Disorders of the Autonomic Nervous System¶
Chapter 451 | Part 13: Neurologic Disorders
KEY CLINICAL POINTS¶
- Autonomic dysfunction (dysautonomia) involves impaired sympathetic, parasympathetic, or enteric nervous system function, with clinical manifestations ranging from orthostatic hypotension (OH) to syncope and neurodegenerative syndromes.
- Key diagnostic tests include heart rate variability to deep breathing, Valsalva maneuver, tilt table testing, quantitative sudomotor axon reflex testing (QSART), and skin biopsy for small-fiber neuropathy.
- Classification of autonomic disorders includes brain involvement (e.g., multiple system atrophy, Parkinson’s disease), spinal cord disorders (e.g., spinal cord injury, multiple sclerosis), and peripheral neuropathies (e.g., diabetic autonomic neuropathy, amyloidosis).
- Management strategies vary by etiology: nonpharmacologic (fluid/salt intake, compression stockings) and pharmacologic (fludrocortisone, midodrine) for OH; symptomatic treatment for POTS and autoimmune autonomic ganglionopathy (AAG).
- Neurodegenerative conditions like synucleinopathies (MSA, Parkinson’s with autonomic failure) and paraneoplastic syndromes are critical differential diagnoses for chronic autonomic failure.
1. DEFINITION & OVERVIEW¶
The autonomic nervous system (ANS) regulates involuntary functions via sympathetic and parasympathetic pathways. Sympathetic activation mediates 'fight or flight' responses, while parasympathetic activity promotes 'rest and digest' functions. The ANS includes central (CNS), peripheral (PNS), and enteric (ENS) components, with the ENS operating independently but regulated by the CNS and PNS.
Table 451-1: Effects of Sympathetic and Parasympathetic Systems on Various Effector Organs¶
| SYMPATHETIC | PARASYMPATHETIC |
|---|---|
| Pupil: Pupillodilation (alpha) | Pupil: Pupilloconstriction |
| Heart: Positive chronotropic effect (beta) | Heart: Negative chronotropic effect |
| Heart: Positive inotropic effect (beta) | Heart: Negative inotropic effect |
| Arteries: Vasoconstriction (alpha) | Arteries: Vasodilation |
| Veins: Vasoconstriction (alpha) | Veins: Vasoconstriction (beta) |
| Tracheobronchial tree: Bronchodilation (beta) | Tracheobronchial tree: Bronchoconstriction |
| Gastrointestinal tract: Decreased motility (beta) | Gastrointestinal tract: Increased motility |
| SYMPATHETIC | PARASYMPATHETIC |
|---|---|
| Bladder: Detrusor relaxation (beta) | Bladder: Detrusor contraction |
| Salivary glands: Scant, thick, viscid saliva (alpha) | Salivary glands: Copious, thin, watery saliva |
| Skin: Piloerection (cutis anserina) | |
| Sweat glands: Increased secretion (cholinergic) | Sweat glands: Decreased secretion |
| Adrenal medulla: Catecholamine release | |
| Glycogen: Glycogenolysis (alpha and beta) | Glycogen: Glycogen synthesis |
| Lipolysis (alpha and beta) |
1.1 ANATOMIC, BIOCHEMICAL, AND PHARMACOLOGIC ORGANIZATION¶
Sympathetic preganglionic neurons originate in the thoracolumbar spinal cord (T1–L2), while parasympathetic preganglionic neurons arise from craniosacral centers (brainstem and sacral cord). Acetylcholine (ACh) is the primary neurotransmitter for both divisions, with nicotinic receptors mediating preganglionic and muscarinic receptors for postganglionic parasympathetic responses. Sympathetic postganglionic neurons use norepinephrine (NE).
1.2 CLINICAL RELEVANCE¶
ANS dysfunction underlies various syndromes, including orthostatic hypotension (OH), postural orthostatic tachycardia syndrome (POTS), and neurodegenerative disorders like multiple system atrophy (MSA) and Parkinson’s disease. It also contributes to autoimmune conditions such as autoimmune autonomic ganglionopathy (AAG) and paraneoplastic syndromes.
2. EPIDEMIOLOGY¶
Autonomic disorders are common, affecting both CNS and PNS. OH prevalence increases with age (5% <50 years, 20% >70 years). POTS is four to five times more common in women, typically presenting between 15–45 years. Neurodegenerative conditions like MSA and Parkinson’s with autonomic failure are less common but have significant morbidity.
2.1 Risk Factors¶
Age, diabetes, neurodegenerative diseases (e.g., Parkinson’s), autoimmune conditions (e.g., AAG), and chronic illnesses (e.g., heart failure, kidney disease) increase risk. Genetic factors (e.g., Fabry’s disease, familial dysautonomia) and environmental triggers (e.g., infections, medications) also contribute.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
Dysautonomia arises from CNS/PNS damage, neurodegeneration (e.g., synucleinopathies), autoimmune processes (e.g., AAG), or metabolic disorders (e.g., diabetes, amyloidosis). Pathophysiology involves impaired sympathetic/parasympathetic signaling, baroreflex dysfunction, and autonomic neuropathy.
Table 451-2: Classification of Clinical Autonomic Disorders¶
| I. Autonomic Disorders with Brain Involvement | II. Autonomic Disorders with Spinal Cord Involvement | III. Autonomic Neuropathies |
|---|---|---|
| A. Associated with multisystem degeneration (e.g., MSA, Parkinson’s with autonomic failure) | A. Traumatic quadriplegia | A. Acute/subacute autonomic neuropathies |
| I. Autonomic Disorders with Brain Involvement | II. Autonomic Disorders with Spinal Cord Involvement | III. Autonomic Neuropathies |
|---|---|---|
| B. Unassociated with multisystem degeneration (e.g., limbic encephalitis) | B. Syringomyelia | B. Chronic peripheral autonomic neuropathies |
| C. Disorders of the hypothalamus (e.g., thiamine deficiency, diabetes insipidus) | C. Subacute combined degeneration | C. Small fiber neuropathy |
| D. Disorders of the brainstem and cerebellum (e.g., Arnold-Chiari malformation) | D. Multiple sclerosis and neuromyelitis optica | D. Combined sympathetic and parasympathetic failure |
| E. Amyotrophic lateral sclerosis (mild, late) | E. Amyotrophic lateral sclerosis (mild, late) | E. Amyloidosis |
| F. Tetanus | F. Tetanus | F. Diabetic autonomic neuropathy |
| G. Stiff-person syndrome | G. Stiff-person syndrome | G. AAG (paraneoplastic and idiopathic) |
| H. Spinal cord tumors | H. Spinal cord tumors | H. Hereditary sensory and autonomic neuropathy |
3.1 Neurodegenerative Causes¶
Synucleinopathies (MSA, Parkinson’s with autonomic failure, DLB) feature α -synuclein aggregates in CNS and PNS. MSA presents with autonomic failure and parkinsonism/cerebellar syndromes. Parkinson’s with autonomic failure includes orthostatic hypotension and bladder dysfunction.
3.2 Autoimmune and Paraneoplastic¶
AAG is an autoimmune disorder with anti-ganglionic ACh receptor antibodies. Paraneoplastic syndromes (e.g., small cell lung cancer) may cause autonomic neuropathy via immune-mediated mechanisms.
4. CLINICAL FEATURES¶
Symptoms vary by etiology: OH (lightheadedness, syncope), POTS (tachycardia on standing), and neurodegenerative syndromes (autonomic failure, parkinsonism). Autonomic dysfunction may also manifest as thermoregulatory abnormalities, sicca syndrome, or gastrointestinal dysmotility.
4.1 Orthostatic Hypotension (OH)¶
Defined as ≥ 20 mmHg SBP or ≥ 10 mmHg DBP drop within 3 min of standing. Symptoms include lightheadedness, syncope, and fatigue. Neurogenic OH is associated with blunted compensatory tachycardia (<15 beats/min).
4.2 Postural Orthostatic Tachycardia Syndrome (POTS)¶
Characterized by ≥ 30 beats/min HR increase within 10 min of standing. Common in young women; associated with autonomic neuropathy, venous pooling, and deconditioning.
5. DIFFERENTIAL DIAGNOSIS¶
Differential diagnoses include cardiovascular causes (e.g., arrhythmias, pheochromocytoma), endocrine disorders (e.g., hyperthyroidism), and psychiatric conditions (e.g., anxiety). Neurodegenerative disorders (e.g., MSA, Parkinson’s) and autoimmune conditions (e.g., AAG) must be considered in chronic cases.
6. INVESTIGATIONS & DIAGNOSIS¶
Diagnostic workup includes autonomic testing (heart rate variability, Valsalva maneuver, tilt table testing), QSART, skin biopsy, and imaging (e.g., MRI for MSA). Laboratory tests assess for diabetes, amyloidosis, and autoimmune markers.
6.1 Autonomic Testing¶
Heart rate variability to deep breathing, Valsalva maneuver, and tilt table testing evaluate baroreflex function. QSART measures sudomotor function, while skin biopsy detects small-fiber neuropathy.
7. MANAGEMENT & TREATMENT¶
Management is tailored to etiology: fluid/salt intake, compression stockings for OH; fludrocortisone/midodrine for neurogenic OH; beta-blockers for POTS; and immunotherapy for AAG. Symptomatic treatment includes addressing comorbidities (e.g., diabetes, autoimmune conditions).
7.1 Pharmacologic Interventions¶
Fludrocortisone (0.1–0.2 mg/d), midodrine (2.5–15 mg TID), and pyridostigmine (30–60 mg TID) are first-line for OH. Ivabradine may be used for refractory tachycardia. Droxidopa is approved for neurogenic OH.
7.2 Nonpharmacologic Measures¶
Increased fluid/salt intake, compression stockings, and exercise (recumbent cycling) improve autonomic function. Avoiding triggers (e.g., heat, alcohol) and managing comorbidities (e.g., diabetes) are critical.
8. PROGNOSIS & COMPLICATIONS¶
Prognosis varies: OH has a 44% mortality rate over 30 months. Neurodegenerative conditions like MSA progress relentlessly, with median survival 7–10 years. Complications include falls, syncope, cardiovascular events, and end-organ damage.
9. SPECIAL CONSIDERATIONS¶
Pregnancy, pediatrics, and elderly patients require tailored approaches. OH is more common in older adults, while POTS is prevalent in young women. Autoimmune conditions like AAG and paraneoplastic syndromes require multidisciplinary management.
10. KEY POINTS & CLINICAL PEARLS¶
- Autonomic dysfunction spans a spectrum from transient OH to chronic neurodegenerative syndromes. 2. Diagnostic tests (e.g., tilt table, QSART) are essential for classification. 3. Management is symptom-driven, with emphasis on nonpharmacologic interventions. 4. Neurodegenerative and autoimmune causes require early recognition to optimize outcomes.