Peripheral Neurology
Many people present to the clinic complaining of both a central problem, e.g. low back, and a peripheral problem, e.g. leg pain, with many presuming the cases are linked together, in this scenario via the sciatic nerve. However, sciatica is one of the most commonly mis-diagnosed symptoms seen in practice, after all ‘true’ sciatica is symptom that something is going on closer to the central nervous system.
Let’s begin at the top and work down and out, after all this is the way the body works. The brain sits at the top receiving and sending messages up and down the spinal cord, which extends from the base of the skull all the way down to the L1/2 vertebrae level. From the spinal cord projects what are called the spinal nerve roots, which exit the spinal canal to the sides, in spaces between the vertebrae. These nerve roots then divide and spread throughout the body supplying the organs, skin, muscle, joints and supportive tissues. So from this we can roughly say that the body works: above, down, inside to outside. These nerves supply the sensation, muscle strength and deep tendon reflexes (e.g. knee jerk), as well as doing a host of other things, but for the purpose of this post we are going to focus on sensation, muscle strength and deep tendon reflexes.
The spinal nerve roots exit the spinal cord, in the case of the nerves which go down the arms and legs, they pass through a plexus where the nerves integrate, and then pass into the periphery supplying a certain area of skin, referred to as a dermatome. A chart showing dermatomal distribution can be seen below. In this chart you will observe that there are two different distributions, the dermatomal distribution being on the left, and what is referred to as cutaneous peripheral nerve distribution on the right, we shall come back to those in a little while. The thing I would like you to notice is that each area is labelled with a letter and number which correspond to the area of the spine and specific level which the nerve originated from, C being from the neck (cervical spine), T being from thoracic spine, L being the lumbar spine and S being sacral. To give an example the L5 nerve root originates from the Lumbar spine below the 5th vertebral segment.
Equal to the dermatomal distribution of sensation each segmental level nerve root is associated with a specific reflex and certain muscles, for example, L4 nerve root is associated with the patella (knee jerk) reflex and innervation to the quadriceps muscle group. Therefore if there were pressure on the L4 nerve root we would expect to see weak quadriceps, reduced patella reflex, altered sensation over the L4 dermatome as well as pain along the dermatome. There are a couple of things to note about nerve root problems, firstly the need to observe the location and pattern of pain, as you will see from the chart below the dermatomes extend from lower lumbar region all the way through to the foot, spiralling around the leg. Secondly, that the nerve root effected is more often than not at the level lower than where the pressure is on the nerve, for example an L5/S1 disc prolapse will affect the S1 dermatome, muscle strength and reflex.
Basically, with the above examples of lower limb problems, we have been discussing ‘sciatica’, as stated above this is easily mis-diagnosed, and in fact is not a diagnosis but a group of symptoms which represent that something is putting pressure on a or some of the nerve roots. The key here as that the symptoms will be of a specific distribution, rather than a vague pain in the back of the leg. Of equal importance is being to identify the diagnosis of what is creating the ‘sciatica’. Most commonly coming from some kind of disc problem, however it could also be a space occupying lesion or degenerative changes in the spine.
Lets now discuss cutaneous peripheral nerves, these nerves are the resultant combinations of the plexus interactions which then descend through the limbs supplying again specific muscle groups and areas of skin. If you observe the chart above you will notice that the sensory distributions, especially in the upper limb are very similar to that of the nerve root level dermatomes, and this is where an in depth clinical knowledge allows us to distinguish between a nerve root problem and a peripheral nerve entrapment. The most common peripheral nerve entrapment is Carpal tunnel syndrome (see image below), where by the median nerve is entrapped under the flexor retinaculum as it passes in to hand to supply sensation to the palmer surface of the hand, index, middle and ½ of the ring finger.
The issue here is that firstly this distribution is very similar to that of the C7 nerve root, so a nerve root problem needs to ruled out, and secondly the median nerve exists from the brachial plexus in the shoulder all the through the arm to hand, passing through or around a number of structures which could potentially entrap the nerve, giving similar symptoms to carpal tunnel syndrome. For example: if the median nerve were to be entrapped under the pronator teres, a muscle at the elbow, the symptoms would be slightly different to those experienced with entrapment at the carpal tunnel, as there would muscle weakness in the muscles supplied by the median nerve before it enters the carpal tunnel.
So how do we differentiate between nerve root problems and peripheral nerve palsies?
Primarily it takes a keen eye for symptoms and a good understanding of the anatomy involved. When dealing with a nerve root problem all the muscles innervated from that level will be affected regardless of the peripheral nerve which goes to that specific muscle, for example if the C5 nerve root was affected in the neck, weakness would occur in the following muscles, Deltoid and teres minor supplied by the axillary nerve, Rhomboid major and minor supplied by the dorsal scapular nerve, supraspinatus and infraspinatus supplied by the suprascapular nerve and finally the biceps supplied by the musculocutaneous nerve. There would also be a reduced deep tendon reflex at the bicep, along with altered sensation over C5 dermatome. Whereas someone suffering compression of the axillary nerve, which can happen in the quadrilateral space (see image below), would expect to experience changes in sensation over the affected cutaneous area, weakness of the deltoid and teres minor, with the preservation of all the other C5 nerve root muscles and reflexes.
