Increased Pressure with CTS

 There are many pressure related studies of the carpal
tunnel in humans [45-47]. Normal pressure has been
recorded to be in the range of 2-10 mm Hg [4]. There are
dramatic changes of fluid pressure in the carpal tunnel with
wrist position; extension increases the pressure 10-fold and
wrist flexion increases it 8-fold [44]. Therefore, repetitive
hand movements have been implicated as one of many risk
factors for CTS. Experimental studies have suggested a
dose-response curve - the greater the duration and amount of
pressure, the more significant is the neural dysfunction [48].
Nerve Injury
 An important step in injury to the median nerve is
demyelination, which occurs when the nerve is repetitively
subjected to mechanical forces [2]. Pressures much higher
than systolic are necessary to produce focal demyelination
[4]. Demyelination of the nerve develops in the compression
site, and can then spread to the entire internodal segment,
leaving the axons intact. A block of nervous transmission
ensues (neuroapraxia). If the compression persists, bloodflow
to the endoneural capillary system may be interrupted,
leading to alterations in the blood-nerve barrier, and
development of endoneural oedema. This starts a vicious
cycle consisting of venous congestion, ischaemia and local
metabolic alterations [2]. Axonal degeneration, macrophage
attraction and activation, release of inflammatory cytokines,
nitric oxide, and development of “chemical neuritis” are all
consequences of this viscous cycle if it continues for a
substantial amount of time [10].
Nerve Tethering
 Nerve fibres have layers of connective tissue: the
mesoneurium, epineurium, perineurium and endoneurium;
which is the most intimate layer. The extensibility of these
layers is critical to nerve gliding, which is necessary to
accommodate joint motion; otherwise nerves are stretched
and become injured [49].
 The median nerve will move up to 9.6 mm with wrist
flexion and slightly less with extension [50]. Chronic
compression results in fibrosis, which inhibits nerve gliding,
leading to injury and therefore scarring of the mesoneurium.
This causes the nerve to adhere to the surrounding tissue,
resulting in traction of the nerve during movement as the
nerve attempts to glide from this fixed position [32]. This is
the basis of the tethered median nerve stress test (TMNST),
which is can be used to diagnose chronic low-grade CTS