Causes of Sciatica

In this article we’ll cover

• Disc problems
• Spondylarthropathies
• Spondylolisthesis
• Tumours
• Spinal Stenosis and Claudication
• Piriformis syndrome
• Strains
• Pregnancy

But first… 

Why sciatica isn’t what you think it is

Often people think of sciatica as a diagnosis. However, it is not, it’s simply a label. Comparatively, it’s like going to a doctor and saying I have a sore head, it hurts me here. Then they diagnose you with a headache. You’d probably think that’s not a lot of use. That is the same with sciatica. As sciatica simply means pain and/or pins and needles in the distribution of your sciatic nerve. That’s it. So the term is not terribly valuable as a diagnosis because it isn’t one; it’s just a label. It tells you where the problem is, but not why you have it.

Exactly which parts of you sciatica can affect

To understand which parts sciatica affect, let’s look at the distribution of the sciatic nerve. So your sciatic nerve is a big nerve – it has some of the longest nerve fibres in your body. It also has tributaries. Imagine your sciatic nerve is like the Amazon river. It’s very big and long and in your case ends up at your toes.  And it originates deep in your pelvis, and it has tributaries.

Those tributaries are called spinal nerve roots. They come directly from the spinal cord. They are small nerves that exit between the vertebrae in your lower back. We have lumbar  (L4), Lumbar 5, Sacral 1 (S1) and Sacral 2. Branches of these spinal nerves  join together and form your sciatic nerve deep in the pelvis. The sciatic nerve then comes out through what’s called the sciatic notch, a sort of groove in your pelvis and then travels through the buttock down the back of your thigh, all through its length, giving off branches of its own. Through the back of your thigh and then down round the side of your knee and down into the lower leg and foot. The areas the sciatic nerve supplies are part of the buttock, the back and outside of your thigh and pretty much everything below the knee.

So if you have pain in the front or inside of your thigh, that is not sciatica. 

Symptoms of sciatica include pain, and pins and needles, which is sometimes accompanied by numbness. By that I mean if you touch the skin, it doesn’t feel right. You might also have feelings of running water, hot and cold sensations, sweatiness/clamminess in the foot; particularly in severe cases of nerve compression sciatica.

How to differentiate the two main types of sciatica

It’s widely accepted that there are two types of sciatica.  So how to differentiate between them?  The two main types of sciatica are ‘nerve compression sciatica’ and ‘non nerve compression sciatica’. We differentiate into two types of sciatica, as each needs a different management approach.  And – in turn – the different types have different causes.

Nerve compression sciatica

Nerve compression sciatica is the more serious of the two. If your sciatic nerve or one of its tributaries is compressed, that’s nerve compression sciatica. Some people would use terms like a ‘pinched nerve’, or ‘trapped nerve’.

Non nerve compression sciatica

Non nerve compression sciatica means that you have symptoms in the distribution of your sciatic nerve, but it is not due to compression of a nerve. The mechanism here is usually what we call referred pain. So let’s imagine you have strained a muscle in your buttock. That buttock muscle has a nerve supply, either from the sciatic nerve directly or one of the tributaries to that sciatic nerve, for example your L4 nerve root. You could feel the pain directly in the buttock, but you could also feel it elsewhere along the distribution of the sciatic nerve. And this is through the mechanism of referred pain. So you might feel pain in your calf, but the pain is actually coming from the strained buttock. That’s just one example.

Underlying causes of Sciatica

Here we focus on the underlying causes of these two types of sciatica.

Nerve compression sciatica causes

Don’t be freaked out by some of the items on this list.  They are very rare.  By a big margin, the most common cause of nerve compression sciatica is disc problems.

Disc problems

Intervertebral discs are sandwiched between the weight-bearing elements of 2 vertebrae.  They are tough, fibrous structures with a gelatinous centre.  At least that’s in a young healthy disc. As you age, the disc slowly dehydrates, losing the gelatinous part over time. They are often described as being like a jam donut.  The pastry around the outside is the equivalent of the fibrous outer ring – called the annulus fibrosus.  And the jam in the middle is the gelatinous bit – called the nucleus pulposus.

Disc problems can come on following an obvious effort, usually in a forward bending movement. But they can also come on as a result of the “last straw effect”.  You apparently have done nothing or very little, but the pain builds up over a few days. In the case of annular strains and herniations, most of the sciatic pain is caused by inflammation affecting the nerve.  As Inflammation tends to build over a few days, so does the pain.  It’s usually worse on day 3 than on day 1. 

Pain pattern with disc problems

Usually – but not always – annular strains and herniations – lead to more pain in the mornings.  This is because inflammation builds over night and because the disc absorbs fluid when you are lying down – this leads to the bulge getting bigger the longer you stay lying down.  The bulge (herniation) irritates the nerve more as it gets bigger. 

Annular strain causing sciatica

As you go through life, you can have tears in the outer fibrous ring.  This is called an annular strain. If a large enough number of these fibres are torn there will be a significant inflammatory swelling – this is the first stage of healing.  If the tear is in the back corner of the disc, the inflammation will be adjacent to the lumbar nerve root as it exits the spinal canal.  

Inflammation around the nerve usually irritates the nerve and can lead to nerve pain.  If that’s L4, L5, S1 or S2 nerve roots, that will result in sciatica. 

Disc herniation causing sciatica

If you have torn enough fibres, this can cause a bulge in the outer disc.  It’s a bit like slashing a tyre – the inner tube bulges through the defect.  This deformation of the disc is referred to as a disc bulge or disc herniation.  This bulge can also irritate the spinal nerve.  As can the accompanying inflammation.  Sciatica can ensue.

Side note… I had recurrent L4L5 disc herniations over a period of 7 years in my 20s.  It was terrible.  No one (and I tried osteo, physio, chiro and massage) was able to help me in any lasting way.  This is what led me to specialilse in low back pain and sciatica.  As a result, I am confident that we have a proven, effective way to rehabilitate disc problems.

Disc prolapse causing sciatica

If more fibres tear, some of the nucleus can actually escape the perimeter of the disc.  This nuclear material can directly compress/squash the nerve root.  Again, this will be accompanied by inflammation.  Again, the spinal nerve can be irritated.  Again, sciatica can develop.  This is like the jam fully escaping the donut. 

Spondylitis causing sciatica

Essentially, this is degeneration of the disc and the bones on either side.  The “itis” refers to inflammation.  Again, because this inflammation can be right next to the spinal nerve, it can lead to sciatica.  “Spondylosis” is degenerative changes in the disc, but without the inflammation.  Spondlyosis is normal with increasing age.

Spondylarthropathies

This is a general term which broadly means a problem with the small joints in the spine.  This isn’t the disc part of the spinal complex.  The facet joints sit behind the disc and behind the exiting spinal nerves. 

Facet joint inflammation causing sciatica

This may be related to degenerative changes in the joint, or the inflammation may be caused by something else.  Whatever the cause of the inflammation, this inflammation can irritate the spinal nerve root, leading to sciatica.

Spondyloarthritis

Degeneration of the facet joint can lead to boney overgrowth, which in turn can close down the space through which the spinal nerve passes.  This may lead to compression and then to sciatica. Think of this as compressing the spinal nerve from the rear; whereas disc problems compress it from the front. 

Spondylolisthesis

Commonly shortened to as a “spondylo”, this is where there is a movement of a vertebra either forwards or backwards on the bone below.  This is associated with a fracture or congenital defect in part of the vertebra.  So, spondlyolisthesis can come on gradually, or very suddenly – in the case of a fractured pars interacticularis (the part of the vertebra). 

When the vertebral body (in front of the fracture) moves forwards or backwards, the spinal nerves at that level can be compressed.  This can result in sciatica in both legs (bilateral).  Although spondylos can cause pain just on one side.

When the bone moves forwards, this is properly referred to as an anterolisthesis.  When it moves backwards, it’s a retrolisthesis.

People with anterolisthesis tend to have more sciatic pain with standing, walking and back-bending activities.  This is not diagnostic, but along with a “step” in the spine is a fair indication (see Clinical Tests for Sciatica). 

Tumours as a cause of sciatica

These are very rare, but possible.  If a tumour is growing adjacent to one of the tributaries of the sciatic nerve – or the sciatic nerve itself – it is possible that this may irritate/compress the nerve, leading to sciatica.

Tumours could also cause non-nerve compression sciatica.  If the tumour was growing in or irritating a tissue that had a nerve supply from one of the spinal tributaries, this could cause referred pain into the sciatic distribution. 

Spinal Stenosis

Spinal stenosis is a narrowing of the spinal canal, most commonly related to old age.  Due to degenerative changes in the spine, the main spinal canal narrows.  This can lead to compression of the spinal nerves, and sciatica.  

In the early stages of spinal stenosis, it is quite possible that non nerve compression could happen.  When the spine degenerates to this extent, it’s not just the canal narrowing.  Other tissues are involved and these can cause pain to be referred into the sciatic distribution.

Spinal Claudication Due to Spinal Stenosis

Spinal stenosis sometimes leads to “spinal claudication”  This is a symptom picture that describes pain that increases with more walking.  Classically this is described as being worse walking downhill. As you walk downhill the extension of the spine compresses the spinal nerves and reduces the blood flow to them.  There’s also reduced blood flow due to the effort of walking  This reduced blood flow (claudication) leads to sciatic symptoms.  When you stop walking, the sciatic pain rapidly reduces. When you start walking again, the pain returns quickly.  Sitting down relieves this pain more than standing still.

We have to differentiate between Spinal Claudication (classically worse going downhill) and Intermittent Claudication (classically worse going uphill).  Intermittent claudication is due to narrowing of the main arteries to the legs.  This requires medical investigations (see Clinical Tests for Sciatica).

Non nerve compression sciatica causes

Piriformis syndrome as a cause of sciatica

Some might argue that this is a form of nerve compression sciatica. But I don’t agree.  The sciatic nerve runs through the piriformis muscle, deep in the buttock. For a number of reasons, the piriformis muscle my “tighten”.  This is presumed to reduce the blood flow to the nerve, leading to altered function of the nerve. Pain and/or pins and needles may then appear i.e. sciatica which can be treated after a thorough diagnosis is made.

Strains leading to sciatica

There are many tissues (muscles, ligaments, fibrous capsules, tendons etc.) in the lower back and hip area.  Many of these receive a nerve supply which originates from L4, L5, S1 or S2.  This nerve sends a message into the spinal cord.  And the next nerve in the pain pathway sends a message up to the brain. Somewhere along the route, the sensation can be interpreted as coming from another area that that spinal nerve supplies.  This is the mechanism behind referred pain. 

So, although you may injure your lower back, you may feel the pain somewhere else in the sciatic distribution.  Some older people might refer to this as a “crossed wire” (a reference to old fixed line telephone services).

Strains may be sudden or come on gradually due to repetitive strain – the last straw effect referred to above. 

Underlying risk factors for sciatica

As you can see, there are a number of causes of sciatica.  And each has its own risk factors – the upstream contributing factors.  For sciatica as a whole, the recognised risk factors are:

• Middle age
• Smoking
• Obesity
• Manual labour
• Lack of physical fitness
• Male sex
• Pregnancy

Each of these risk factors will have more or less effect in each of the causes of sciatica (disc, spondylitis etc).

Side note – Once, when I was doing a presentation – someone in the audience asked whether the link to smoking was caused by always carrying the “pack of fags” in the same back pocket (rather than swapping between back pockets).  I don’t think it’s a postural/weight distribution thing.  More likely, people who smoke tend to heal more slowly and are at greater risk of all sorts of pains as a result – not just sciatica.

Pregnancy and sciatica

Pregnancy is worth a special mention.  It’s possible that the developing baby can “sit” on the sciatic nerve deep in the pelvis near its exit through the sciatic notch.  Of course, with the changing alignment of the spine and pelvis, this increases the risk of other causes of sciatica.  Disc problems and strains are common during pregnancy, leading to nerve compression and non nerve compression sciatica respectively.

Disc problems in the second and third trimesters are often a real challenge to improve due to the increasing size of the abdomen as the baby grows. 

IF the baby is sitting on your sciatic nerve, he/she may be encouraged to move by gentle manual techniques.  Something many old-timer osteopaths were taught how to do (people who have been practising for 30+ years).  Otherwise, you need to consult someone who specialises in pregnant women.