autonomic neuropathy
Summary
Autonomic neuropathy is damage to the autonomic nerve fibers controlling involuntary functions such as heart rate, blood pressure, digestion, bladder function, and sweating. It is most commonly caused by long-standing diabetes mellitus but can also result from amyloidosis, Guillain-Barré syndrome, Parkinson disease, or paraneoplastic syndromes. Clinical manifestations include orthostatic hypotension, gastroparesis, erectile dysfunction, resting tachycardia, and neurogenic bladder.
Detail
Autonomic neuropathy results from dysfunction of the sympathetic and/or parasympathetic nervous system fibers, often due to microvascular damage (as in diabetes) or infiltrative/degenerative processes. In diabetic autonomic neuropathy, chronic hyperglycemia leads to nonenzymatic glycosylation and microvascular injury (vasa nervorum ischemia) affecting small unmyelinated autonomic fibers, similar to the mechanism causing diabetic peripheral neuropathy and retinopathy.
Key clinical manifestations by system: - Cardiovascular: Orthostatic hypotension (due to impaired baroreceptor reflex and vasoconstriction), resting tachycardia (early sign due to vagal denervation), fixed heart rate that doesn't vary with respiration or exercise (loss of heart rate variability), and silent myocardial ischemia (due to impaired pain sensation from cardiac autonomic nerves) — a major risk in diabetics. - Gastrointestinal: Gastroparesis (delayed gastric emptying causing early satiety, nausea, vomiting), diabetic diarrhea (often nocturnal), constipation, esophageal dysmotility. - Genitourinary: Erectile dysfunction (often an early sign in diabetic men), neurogenic bladder (detrusor dysfunction leading to urinary retention or incontinence). - Sudomotor: Anhidrosis (impaired sweating, especially distally) or gustatory sweating. - Pupillary: Decreased pupillary light reflex.
Diagnosis involves clinical history, orthostatic vital sign measurement (drop in BP ≥20/10 mmHg upon standing), heart rate variability testing with deep breathing, and tilt-table testing. Diabetic patients are often screened annually for autonomic and peripheral neuropathy.
Management is largely symptomatic: fludrocortisone or midodrine for orthostatic hypotension, prokinetics (metoclopramide) for gastroparesis, and strict glycemic control to slow progression in diabetics. Other causes to consider include amyloidosis (AL or transthyretin-related), Guillain-Barré syndrome (acute dysautonomia), multiple system atrophy (Shy-Drager syndrome), paraneoplastic autonomic neuropathy (often anti-Hu antibodies), and toxic/drug-induced causes (chemotherapy agents like vincristine).
High-yield board associations: diabetic autonomic neuropathy causing resting tachycardia + orthostatic hypotension + gastroparesis; amyloidosis presenting with autonomic dysfunction plus peripheral neuropathy and cardiomyopathy; multiple system atrophy presenting with parkinsonism plus severe orthostatic hypotension distinguishing it from Parkinson disease.
Sources
- First Aid for the USMLE Step 1
- Harrison's Principles of Internal Medicine
- Kaplan USMLE Step 2 CK Neurology
- UpToDate: Diabetic autonomic neuropathy
Reviewed by AnkiBoss editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.