Feature Article - Sensitivity of Neural Tissue to Movement (Mechanosensitivity)

Mechanosensitivity is likely to be a key factor in many patients' neurodynamic problems, such as those that produce pins and needles, numbness related to nerve entrapment or simply nerve-like symptoms with daily activities. The problem may exist in both upper and lower extremities such as with conditions involving carpal tunnel syndrome, elbow pain, thoracic outlet, cervical and lumbar nerve root and sciatica problems and foot pain.

What is Mechanosensitivity?

Michael Shacklock has defined it in specific terms in his book Clinical Neurodynamics.

OVERVIEW

Mechanosensitivity is the mechanism by which neural structures can become a source of pain with movements and postures. This is because it allows the production of afferent impulses from the neural structure for processing in the central nervous system.

DEFINITION

Mechanosensitivity is how easily the neural tissues become electrically active (produce impulses) when mechanical force is applied to the neural tissues. The more sensitive the nerve is, the less force is needed to produce activity and the more intense is the response.

Causes of Abnormal Mechanosensitivity

Various causes of mechanosensitivity have been shown to occur but the main one is by inflammatory changes in the neural tissue (Eliav et al 2001; Dilley et al 2005). During inflammation (which may be caused by compression or irritation by neighboring musculoskeletal (interface) tissues), blood flow in the nerve or nerve root increases and swelling may occur. Swelling in nerves can be readily observed with the use of high resolution ultrasound.

Figure A. Normal Median Nerve at the Wrist © NDS 2008Figure A. Ultrasound scan of a cross section of the median nerve at the wrist - normal nerve (see asterisk * in picture).
From Shacklock 2005, Clinical Neurodynamics, Elsevier, Oxford.*


Figure B. Swollen Median Nerve at the Wrist © NDS 2008Figure B. Abnormal nerve in case of carpal tunnel syndrome. It looks more like a kidney bean than a soft pliable nerve! The nerve is swollen, darker and has increased fluid in it compared with A. It is also producing upward displacement of the transverse carpal ligament due to pressure in the tunnel. The ligament bow strings upward more than in A. This severity of swelling is sometimes visible on the ventral aspect of the wrist over the carpal tunnel.

Figure AFigure A. MNT1 for a patient with a neural problem in upper extremity. Range of elbow extension (approx. 90°) may reflect the threshold of sensitivity with applied force.

Figure BFigure B. MNT1 again after treatment. There is an increase in range of motion of elbow extension at the onset of symptoms which may reflect an improvement in neurodynamics (mechanics or physiology), particularly mechanosensitivity - increased movement, more force, less nerve response correlating with reduced sensitivity.

Changes in nerve palpation immediately after treatment may also indicate an alteration in mechanosensitivity in the nerve. These changes may be reflected in reducing pain or pins and needles with palpation at a pressure similar to before and after treatment.

Lower limb equivalents of this phenomenon would consist of increases in the straight leg raise ROM or the knee extension component of the slump test.

Figure CFigure C. Palpation of the median nerve at the wrist proximal to the carpal tunnel where the nerve sometimes becomes swollen and painful. Note that the palmaris longus digitorum tendons have been displaced manually to gain access to the nerve.

What Might the Cause of the Improvement Be?

This is open to speculation but it is likely to be influenced by what treatment has been applied.

It could be that:

• the interface pressure on the nerve has been released through a manual technique
• it has something to do with the mechanical function of the tissues around the neural structure producing less irritation of the neural structure
• the treatment might have produced altered blood flow or sensitivity of the neural structure through direct mechanical effects on the nerve
• the central pain mechanisms may also be a factor, but these are not the subject of this article