Definition and Overview
The sympathetic skin response (SSR) is an autonomic function test that measures changes in skin electrical resistance caused by alterations in sympathetic nerve activity of cutaneous sweat glands in response to various stimuli, including electrical stimulation, deep breathing, and sudden auditory stimuli.
SSR reflects the function of the sympathetic cholinergic pathway innervating eccrine sweat glands. A normal response requires integrity of the efferent pathway from the brainstem through the spinal cord lateral horn to the postganglionic sympathetic fibers [1].
Principles and Measurement Methods
Test Principle
When a stimulus is applied, sympathetic nerve activity to the cutaneous sweat glands increases, causing changes in skin surface electrical conductance. These potential changes are recorded using surface electrodes.
Stimulation Methods
- Electrical stimulation: Single electrical pulse (0.1-1 ms, 10-30 mA) applied to the wrist or ankle
- Deep breathing stimulation: Deep inspiration or Valsalva maneuver
- Auditory stimulation: Sudden loud sound
Recording Method
The active electrode is placed on the palm or sole, and the reference electrode on the dorsum of the hand or foot. A potential change is observed after a latency of approximately 1.5-3 seconds following the stimulus [3].
Measurement Parameters
- Latency: Time from stimulus to response onset
- Amplitude: Magnitude of the potential change
- Waveform morphology: Monophasic or complex waveform
- Response presence: Absent or present
Clinical Significance
Diabetic Autonomic Neuropathy
SSR abnormalities (absent response or prolonged latency) are observed in approximately 72-87% of patients with diabetic autonomic neuropathy [4]. SSR abnormalities may precede clinical symptoms, making it useful for early detection.
Small Fiber Neuropathy
SSR assesses the sympathetic pathway through unmyelinated C fibers and small-diameter A-delta fibers. SSR abnormalities may be present even when nerve conduction studies are normal, making it a useful adjunctive tool for diagnosing small fiber neuropathy [2].
Parkinson Spectrum Disorders
SSR abnormalities have been reported in Parkinson disease, multiple system atrophy (MSA), and pure autonomic failure (PAF). SSR loss is more pronounced in MSA and PAF compared to Parkinson disease, serving as a reference for differential diagnosis.
Anhidrosis
SSR is utilized in evaluating the cause of focal or generalized anhidrosis. Absent SSR in the affected area suggests peripheral sympathetic fiber damage.
Diagnosis
Normal Reference Values
- Palm: Latency 1.5-2.5 seconds (upper extremity stimulation)
- Sole: Latency 2.0-3.0 seconds
- Age-adjusted normative values are needed, as latency increases with age
Abnormal Findings
Criteria for abnormal SSR:
- Absent response
- Prolonged latency (exceeding normal reference values)
- Significantly reduced amplitude (criteria vary between studies)
Test Limitations
SSR is affected by various factors including habituation from repeated stimulation, skin temperature, arousal state, anxiety, and medications (anticholinergics, etc.). The lack of standardized test protocols and normative values results in inter-examiner variability [2]. Therefore, interpretation in conjunction with other autonomic function tests rather than as a standalone test is recommended.
Treatment
SSR is a diagnostic test, so no direct treatment exists for the test itself. When SSR abnormalities are confirmed, treatment of the underlying condition (diabetic autonomic neuropathy, small fiber neuropathy, Parkinson spectrum disorder) takes priority.
In diabetic autonomic neuropathy, improved glycemic control is important for preserving autonomic function, while small fiber neuropathy requires individualized treatment based on the underlying cause.