Workbook questiond 6 - Nerve entrapment syndromes Flashcards
Most peripheral nerves, such as the median nerve are commonly referred to as “mixed spinal nerves”. Identify 3 general characteristics of spinal nerves that give rise to use of the term “mixed spinal nerves”.
a) Directionality in which nerve impulses are conducted- afferents (conduction from sense organs to the CNS) and efferents (conduction from the CNS to the PNS)
b) Functional Modalities represented within the nerve, sensory, motor and autonomic
c) General or Special Modalities. This is really only true in the case of cranial nerves (e.g. visceral efferent; special sensory; general sensory
C) Myelination or lack of. Nerves are strictly divisible into two categories; either myelinated or unmyelinated. Myleinated axons can be further teased into lightly myelinated, or heavily myelinated.
D) The speed with which nerve impulses are conducted. The speed of nerve impulse conduction is heavily influenced by the level of myelination of the nerve in question. The speed of conduction in myleinated axons is directly proportional to the fibre cross-sectional diameter, whilst in unmyelinated axons it is
E) Thickness of axonal fibres. Whilst it is true that myelinated axons are more likely to have thicker fibres than unmyelinated axons, it is interesting to note that the cross-sectional diameter of an axon is directly proportional to the size of its cell body.
What is the explanation for the various waves of a compound action potential
The various waves of the compound action potential suggest that there are differences in the speed with which impulses are conducted in human peripheral nerves. In particular, it suggests that there are 3 general waves to the compound action potential, thus, implying that there must be 3 general categories with respect to relative speeds of impulse conduction.
Additionally, the first major wave is itself complex but relatively narrower with respect to the latter 2, with the last wave appearing much wider. The first general wave suggests that the speeds of conduction within the nerve fibres constituting it are tightly packed whilst for the latter two, these are more widely distributed, with the last wave, particularly wider.
When carrying out nerve conduction investigations of a specific mixed spinal nerve, which sensory modalities would you expect to be activated at just threshold intensities of stimulation (i.e. minimal intensities). Justify your answer.
Proprioceptors and in particular, muscle spindle afferents because these are the most heavily myelinated nerves in the body. Since myelin reduces the capacitance of axons, it follows that the most heavily myelinated axons would be easiest to bring to threshold by electrical stimulation since the will require relatively less current to activate them.
When carrying out nerve conduction investigations of a specific mixed spinal nerve, which sensory modalities would you expect to be activated last (i.e. maximal intensities of stimulation). Justify your answer
C-fibres (hence slow pain fibres). These are unmyelinated axons of the body. Their lack in myelin means that their axons have a relatively high capacitance, thus, making them particularly difficult to bring to threshold by electrical stimulation. As such, they are likely to be the last axons of the body to be activated by electrical stimulation.
History in this patient suggested that three weeks prior to the onset of pain, the affected hand became functionally weak. She had difficulty putting her signature to documents, gripping cutlery correctly during meals and finally her grip had weakened leading to dropping a mug full of tea. What is the explanation for these difficulties in this patient?
1) The nerve is under constant compression within the carpal tunnel by the progressively growing ganglion. This gives rise to a nerve compression syndrome. When nerves are gradually compressed, the most myelinated axons are most susceptible to the effects of compression, presumably due to disruption of their blood supply, leading to deprivation of nutritional substrate. In this case, the proprioceptive afferents are thus, being selectively knocked out of action, leading to the patient being unable to carry out skilled fine movements like putting her signature to documents. The difficulties may be arising due to lack of afferent feedback from muscle spindle afferent axons that have succumbed to compression 2) Whilst it is theoretically possible that although the motor deficits in this patient could be explained by defective signalling in proprioceptive afferents, the possibility of the compression of the nerves having progressed to also directly disrupt the signalling of motor axons has to be seen as an equally plausible explanation for the emergence of these motor deficits.
The patient described the problematic hand as not painful but as if “sometimes it was not mine”
What is the explanation for this lack of awareness of the hand here?
This is likely to have arisen from compression of proprioceptive elements of the nerves leading to a consequent lack of proprioceptive feedback from the hand. Thus, the brain was no longer aware of proprioceptive feedback from it, hence its alienation.
What is the explanation for a) Numbness? B) The sensation of pins and needles.
Anaesthesia due to compression of the nerve and its attendant blood supply.
Parasthesia due to compression. The pressure on the axons of the nerve due to compression may have led them to fire action potentials due to this non-physiological stimulus. Action potentials fired in this way would not have been encoded correctly and as such, when they occur, the nervous system is unable to make sense of them, hence the strange feeling of pins and needles.
History in this case indicates that when pain started to be noticeable, it seemed tolerable for a while but gradually worsened. What is the explanation for this progression in the perception of pain in this patient?
The gradual compression of the nerve led to a gradual and progressive increase in the recruitment of pain fibres. This would have given rise to the progressive increase in the severity of pain. (NB: The central phenomenon of acute pain progressing to chronic pain due to changes in the central wiring across pain pathways in the brain and spinal cord is avoided here as this may complicate this simple case scenario)
What does the term “ganglion” refer to in a carpal tunnel case?
A tumour growing axons of a peripheral nerve
What treatment strategies might you suggest would resolve this problem a) acutely and b) long-term?
Acutely- conservatively by trying to manage the pain in the hope that the condition stabilises.
Chronically- if this case does not settle due to progression of the growth, surgical removal of the growth may be inevitable for thre reasons: a) To relieve pain and suffering b) Reduce the possibility of this acute pain becoming a case of chronic pain; c) Surgical removal of the growth would avail it for biopsy.
How common are soft tissue injuries as the cause of lower back pain?
Identify soft tissues of the back that are likely to be implicated in such a case.
Most Common. Muscle Tissue; Ligaments of the sacro-iliac joint; tendons of muscles
What would be the characteristics of radiation of pain in lower back pain?
Local tenderness; Stiffness of back muscles: Stiffness of trunk musculature.
What would be the characteristics of radiation of lower back pain in entrapment syndrome which involved the sciatic nerve?
Shooting pain radiating down the leg and buttock. The pain here would trace the physical course taken by the anatomical layout of the nerve
Name the likely sites in which the sciatic nerve or its roots might be trapped.
a) Intervertebral Foramen of the spinal cord (Root values L2-L4);
b) Greater Sciatic Foramen;
c) As the nerve passes though the substance muscle of the buttock and in particular, pyrifromis
What further tests (non-imaging) might you carry out in order to ascertain involvement of the sciatic nerve?
Straight-Leg raise test
