syncope, and revealed significant abnormalities frequently occurred on head-up tilt table testing.
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