By Michael Rubin, MD
Professor of Clinical Neurology, Weill Cornell Medical College
SYNOPSIS: Unusual patterns of weakness in the various motor neuron disorders may point to a specific diagnosis of amyotrophic lateral sclerosis, if supported by electrophysiological evidence of chronic denervation.
SOURCE: Hamada Y, Kanbayashi T, Takahashi K, et al. Weak shoulder and arm sparing signs in amyotrophic lateral sclerosis. Muscle Nerve 2022;65:311-316.
Selective muscle involvement in amyotrophic lateral sclerosis (ALS) is well described, encompassing the split hand syndrome (hand muscle wasting preferentially affecting the lateral hand muscles, including the abductor pollicis brevis and first dorsal interosseous, with relative sparing of hypothenar muscles), split-hand plus sign (flexor pollicis longus relatively spared compared to thenar muscles), split-leg sign (ankle dorsiflexion preferentially affected over plantar flexion), split-elbow sign (preferential weakness of biceps over triceps), split-finger sign (flexor digitorum profundus to index finger more severely affected than to the little finger), and relative preservation of finger flexion compared to finger extension. Presently, two new signs are described, which may help differentiate ALS from multifocal motor neuropathy (MMN).
ALS patients seen between 2015 and 2018 were identified using an electrodiagnostic database from the Departments of Neurology, Teikyo University School of Medicine, Tokyo, and Kitasato University School of Medicine, Sagamihara, Japan, and retrospectively reviewed. Inclusion criteria included clinical and electrodiagnostic diagnosis or suspicion of ALS using Gold Coast criteria1 (see “Commentary”), careful examination and Medical Research Council (MRC) grading of arm muscles, exclusion of other disorders, and follow-up confirmation of ALS diagnosis. For comparison, patients from the same time period with cervical spondylotic amyotrophy (CSA) and MMN also were identified. “Weak shoulder” sign was considered present when the deltoid was weaker than the biceps and triceps, or weaker than one but no stronger than the other. Additionally, when both the first dorsal interosseous (FDI) and the deltoid were weaker than biceps and triceps, the “arm sparing” sign was considered present. Statistical analysis comprised the Mann-Whitney and McNemar tests, with P < 0.05 considered significant.
Among 130 ALS patients identified (76 men and 54 women) with mean age of 66.6 years, Gold Coast criteria were fulfilled in 93 (72%). By comparison, 64 patients with CSA were identified (55 men and nine women), mean age 60.9 years, and 16 patients with MMN (11 men and five women), with mean age of 44.3 years. ALS patients demonstrated the weak shoulder sign and the arm sparing sign in 73% and 55%, respectively, with the weak shoulder sign more sensitive for ALS than any of the other previously reported signs, such as the split finger or elbow sign. Neither the weak shoulder sign nor the arm sparing sign was observed in MMN patients, but proximal CSA patients had higher positive rates for the weak shoulder sign than the ALS group, with the arm sparing sign seen in only one CSA patient. As opposed to MMN, the weak shoulder sign is specific and highly sensitive for ALS, while the arm sparing sign is similarly highly specific.
COMMENTARY
Absent any definitive biomarkers, the diagnosis of ALS remains clinical in nature with electromyography (although not 100% sensitive) an essential diagnostic aid. Gold Coast criteria for diagnosis of ALS comprise progressive motor disability, upper and lower motor neuron dysfunction in one body region or lower motor neuron dysfunction in two body regions, exclusion of other causes, hyper-reflexia, pathological reflexes (Hoffman, Babinski, crossed adductor, snout), spasticity, and electrophysiologic evidence of lower motor neuron dysfunction in the form of positive waves or fasciculation potentials, with evidence of chronicity in the form of neurogenic, large motor units. Importantly, a compassionate outline of proposed care also is needed when conveying the diagnosis.2
REFERENCES
- Shefner JM, Al-Chalabi A, Baker MR, et al. A proposal for new diagnostic criteria for ALS. Clin Neurophysiol 2020;131:1975-1978.
- Turner MR, UK MND Clinical Studies Group. Diagnosing ALS: The Gold Coast criteria and the role of EMG. Pract Neurol 2022; Jan 6. doi: 10.1136/practneurol-2021-003256. [Online ahead of print].
