Thoracic outlet syndrome due to accessory shoulder muscle
Neurogenic thoracic outlet syndrome (NTOS) is a difficult to diagnose condition that is characterized by brachial weakness, pain and sensory deficits. It has a prevalence of 8% in in a healthy population, affecting women four times more frequently than men. The subclavius posticus muscle (SPM) is an irregular muscle first described in the early 19th century that is shown to originate on the medial aspect of the first rib and attach on the superior border of the scapula. In this current case study the authors attempted to shed new light on NTOS by presenting a patient with NTOS due to dynamic neurogenic occlusion from this aberrant muscle.
A 32 year old healthy male presented with intermittent reduced hand strength with no signs of steady weakness or pain. Electromyography showed a diminished conduction suggesting carpal tunnel syndrome (CTS). Previous research had shown NTOS may facilitate the occurrence of CTS. Consistent weakness was found in pronation, radial deviation and elevation of the hand during arm abduction. From this the authors predicted a possible brachial plexus pathology and verified the presence of SPM on MRI.
During arm abduction and weight bearing the costoclavicular space narrowing is exaggerated and therefore the authors state that muscle strength testing needs to be performed in the positions that the patient reports weakness. Despite this being the second case of NTOS due to SPM previous anatomical studies have verified the presence of SPM in 8.9% of the population. Taken together, the case presented is clinically valuable due to the relative prevalence of this accessory muscle and its possible contribution to a commonly made clinical diagnosis.
New research is beginning to emerge associated with the anatomical variation found in TOS patients. Further studies are needed to validate more accurate diagnostic criteria to better identify the contributing mechanism to NTOS.
> From: Muellner et al., BMC Res Notes 8 (2015) 351. All rights reserved to The Author(s). Click here for the Pubmed summary.