Burning Feet: Detection of small-fiber neuropathy by sudomotor testing

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Muscle Nerve. 2006 May 22

Detection of small-fiber neuropathy by sudomotor testing.

Low VA, Sandroni P, Fealey RD, Low PA.

Department of Neurology, Mayo Clinic, Guggenheim 811, 200 First

Street SW, Rochester, Minnesota 55905, USA.

The symptoms of burning sensation affecting the feet, thought to be

due to a distal small-fiber neuropathy (DSFN) affecting somatic

unmyelinated fibers, are usually accompanied by vasomotor or

sudomotor changes suggestive of involvement of autonomic fibers. We

therefore examined the relationship between pattern of anhidrosis and

DSFN and its etiology, comparing patients with " pure " DSFN (with

normal nerve conduction) to those with clinical DSFN (minor

conduction abnormalities).

We reviewed 125 cases with a clinical phenotype of DSFN. These

patients had distal burning discomfort, variable sensory deficits,

and intact motor function. All had undergone assessment with

thermoregulatory sweat test (TST), autonomic reflex screen (ARS), and

nerve conduction studies and electromyography (NCS/EMG).

TST showed a distal pattern of anhidrosis in 74%. The quantitative

sudomotor axon reflex test (QSART) was abnormal in 74%, with 80% of

those having a length-dependent pattern of anhidrosis/hypohidrosis.

In total, 93% of patients had a distal pattern of abnormality on

QSART or TST.

The Composite Autonomic Severity Score (CASS) was used to quantify

the severity and distribution of autonomic deficits: 98% had CASS

abnormality (sudomotor, 98%; adrenergic, 43%; cardiovagal, 35%). EMG

was normal or showed unrelated abnormalities in 75%. The most common

etiologies of DSFN were idiopathic (73%), presumed hereditary (18%),

and diabetes (10%).

Sudomotor examination is thus a highly sensitive detection tool in

DSFN. Autonomic involvement is mainly distal, and additionally may

involve adrenergic and the long cardiovagal fibers.