Radiculopathy is simply a medical term for nerve damage. The word 'sciatica' is best avoided. Sciatica is a purely descriptive term to describe pain in the region of the sciatic nerve. Many nerves in the lumbar spine can be damaged as a result of degenerative disc disease (DDD) or Internal Disc Disruption (IDD). Only the L5 and S1 (L4 to a variable extent) nerve roots contribute to the sciatic nerve. The precursor to radiculopathy is radiculitis which simply means irritation of a nerve root. The work radicular simply means “from the nerve root”. The diagnosis of radiculitis can be made after a few simple questions and clinical examination. If on a clinical examination findings of muscle weakness, altered sensation or loss of reflexes are made then the diagnosis is radiculopathy. The clinical examination is a very crude method to determine if radiculopathy exists however as other factors often make muscles appear slightly weak and reflexes are often absent for all sorts of reasons. Also, a nerve has to be considerably damaged for the weakness in the muscle it supplies to be clinically evident. Therefore, nerve conduction studies with EMG are needed to make the diagnosis. EMG is electromyography. This means tiny acupuncture-like needles / electrodes are placed in various muscle groups and the muscles are activated by the patient. The waveform of muscle activity recorded by these electrodes can discern whether or not the nerve supplying them is damaged, and to what extent. This study can also discriminate between nerve damage which is static (or chronic), or ongoing (or acute). The test can also be performed in a dynamic mode which means testing the function of the nerves with the spine in different positions to see if there is evidence of nerve damage with different postures. It is an extremely important test and used frequently in spine surgery patient assessment.
Figures 1 and 2 show anatomical dissections of the lumbar plexus from the front and the back. You can see that the individual nerve roots have a complex and intimate relationship to the discs. At any point, any individual nerve root can be irritated or physically compressed. You will recall from the Causes of Back and Neck Pain page that I alluded to the fact that back pain and radicular pain are two very separate entities that have a common cause? Here we will see why I said this.
The common cause is degenerative disc disease (DDD). Everything stems from degeneration in the disc (and thus the motion segment as a whole – degeneration leading to collapse, instability, deformity etc.). Whilst the causes of back pain have been discussed in detail on the Causes of Back and Neck Pain page, the causes of radicular pain are nerve irritation and nerve compression. Nerves can also be compressed well outside of the spine. The sciatic nerve for example can be compressed by a tight piriformis muscles in the pelvis, or by altered anatomy in the pelvis. With respect to purely spinal pathology however, examine the L5 nerve root on the diagram to the right. The L5 nerve root can be irritated by an Internal Disc Disruption at the L3 disc (see Causes of Back and Neck Pain page –the section on internal disc disruption), by a disc lesion from a degenerate L4 disc (as the nerve is sprouting to the side about to exit under the pedicle) and by a disc lesion from a degenerate L5 disc (as the nerve is exiting through the neuroforamen). So, this one nerve root has the potential to be damaged in three different places, from three different discs. In fact, the L5 nerve root is more frequently is damaged in two places than one. In fact, disc degeneration, instability, collapse or deformity at any level can damage a whole number of nerves. If you understand this concept you understand already more about radiculopathy than most spine surgeons. One of the commonest comments on MRI scan reports say something like "no evidence of nerve compression", "the nerve roots exit freely" or "no radiological cause is seen to explain the patient’s radicular symptoms". The MRI scan really needs to be interpreted by a spine surgeon knowledgeable in the causes of radicular pain.
The L5 nerve root is the commonest nerve to suffer from degenerate conditions in the lumbar spine. This is the nerve root which is responsible for a lot of foot function. The action of dorsiflexion of the ankle and extension of the big toe (pointing the ankle and toes towards the head) is mostly L5 mediated. If this nerve is damaged severely the patient will have weakness of these muscles, pins and needles or even numbness in the outer calf and top of the foot, and an abnormal gait because dorsiflexion of the ankle is needed to perform a heel-strike (the first phase of the gait cycle). The leg pain that patients experience is highly variable. Don't forget that not all leg pain is radicular.
Figures 3, 4 and 5 show how variable the distribution of radicular pain is in the population according to research performed in 1985. Most consistent are the red zones i.e. L4 tends to refer to the front of the shin, L5 tends to refer to the outer calf, and S1 tends to be real classic back of thigh and outer foot. Interestingly all nerve roots can refer to the buttock. Pain that does not refer below the knee is rarely radicular in its origin except to say that each nerve has a dorsal and ventral root.
The dorsal roots of the L5 and S1 nerve root can refer pain just to the buttock and around the bones of the pelvic girdle. In Figure 5 you can see that each nerve has a ventral root – ventral just means "...to the front..." – and a dorsal root – dorsal just means "to the back". The ventral roots run down the leg because during embryological development the legs are developed at the front of the embryo. The dorsal roots runs to the skin over the back of the spine, supply the muscles at the back of the spine and from an embryological point of view the buttock area is included in this.
Nerves comprise an axon and a cell body. The axon is the long structure that travels from the spinal cord to the skin and muscles it innervates. It carries impulses to and from the spine. The cell body is the powerhouse of the axon. It contains the mitochondria (energy generation), DNA and other such important structures that drive the axon. The cell bodies are contained in the dorsal root ganglion (DRG).
As you can see in Figure 6 and 7 (below) the dorsal root ganglion is perfectly positioned to get damaged. In fact, it couldn't be better positioned as a target. If DDD causes damage to an axon it will regenerate. If the DRG is damaged however the nerve may not recover at all. Reconstructive spine surgery aims to reverse whatever process is damaging the nerve. Usually this means stabilising a segment of the spine using either fusion or motion preservation technology (see more about this in the pages on spine surgery). If no radiculopathy is present (i.e. only radiculitis) then the radicular pain usually disappears quite quickly. If the nerve is frankly radiculopathic however, the axon has to regenerate. Sometimes this is a painful process but not always. If the DRG is damaged, then the prognosis is often poor for resolution of the radicular pain.
So why is a damaged nerve painful? Many theories exist. Two incredibly bright individuals called Professor Patrick Wall, and Professor Ronald Melzack, who I had the pleasure of working with at St Thomas's Hospital in London developed the “gate theory of pain” in 1965 via (one can only call them) 'peculiar' means. Pain is an incredibly important sensation to feel. It is the warning sensation that tells you when you're doing something bad to your body. It is mediated by specialised pain receptors and transmitted to the spine via specialised axons called C-fibres or delta fibres. So for example if you were to put your toes onto some hot coals in a fire then specialised pain receptors operate and transmit to the brain, via the spinal cord, a warning that makes you reflexively look at the area from which the signal generated and, hopefully, move your foot out of the fire.
Descartes understood this many hundreds of years ago – see Figure 8 (below). However, the brain is not infallible, and it can get confused – when a nerve in the spinal cord is compressed or irritated, no pain receptors are being stimulated, and no C or delta fibres get activated. Remember we are talking about the mechanisms of radicular pain here, not back pain – back pain mediated via the sinuvertebral nerve certainly does involve discreet pain receptors and fibres. All axons which are being compressed start to activate (normal motor and sensory ones) and the brain doesn't know what to make of the situation, so it defaults to the safest sensation which is to feel pain. In other words, "something is wrong, and I don't know what it is, so let's make the sensation a painful one in the hope that this body will examine the area and do something about it." If this seems an oversimplification or to trivialise a complex situation, you have my apology – but some of the brightest physiologists on the planet explain things this way and if you are interested in a good intellectual discourse on this subject, similarly described, then Melzack's and Wall's book on the subject is fascinating and easy to read.
Traditional painkillers which work by blocking signals from pain receptors obviously will not be very effective in this situation. Anti-inflammatories can relive some inflammation around whatever may be compressing a nerve and so have some effect but will not change the impulses travelling to the brain. The only medications that have an effect on radicular pain are neuromodulators. This group of medications are traditionally used as anti-depressants and anti-epileptics (both of these conditions require neuro-modulation) in higher doses; in lower doses they can neuro-modulate radicular pain, but the side effects can be unpleasant.
Not all leg pain is radicular pain. In much the same way that the pain of a heart attack can be felt in the left arm, and the pain from gastro-oesophageal reflux disease can be felt in the shoulder tips, pain from a degenerate disc can be felt in a very diffuse manner over the front of both legs or even around the front of the abdomen. We have talked mostly about radicular pain and the dermatomal map for the L4, L5 and S1 nerves as they are the most commonly affected. The L3 nerve tends to give pain to the knee joint and the front of the thigh. The L2 nerve root tends to give pain in the groin. L1 radicular pain is extremely rare and if present would probably give pain in the genitals and front of the abdomen. Pain in the genitals and the front of the abdomen is common with DDD however as a result of the unique relationship between the L5/S1 disc and the sympathetic plexus which send gray rami communicantes via the sympathetic plexus to the L1/2 region – see Causes of Back and Neck Pain page.
The mechanisms of radicular pain production in the neck are very similar in their principles to those of the lumbar spine. The most commonly degenerate discs in the cervical spine are C4/5, C5/6 and C6/7.
The real confusing thing about the cervical spine is the numbering of the nerves. There are 7 cervical vertebrae but 8 nerves. This is simply a result of no anatomist wanting to describe the nerve that exits the base of the skull as C0. Therefore, C8 is the last cervical nerve and it exits below the C7 vertebra. This leaves the conventional numbering system for the thoracic spine to continue logically so that T1 exits under the pedicle of theT1 vertebra. If this is confusing simply forget all about it. In terms of understanding your pathology it doesn't matter. As a spine surgeon, of course, this arrangement has to be perfectly understood in order to diagnose and treat the correct level.
As you can see Figure 9 (below) the C4 nerve tends to send pain to and around the shoulder girdle, C5 over the deltoid muscle, C6 down the outside of the forearm and the thumb, C7 innervates only a small patch of skin over the palm of the hand and the middle finger. C8 gets the little finger and a portion of the inner aspect of the forearm. T1 is very rarely radiculopathic, in fact I have never seen it, and innervates the inner part of the upper arm and a portion of the chest wall.
In Figure 10 (below) you can see that the relationship between nerves roots in the neck and the spinal cord is very different to that in the lumbar spine. In the spinal column the actual spinal cord (i.e. the neural component) ends at about T12/L1. The nerve roots stream down from this level in a way which has been described as looking like a horses tail (cauda equina is a Latin term for horses tail) until they reach their exit foramen. No part of the spinal cord exists below this level although for descriptive purposes it is often so much easier to think that it does. In the cervical spine however, directly behind the cervical vertebrae is the spinal cord proper and all the parts of the central nervous system that control motor function to the legs, relay sensory feedback to the brain and also bladder and bowel function, sexual function etc. This means that disc disease in the neck can cause a pathological entity called myelopathy. Myelopathy simply means 'spinal cord disease'. In a very similar semantic relationship to radiculitis and radiculopathy, myelitis also simply means 'spinal cord irritation'. There are a very disabling group of conditions called transverse myelitis, as well as other names, which are usually infective in their origin which significantly interfere with the function of the cord with neurological deficits. Myelopathy, if it exists is extremely serious. If left untreated patients often end up wheelchair bound within 10 years. Cervical Spondylitic Radicular Myelopathy (CSRM) is the diagnosis when multiple discs in the cervical spine degenerate and damage both the spinal nerves and the spinal cord over multiple levels – it is something we see commonly.
There are 12 thoracic vertebrae and 12 thoracic nerves. Each nerve travels under a rib and innervates a strip of skin associated with that rib with considerable overlap with the ones above and below. Thoracic DDD is rarely symptomatic as a result of the kyphotic configuration to this region and the protection afforded to the discs by the rib cage to the sides and the sternum to the front. However, trauma, infection, tumours, arthritis of the facet joints, and rarely – primary discogenic disc disease can still affect the discs in the thoracic spine. It is now possible to use minimally invasive techniques to safely treat conditions in the thoracic spine, but the results are only good if one disc can be proven to be the pain generator. Frequently seen is the condition where the three or four discs at the apex of the thoracic kyphosis degenerate and form painful osteophytes, which can mimic radiclopathy. This condition usually self-resolves and auto-fuses 18-24 months after the onset of pain. More traditional surgery used to involve whole rib resection, deflation of the lung and long stints in intensive care to treat so surgery was rarely performed. Nowadays, the same job, if required, can be accomplished through a small 3-4cm incision with maybe a night in intensive care just to be safe using the extreme lateral approach.