The Sporting Nerve Part 1 | The Sporting Nerve Part 2 | Piriformis | Contralateral Tests
Sciatica: Is Extension the Right Choice? | Trouble Shooting with Scapular Stabilisation
Wrist Technique during Median Neurodynamic Testing | Tarlov's Cysts
Asymmetry and Diagnosis: Are We Neurodynamic Mirror Images?
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.
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.
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.
Two things are important:
- Nerves can become inflamed and show increased responses to mechanical forces at low levels of stretch (Bove et al 2003; Dilley et al 2005).
- Inflammation in nerve can produce increased sensitivity whilst the axons can still conduct normally through the site of inflammation (Eliav et al 2001).
This may manifest as nerve pain with movement, much like an inflamed muscle or joint would, even though the nerve may be conducting quite normally ie. no neurological loss.
Mechanosensitivity has been shown to occur for many years but it is relatively recently that our understanding of it can be applied more comprehensively in clinical practice.
Key clinical implications of mechanosensitivity are as follows:
- Daily movements, palpation of nerves and neurodynamic tests may produce symptom responses that are related to mechanosensitivity.
- Hypotheses about mechanosensitivity are used to deduce what may be happening in the physiology of the neural structure with physical testing and treatment.
- These hypotheses are used to monitor progress.
For instance, if the median neurodynamic test 1 (MNT1) is used to reproduce symptoms in the upper limb. Symptoms (eg. pins and needles) occur at a certain range of motion of elbow extension (ROM-EE). The ROM-EE at which the symptoms start may be interpreted as the threshold for mechanosensitivity for that patient.
After treatment, whatever the technique, if the ROM-EE has increased, the clinical hypothesis is that the mechanosensitivity has decreased and the neurodynamics may have improved. This may mean that the force at which the nerve started to become active was greater therefore indicating that the physiology or mechanics, or both may have improved and the sensitivity is less.
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
Mechanosensitivity is a key issue with neural problems. This phenomenon is used to make hypotheses about the effects of treatment on nerve physiology, particularly inflammation and sensitivity of the neural structure. If the ROM of a neurodynamic test changes, this might reflect changes in the neural sensitivity, among other things. Through an understanding of mechanosensitivity, the therapist can make judgments about what is effective treatment.
Bove, G, Ransil B, Lin H-C, Leem J-G 2003 Inflammation induces ectopic mechanical sensitivity in axons of nociceptors innervating deep tissues. Journal of Neurophysiology 90: 1949–1955
Dilley A, Lynn B, Pang S 2005 Pressure and stretch mechanosensitivity of peripheral nerve fibres following local inflammation of the nerve trunk. Pain 117 (3):462-472
Eliav E, Benoliel R, Tal M 2001 Inflammation with no axonal damage of the rat saphenous nerve trunk induces ectopic discharge and mechanosensitivity in myelinated axons. Neuroscience Letters 311: 49-52