Autonomic pathophysiology
The autonomic nervous system (ANS) unconsciously controls critical body functions and coordinates our responses to stimuli. The two main branches of the ANS are the sympathetic system, which drives the “fight or flight” response, and the parasympathetic system, which produces the “rest and digest” response. These branches are integrated with the Central Nervous System (CNS) through the central autonomic network (CAN), an internal regulation system consisting of subcortical and cortical structures that coordinates the various responses. The pathophysiology behind autonomic disorders after acquired brain injury remains unknown, but the most frequently cited theories suggest disconnection within the CAN and include the excitatory:inhibitory ratio model, which proposes that injury at the brainstem or above leads to reduced higher-level inhibitory input and results in overamplification of even mild afferent input and the development of allodynic hypersensitivity. It is important to note that although radiographic evidence often shows brainstem damage or diffuse axonal injury in patients with severe forms of autonomic dysfunction, no single lesion location has been identified as the cause of the dysfunction, and syndromes can been seen after both focal and diffuse injury.
A spectrum of autonomic dysfunction syndromes
Although the autonomic dysfunction at the most severe end of the range is the most widely recognized of the post-brain injury autonomic disorders, a spectrum of autonomic disorders ranging in severity and symptoms has been noted corresponding with the variation in brain injury severities and patterns. A number of case reports and case series have showed that a subset of patients develop autonomic dysfunction after even mild traumatic brain injury (mTBI). The most frequently cited abnormalities include heart rate variability, abnormal tilt testing, baroreflex dysfunction, and syncope. Development of postural orthostatic tachycardia syndrome (POTS) has also been reported after TBI. Some post-TBI symptoms, such as temperature dysregulation and excessive or gustatory sweating, are also seen in non-TBI autonomic disorders and may be a result of mild autonomic dysfunction. More exploration of the mild range of autonomic dysfunction after acquired brain injury is needed to better understand the prevalence and impact of these disorders.
Paroxysmal sympathetic hyperactivity
The most widely recognized and most severe type of autonomic dysfunction after traumatic brain injury is paroxysmal sympathetic hyperactivity (PSH). PSH is a syndrome of paroxysmal, transient increases in sympathetic activity that can occur after severe brain injury. It is characterized by episodes of catecholamine elevation, severe hemodynamic alterations, and motor overactivity in response to even minor stimulation. This syndrome has been previously known by several other names, including sympathetic storming , autonomic dysfunction syndrome , and paroxysmal autonomic instability with dystonia , but increasingly preference has been given to the term PSH because of its specificity and more accurate portrayal of the clinical syndrome. , ,
Epidemiology
A review of published cases of PSH by Perkes et al. showed that most occurred after TBI (79.4%) followed by hypoxic brain injury (9.7%) and then stroke (5.4%). Hemorrhagic stroke was associated with four times as many cases of PSH as ischemic stroke. The incidence of PSH after severe TBI is variable, with reports of 8% to 33%. , This variability in incidence is likely a result of differences in diagnostic criteria and time since injury. As an example, one study showed 92% of patients had some degree of sympathetic hyperactivity within the first week after injury, but only 24% of patients met criteria for PSH at day 7 postinjury, and only 8% continued to meet criteria for PSH at day 14. Time to PSH onset varies, but emergence of signs and symptoms is often noted after sedation withdrawal. Although the majority of patients show reduction in sympathetic hyperactivity within the first few weeks after injury, studies have shown that a subset of patients develops a chronic form of PSH in which a degree of sympathetic hyperactivity can last as long as 2.5 to 6 months.
Diagnosis and clinical features
Despite development of diagnostic assessment tools and a consensus statement, diagnosis of PSH remains one of exclusion. The signs and symptoms of PSH overlap with those of seizures, sepsis, neuroleptic malignant syndrome, serotonin syndrome, malignant hyperthermia, untreated pain, and alcohol or sedative withdrawal, and these potential diagnoses must be considered before attributing all symptoms to PSH. , , Although there is no single standard set of criteria used consistently to diagnosis PSH, there is a key group of features that are common among assessment measures. , , , , These common clinical features are episodic tachycardia, tachypnea, hypertension, hyperthermia, diaphoresis, and posturing. Common ranges for these parameters are noted in Table 24.1 .
| Symptom | Normal | Mild | Moderate | Severe |
|---|---|---|---|---|
| Heart rate | <100 | 100–119 | 120–139 | ≥140 |
| Respiratory rate | <18 | 18–23 | 24–29 | ≥30 |
| Systolic blood pressure | <140 | 140–159 | 160–179 | ≥180 |
| Temperature | <37.0 | 37.0–37.9 | 38.0–38.9 | ≥39.0 |
| Sweating | None | Mild | Moderate | Severe |
| Posturing | None | Mild | Moderate | Severe |
Additional factors that increase the diagnostic likelihood of PSH include , :
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Episodes begin after brain injury
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Symptoms occur simultaneously
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Symptoms are paroxysmal
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Symptoms are triggered by even benign or mildly nociceptive stimuli
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Symptoms are severe
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Duration of syndrome 3 or more consecutive days
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Duration of syndrome 2 or more weeks after brain injury
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Two or more episodes daily
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Syndrome persists after treatment of other potential diagnoses
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No other presumed etiology
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No parasympathetic features during episodes
Motor overactivity commonly fluctuates with paroxysms, may be asymmetrical, and includes varying patterns, such as decorticate and/or decerebrate posturing, spasticity, rigidity, and dystonia.
Additional features that may be seen in PSH include:
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Poor gastrointestinal tract absorption
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Elevated white blood cell count without infection
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Elevated catecholamine levels
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Asymptomatic arrhythmias
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Neurogenic lung disease
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Pupillary dilatation
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Excessive salivation
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Crying
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Hiccups
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Yawning
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Sighing
Complications
There are both individual and population-level implications caused by PSH. Compared with symptoms seen in other severe TBI patients, PSH is associated with longer comas, lower Functional Independence Measure scores at discharge, longer hospital stays, greater healthcare costs, and increased morbidity and mortality rates. , , , ,
Factors such as arrhythmia, tachypnea, hyperglycemia, hyperthermia, and hypernatremia increase the risk for impaired cerebral tissue oxygenation and secondary brain injury. Persistent hypertension increases the risk of secondary brain injury through cerebral edema, expansion of hemorrhage, or anoxic injury from cardiac dysfunction. Cardiopulmonary disorders after TBI are discussed in depth in Chapter 18 .
Many additional complications after severe TBI arise because of the sympathetic overactivity of PSH. Fluid loss from diaphoresis can cause hypernatremia, renal insufficiency, and thickened pulmonary secretions. If appropriate dietary supplementation is not maintained, the elevated metabolic state can cause weight loss, malnutrition, muscle atrophy, renal failure, and pressure injuries. Untreated motor overactivity can cause permanent contractures. The relative risk of developing symptomatic heterotopic ossification is 59 times higher in TBI patients with PSH compared with those without.
Management
The primary goal of PSH management is to reduce secondary morbidity and mortality after TBI. As part of that goal, an important first step in management of PSH is to establish the appropriate diagnosis. Other potential diagnoses such as seizures, sepsis, substance withdrawal, and overlap syndromes must be considered, treated, and excluded in tandem with treatment for presumed PSH.
Guidance for PSH management is currently limited by incomplete understanding of the underlying pathophysiology and the lack of large prospective studies on the subject. Management of PSH remains focused on symptom treatment and often includes both pharmacologic and nonpharmacologic interventions.
Nonpharmacologic management
Nonpharmacologic treatment to prevent secondary complications includes physical and occupational therapy and dietary consultation. Environmental modifications can be used to prevent common triggers of episodes, including touching, repositioning, passive movement, endotracheal tube suctioning, constipation, urinary retention, catheter manipulation, and pressure injuries.
Environmental modifications to prevention and treat triggers include:
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Appropriate positioning
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Preventing and treating constipation and urinary retention
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Managing and preventing pain
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Preventing pressure injury
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Limiting unnecessary environmental stimuli
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Cooling blankets
Pharmacologic management
Pharmacologic management is often categorized into abortive and preventive treatments. A comparison of pharmacological treatment options is shown in Table 24.2 .
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