Autonomic dysreflexia (AD) is a life-threatening condition prevalent amongst patients with high spinal cord injury (SCI) and may occur any time after injury. It is a syndrome characterized by an exaggerated reflex increase in blood pressure, usually accompanied by bradycardia in response to a stimulus originating below the level of SCI. AD commonly affects patients with SCI above the major sympathetic nervous system splanchnic outflow, which is usually above T6 6,10. The syndrome can be associated with complete or incomplete injury. The higher the lesion, the more severe the clinical manifestation of AD as measured by the rise in blood pressure 8,10.
It is a distinct entity, is episodic and a rise in blood pressure is a sentinel sign. The intensity of AD can vary from asymptomatic, mild discomfort and headaches to a life-threatening emergency when systolic blood pressure can reach 300 mmHg 9. Untreated episodes of autonomic dysreflexia may have grave consequences, including intracranial hemorrhage, cardiac complications, retinal detachment, cerebral edema leading to seizures, coma and death 14.
The incidence is ~70% (range 48%-91%) of individuals with tetraplegia and high paraplegia 3, with some reporting over 75% of patients with high SCI (above T6) experience AD by one year 8. AD has also been reported after posterior fossa tumors 3. AD generally occurs after the period of spinal shock, commonly between 6-12 months. It is uncommon before 2 months after injury 8.
- severe pounding frontal and occipital headaches
- feeling of anxiety/impending doom
- mydriasis if lesion is above T1
- Horner's syndrome if injury is at T1: loss of sympathetic input leading to meiosis, ptosis, anhydrosis
- nasal congestion
- pallor initially followed by flushing of the face and neck along with sweating in areas above and around the SCI
- piloerection: due to sympathetic stimulation of hair follicles below the level of injury
- rise in systolic blood pressure by 20-30 mmHg above baseline
- reflex bradycardia
- cardiac arrhythmias, e.g. atrial fibrillation
- contraction of the urinary bladder and large bowel
- seminal fluid emission
The classic syndrome of AD involves excessive peripheral responses that are normally mediated by catecholamines below the level of SCI 7. The mechanism involves a strong stimulus that enters the spinal cord through intact peripheral nerves. The stimulus ascends through the spinothalamic tract and posterior columns below the level of the SCI. This initiates reflex release of catecholamines from the pre-ganglionic neurons in the intermediolateral cell column below and subsequent release of norepinephrine and dopamine resulting in regional vasoconstriction of the GI tract (subdiaphragmatic and splanchnic vasculature) 5. Peripheral vascular resistance increases with associated increase in cardiac output resulting in increased blood pressure. Compensatory reflex bradycardia is seen as the brainstem vasomotor centers attempt to reduce blood pressure.
Possible mechanisms accounting for AD are 7,9,11:
- loss of supraspinal control
- denervation hypersensitivity of sympathetic spinal, ganglionic or peripheral receptor sites
- decreased neuronal uptake and increased synaptic levels of norepinephrine
- formation of abnormal synaptic connections due to axonal sprouting
As AD is uncommon early after SCI suggests that this phenomenon develops over time and is produced mainly by peripheral or central receptor hypersensitivity rather than blockade of descending inhibitory tracts. As the level of SCI plays such a critical role in AD, this has conventionally supported the blockade of inhibitory tracts as the main mechanism.
However, it may be just that receptor hypersensitivity or synaptic overload of norepinephrine does not occur below the critical T6 level, as there may not be sufficient blood vessel constriction below this neurological level to cause an appreciable rise in blood pressure 5.
Stimuli and conditions associated with the development of AD 9,13:
- bladder distension: most common cause
- bowel distension
- urinary tract infection, epididimytis
- renal calculus
- heterotopic ossification
- gastro-oesophageal reflux
- pressure ulcers
- ingrown toenail
- labor and delivery (in women)
- ejaculation (in men)
- procedures, e.g. radiological imaging, surgery, urodynamic studies
- surgery, e.g. during anesthetic induction; intrathecal anesthesia interrupts this autonomic reflex arc
- pulmonary embolism
- intracerebral hemorrhage, in particular, subarachnoid hemorrhage
- retinal haemorrhage
- acute myocardial infarction
- coma and death
- indirect complication: pressure ulcers of the skin due to denervation and arteriovenous shunting leading to ischemia14
Treatment and prognosis
Baseline blood pressure after cervical or high thoracic SCI may fall by approximately 15-20 mmHg compared to able-bodied individuals 2,9. Hence, a rise in blood pressure of over 20-30 mmHg or systolic pressure of over 150 mmHg represents significant hypertension 13. There may be associated reflex bradycardia.
The management of an acute episode of AD generally focuses on identifying and eliminating the cause 1. Positioning the patient in an upright sitting position with the legs dangling over the bedside can help reduce venous return, hence leading to a posture-related drop in blood pressure. Tight clothing and constrictive devices should be removed while the blood pressure and pulse are monitored. A quick survey of instigating causes should be done beginning with the urinary system. This involves checking if the urinary catheter (if present) is not kinked and performing a bladder scan to check volumes and to insert a urinary catheter 13.
Other causes such as rectal impaction must be sought 9. However, if a bladder and bowel cause is ruled out, pharmacological management should be initiated. This is done while looking for other less common causes. Rapid onset, short duration anti-hypertensive agents including nitrates (sublingual GTN spray), nifedipine, prazosin, hydralazine and intravenous diazoxide may be used1.
Headaches can persist for hours after the rise in blood pressure is treated. This responds to simple analgesia. Regional, spinal and epidural anesthetic may be used as pre-treatment for episodes that are anticipated with diagnostic or surgical procedures (e.g. repeat radiological imaging, urodynamic studies and catheter change) 4. Epidural anesthesia is effective in the obstetric setting 12.
Patients who experience recurrent episodes of AD present difficult diagnostic and therapeutic challenges. The triggering mechanism is often obscure or the putative trigger is not easily remedied. In these situations, a suppressive anti-hypertensive agent is usually needed (such as alpha-adrenergic receptor blockers) 1.
Prevention of recurrent episodes of AD includes satisfactory bladder, bowel and skin care. Also vital is patient and carer education (including health professionals who may deal with patients with SCI) regarding presentation, causes, prevention and management of this condition.
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