What are the changes in sweating patterns in patients with autonomic neuropathy?

Changes in Sweating Patterns in Autonomic Neuropathy

Autonomic neuropathy causes sudomotor dysfunction that manifests as either increased or decreased sweating, with the most common pattern being distal anhidrosis (reduced or absent sweating) in the feet and lower extremities, often accompanied by compensatory hyperhidrosis in the upper body and trunk. 1

Primary Sweating Abnormalities

Anhidrosis (Decreased Sweating)

• Distal anhidrosis affecting the feet and lower legs is the hallmark finding, occurring because postganglionic, thin, unmyelinated cholinergic sympathetic C-fibers that innervate sweat glands are damaged in a length-dependent pattern 1, 2
• The pattern typically follows a "stocking" distribution, beginning in the feet and progressing proximally as neuropathy advances 3, 4
• Anhidrosis is associated with dry, cracked skin in the extremities and increases risk of foot ulceration in diabetic patients 1
• In severe cases with ≥25% body surface anhidrosis, patients demonstrate significantly impaired thermoregulation with more rapid core temperature rise during heat exposure 5

Compensatory Hyperhidrosis

• When distal anhidrosis develops, many patients experience compensatory hyperhidrosis in unaffected areas, particularly the upper trunk, face, and neck 6
• This compensatory pattern occurs as the body attempts to maintain thermoregulation despite loss of distal sweating capacity 6

Mixed Patterns

• Some patients exhibit patchy or irregular sweating patterns rather than purely distal anhidrosis 1
• The American Diabetes Association notes that sudomotor dysfunction can present with "either increased or decreased sweating" depending on which autonomic pathways are affected 1

Clinical Significance

Impact on Quality of Life

• Sudomotor dysfunction impairs quality of life through skin discomfort, itching from dry skin, and heat intolerance 1
• Dry, cracked skin from anhidrosis creates entry points for infection and increases foot ulceration risk in diabetic patients 1

Diagnostic Value

• Sweating abnormalities are highly sensitive markers of autonomic neuropathy, with sweat testing abnormal in 80-90% of patients with small fiber neuropathy 7
• Sudomotor testing detects autonomic dysfunction earlier and more frequently than cardiovascular autonomic tests, as sympathetic sudomotor fibers are affected before cardiac autonomic nerves 7
• The indicator plaster test shows 59-82% sensitivity for detecting autonomic neuropathy, though specificity is lower at 27-46% 1

Diagnostic Testing Options

Quantitative Sudomotor Axon Reflex Test (QSART)

• QSART is the gold standard for evaluating postganglionic sudomotor function, measuring sweat production after acetylcholine iontophoresis 1
• Sensitivity ranges from 54-90% in small fiber neuropathy, improving to 98% when combined with thermoregulatory sweat testing 1
• Limitations include inability to detect preganglionic lesions, requirement for specialized equipment, and high cost 1

Thermoregulatory Sweat Test (TST)

• TST evaluates both central and peripheral sudomotor pathways by raising core temperature to 38°C and measuring body surface anhidrosis 1
• Sensitivity is 72-96% for detecting neuropathy, with results expressed as percentage of anterior body surface showing anhidrosis 1
• TST abnormalities correlate with neuropathy severity scores and intraepidermal nerve fiber density 1

Sympathetic Skin Response (SSR)

• SSR measures electrodermal activity as a surrogate for sudomotor function 1
• Shows 87.5% sensitivity and 88.2% specificity for diagnosing diabetic autonomic neuropathy 1
• Important caveat: responses may be absent in healthy individuals over age 50, limiting specificity 1

Indicator Plaster Test

• Simple screening method using cobalt compound that changes color (blue to pink) with foot perspiration after 10 minutes 1
• Correlates well with neuropathy disability scores, cold detection thresholds, and reduced intraepidermal nerve fiber density 1
• Sensitivity 85-95% for peripheral neuropathy but lower (59-82%) for autonomic neuropathy specifically 1

Pathophysiology

• Sweat glands are innervated exclusively by postganglionic, unmyelinated cholinergic sympathetic C-fibers 1
• These same C-fibers contribute to microvascular blood flow regulation, explaining why sweating abnormalities often accompany circulatory instability in the feet 1, 2
• C-fiber dysfunction occurs early in diabetic peripheral neuropathy, making sudomotor testing valuable for early detection 1
• The length-dependent pattern of nerve damage explains why anhidrosis begins distally and progresses proximally 3, 4, 5

Common Pitfalls

• Do not dismiss normal cardiovascular autonomic testing as excluding autonomic neuropathy—sudomotor fibers are affected more frequently and earlier than cardiac autonomic nerves 7
• Avoid performing SSR in patients over 50 years without recognizing that absent responses may be normal for age 1
• QSART results are highly sensitive to temperature, humidity, hydration status, and caffeine intake—standardize testing conditions 1
• Recognize that "changes in sweat function" on screening questionnaires require specific inquiry about both decreased sweating (anhidrosis) and increased sweating (hyperhidrosis) patterns 1