The split-elbow index: A biomarker of the split elbow sign in ALS

Objective

The split elbow sign is a clinical feature of amyotrophic lateral sclerosis (ALS), characterised by preferential weakness of biceps brachii muscle compared to triceps. A novel neurophysiological index, termed the split elbow index (SEI), was developed to quantify the split-elbow sign, and assess its utility in ALS.

Methods

Clinical and neurophysiological assessment was prospectively undertaken on 34 ALS patients and 32 ALS mimics. Compound muscle action potential (CMAP) amplitude was recorded from biceps brachii and triceps muscles from which the SEI was calculated using the following formula:

$\mathrm{SEI}=\frac{\mathrm{CMAPamplitudeBICEPSBRACHII}}{\mathrm{CMAPamplitudeTRICEPSBRACHII}}$

Results

The split elbow sign was significantly more common in ALS patients when compared to ALS mimic patients (P < 0.05). The SEI was significantly reduced in ALS patients when compared to ALS mimics (P < 0.01). This reduction was evident in spinal and bulbar onset ALS. A SEI cut-off value of ≤0.62 exhibited a sensitivity of 71% and specificity of 61%.

Conclusions

The split elbow sign is significantly more common in ALS patients, and was supported by a reduction in the SEI.

Significance

The SEI may be utilised as a surrogate biomarker of the split elbow sign in future ALS studies.