The USMAR (ulnar SNAP/CMAP ratio) distinguishes ganglionopathy from sensory polyneuropathy.
USMAR = SNAP ulnar (µV) / CMAP ulnar (mV)
Right (R)
Left (L)
What is USMAR and how to interpret it
The USMAR (ulnar SNAP/CMAP ratio) compares sensory and motor amplitudes of the ulnar nerve. A reduced ratio favors ganglionopathy; preserved ratio with low SNAP favors polyneuropathy.
The USMAR index (Ulnar Sensory-Motor Amplitude Ratio) is a quantitative electrophysiological biomarker used to differentiate sensory neuronopathies (ganglionopathies - GNP) from predominantly sensory polyneuropathies (PNP). This calculator processes ulnar nerve amplitudes entered by the examiner and stratifies the risk of restricted dorsal root ganglion involvement.
1. What Does the Index Reflect? (Pathophysiology)
Sensory neuronopathies (ganglionopathies)
Damage occurs directly in the dorsal root ganglion (sensory neuron cell body), producing exclusively sensory injury. SNAP amplitude falls dramatically and disproportionately while the motor pathway remains largely preserved. The result is a very low ratio.
Polyneuropathies (predominantly sensory)
Even in primarily sensory presentations, there is usually some degree of subclinical motor involvement. Because CMAP (the denominator) also declines together with SNAP, the proportion tends to stay more balanced - yielding an index higher than in ganglionopathies.
2. Why Use the Ulnar Nerve?
Ulnar nerve selection is strategic: it avoids confusion from carpal tunnel syndrome. The median nerve is often focally compressed at the wrist, reducing amplitudes independently and potentially causing distorted indices and false positives.
3. The Formula
USMAR = Ulnar SNAP amplitude (µV) / Ulnar CMAP amplitude (mV)
4. Result Interpretation
USMAR < 0.71 - Abnormal (ganglionopathy)
Cutoff for abnormality. 94% sensitivity and 90% specificity for GNP vs. PNP.
USMAR ≥ 0.71 - Normal / polyneuropathy
Preserved ratio indicates proportional sensory and motor damage, compatible with polyneuropathy or healthy controls.
Means observed in the original study
0.3
Ganglionopathy (GNP)
1.5
Polyneuropathy (PNP)
4.6
Healthy controls
5. Clinical Application
A considerably low USMAR (< 0.71) should prompt evaluation for autoimmune (e.g. Sjögren syndrome), paraneoplastic, or toxic causes (e.g. cisplatin). The index is a screening tool - always interpret in full clinical context.
References:
· França MC Jr, Elias Junior J, Boa Sorte NC, Marques Júnior W. Ulnar sensory-motor amplitude ratio: a new tool to differentiate ganglionopathy from polyneuropathy. Arq Neuropsiquiatr. 2013;71(7):461–464. doi:10.1590/0004-282X20130063.
· Camdessanché JP et al. The pattern and diagnostic criteria of sensory neuronopathy. Brain. 2009;132:1723–1733.
Frequently asked questions
Does USMAR differentiate ganglionopathy from polyneuropathy?
Yes. In sensory-motor ganglionopathy, ulnar SNAP is disproportionately low vs CMAP; in diffuse axonal polyneuropathy both fall proportionally.
How to calculate USMAR?
USMAR = ulnar SNAP amplitude / ulnar CMAP amplitude at the same segment.