A key element of examination of the physical health and sensitivity of the nervous system (neurodynamics) is structural differentiation (SD). Based on the fact that the nervous system is a continuum, the aim of SD is to provide a clinical diagnosis of whether or not the nervous system is physically compromised in a pain state.
Two examples are below. If spinal flexion hurts in the lumbar spine and neck extension relieves evoked responses, then the inference is that the nervous system is physically compromised, although further clinical data will be need to ascertain where the compromise is. Note that the lumbar and thoracic spines have not been moved only the neck. In the second example, stretch of the upper trapezius muscle could be mechanically loading neural and muscle tissue. If the response is enhanced with the elbow extended, the inference is that neural tissues may be physically compromised.
Fig 10.4 From Butler DS 2000 The Sensitive Nervous System, Noigroup, Adelaide
Michel Coppieters (2006) and colleagues at the University of Queensland carried out a nice piece of cadaveric research on structural differentiation at the ankle. They demonstrated that hip flexion will increase the strain in the tibial nerve at the ankle without altering the strain on the plantar fascia – can you see the diagnostic possibilities here with plantar fasciitis? Clinicians who use the slump test to assist in analysis of plantar fasciitis may have noted that neck movement can alter “plantar fasciitis” symptoms.
It would be nice if it was all that easy! Remember – SD is crude, it calls for skilled manual handling and results need to be taken with other clinical data. And while clear cut responses may occur in pain states involving peripheral neural tissues, it will be a bit more complex and a clinical headache where central sensitisation is a clinical feature. For example, traditional SD teaching is that if ankle dorsiflexion increases lumbar or pelvic responses evoked by a SLR, then “its neural – something is tight or stuck” . However, with central sensitisation, the addition of ankle dorsiflexion could just provide more normal movement input adding to an exiting barrage into the CNS. In future blogs, I will discuss central sensitisation and physical examination of neural tissues.
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Coppieters, M. W., A. M. Alshami, et al. (2006). "Strain and excursion of the sciatic, tibial and plantar nerves during a modified straight leg raise test." J Orthop Res 24: 1883-1889.