Many studies show that; given a more proximal root compression (for example root compression at cervical transverse foramina) less involvement of the median nerve across the carpal tunnel was required to produce symptoms of CTS (carpal tunnel syndrome).
The surgical outcome of carpal tunnel release in this double crush group is poorer than in that group with isolated carpal tunnel involvement. It is important to preoperatively identify those patients who may have double crush lesions and thus anticipate a less than optimal result from surgical release of the peripheral nerve.
So let us review what a double crush syndrome means?
The double-crush syndrome was initially described by Upton and McComas in 1973. Multilevel lesions along a peripheral nerve trunk do occur. In the double crush syndrome as postulated by Upton and McComas, the presence of a more proximal lesion does seem to render the more distal nerve trunk more vulnerable to compression.
Concrete support of existence of double crush syndromes from MRI studies:
Pierre-Jerome C et al assessed the coexistence of narrowed cervical foramens and cervical canal stenosis in patients with carpal tunnel syndrome (CTS) through MRI studies.
Study of MRI of 120 wrists and 480 foramens in 60 age and sex matched subjects (30 patients with CTS and 30 controls) revealed that; there was no difference in the size of the cervical canal. The higher incidence of narrowed cervical foramens in the patients and its concordance with affected nerve roots on the same side as the CTS symptoms support the hypothesis of a double-crush phenomenon.
While the exact pathophysiologic mechanism of this interaction is not yet elucidated, it most likely relates to disturbances in axonal flow kinetics and the disruption of the neurofilament architecture.
MacKinnon and Dellon have expanded the description of this syndrome to include
a) Multiple anatomic regions along a peripheral nerve
b) Multiple anatomic structures across a peripheral nerve within an anatomic region
c) Superimposed on a neuropathy, and
d) Combinations of the above.
e) Further Zahir KS et al has reported an unusual case of symptomatic nerve compression caused by two non-anatomic structures within an anatomic region.
According to the hypothesis of Upton and McComas:
1. Non-symptomatic impairment of axoplasmic flow at more than one site along a nerve might summate to cause a symptomatic neuropathy. This was suggested by their clinical observation that the majority of their patients had a median or ulnar neuropathy associated with evidence of cervicothoracic root lesions.
Other researchers have since reported series of patients supporting the frequent association of a proximal and distal nerve compression syndrome, including carpal tunnel syndrome associated with cervical radiculopathy, brachial plexus compression, and diabetic neuropathy.
2. They also hypothesized that one of the constraints on axoplasmic flow could be a metabolic neuropathy, and this is supported by the high association of diabetes and carpal tunnel syndrome.
Presentation of a unique case study by Putters JL:
Bilateral thoracic outlet syndrome with bilateral radial tunnel syndrome: a double-crush phenomenon.
A case of bilateral thoracic outlet syndrome combined with bilateral radial tunnel syndrome is reported. Persisting complaints in the upper extremities after bilateral first-rib resection and scalenotomy were due to radial nerve entrapment in the radial tunnel. Although this bilateral double-crush phenomenon is extremely rare, and has not been reported previously, persistence of symptoms after initial treatment of a nerve entrapment is an indication to search for another site of compression.
NB: This review is based on articles by Osterman AL, Zahir KS et al, Pierre-Jerome C et al and Putters JL in pubmed.