Neurogenic thoracic outlet associated with carpal tunnel syndrome: is this a clinical example of double crush hypothesis?

ABSTRACT

The aim of this study was to assess the possible contribution of the double crush hypothesis [DCH] for the association of carpal tunnel syndrome [CTS] with neurogenic thoracic outlet syndrome [NTOS]which -if confirmed- can explain some of the not uncommonly persistent CTS cases despite being confirmed and properly treated, particularly that NTOS is potentially treatable. 137 CTS patients [91 [66.4%] females, 46[33.6%] males], with mean age 39.4, ranging from 23 to 48 years, were included in the study based solely on electrophysiological criteria of CTS [distal motor latency to abductor pollicis brevis > 4 ms, 3 rd digit to wrist orthodromic sensory conduction velocity < 45m/s, or orthodromic median/ulnar latency difference of the 4[th] digit > 0.4 ms]. Patients who proved suffering peripheral neuropathy or entrapment of ulnar nerve were excluded. Twenty apparently healthy individuals, age and sex matched with patients were included as a control group. The patients and control groups were subjected to clinical neurological evaluation. Electrophysiological work up including motor/sensory conduction study of median and ulnar nerves on both sides, and bilateral medial antebrachial cutaneous nerve [MABCN] antidromic sensory, considering side to side MABCN sensory nerve action potentials [MABCN SNAP] amplitude ratio of >2.0 as abnormal. Electrophysiological criteria used for confirming [NTOS], were low median compound motor action potentials [CMAP], low ulnar SNAP, low or normal ulnar CMAP, normal or reduced interference pattern of C8 T1-innervated muscles, and MABCN SNAP interside amplitude ratio >2.0[the latter was used as a mandatory inclusion criterion]. Patients with atypical upper limb pain have undertaken cervical plain X ray. Control group was subjected to complete neurophysiological studies. Student t test was used to compare means of two groups. Ulnar nerve SNAP amplitudes were found normal. Reduction of median CMAP was reported in 19 patients [13.86%] and it was bilateral in 7 [36.84%]. Antidromic MABCN SNAP interside amplitude ratios showed values <2.0 and mean +/- SD was 1.273 +/- 0.221. Needle examination showed incomplete interference pattern in abductor pollicis brevis muscles in 11 patients [8%]. 3 patients [5.26%], 2 males, and 1 female had bilateral bony cervical ribs but non had evidence of NTOS. Neurophysiologically confirmed CTS was not proved to associate NTOS, and the hypothesized relationship between them could not be obtained. This might inspire us to revisit DCH for re-evaluation. Finally, NTOS is still a rare medical condition and scrutinizing suspected cases with thorough clinical assessment, and electrophysiological work up is a must