One of the biggest misconceptions in spine care is that a bigger slipped disc means a worse outlook. In fact, larger herniations often have a better chance of reabsorbing on their own.
If you have been told your disc herniation is large, the natural assumption is that this is worse news than a small one. It seems logical: more material, more compression, more need for an operation.
It is one of the most persistent misconceptions in spine care, and it is not true.
What actually determines whether you need surgery
The decision to operate comes down to the severity of your symptoms, not the size of the disc, and specifically to whether there are neurological problems: weakness in the legs, or difficulty controlling your bladder or bowel.
The severity of pain often does not correlate with how big the disc is.
A study published in the Global Spine Journal examined patients with herniated discs, nerve compression, and symptoms travelling down the legs. A year after diagnosis, 91% of them had been managed without surgery. They simply did not need it.
The striking finding was this: among those who did need surgery and those who did not, the average herniation took up about 31% of the spinal canal in both groups. The size was the same. The outcome was not.
How discs are built
To understand why large herniations shrink, it helps to know what a disc is made of.
The inner component is the nucleus, which when we operate on it looks much like a lump of crab meat. Around it are many layers of the annulus, a fabric-like lining of fibres holding the nucleus in place.
That outer fabric can thin as layers begin to tear, allowing the nucleus to bulge while the lining remains essentially intact. A bulge or protrusion can therefore be quite sizeable with the inner substance still well contained.
An extrusion is different. Here the outer lining tears fully and the nucleus pokes outwards, sometimes breaking away entirely to form a sequestration, a free fragment. These too can be large.
The three ways herniations shrink
Research has identified three mechanisms.
Mechanical. A fragment pokes outwards and then, with ordinary movement over time, simply migrates back into place.
Dehydration. The nucleus contains a great deal of water. As the lining tears and that water gradually leaks out, the nucleus shrinks, and the bulge reduces with it.
Immune response. This is the mechanism that has attracted the most research, and it explains the counterintuitive finding.
Once a disc has extruded, with the outer lining torn completely and the nucleus poking out, chemicals released from within the disc activate an immune response. Cells called macrophages arrive and begin breaking the material down. The same happens with a sequestration, where a fragment has broken away completely.
In other words, the more the disc material is exposed to the body’s immune system, the more effectively the body can dissolve it. A contained bulge is, in a sense, protected from that process. A large extrusion is not.
What this looks like on a scan
An MRI can give clues about the composition of a herniation. The material poking out is the inner nucleus, and a change in colour in a teardrop-shaped component suggests an extrusion, or a fully broken-free fragment.
In the study, following the same patient a year later showed the herniation had completely shrunk, with symptoms improved accordingly.
What this means for treatment
Surgery is therefore rarely offered as a first-line measure.
Where symptoms have only just started, particularly back pain radiating down the legs, I usually give it time, perhaps a few weeks, to see whether it settles. In the majority of patients, it does.
The exceptions
There are two situations where waiting is wrong.
Weakness in the legs from nerve compression needs prompt attention, because the priority becomes preserving nerve function.
Cauda equina syndrome is a medical emergency. It is characterised by pain down both legs with saddle numbness, meaning numbness in the groin area around the genitals, together with either incontinence or retention of urine and difficulty passing it. On MRI all the nerves are compressed. This requires immediate decompression to preserve their function.
The key takeaway
It is not the size of the herniation that determines whether you need surgery, or whether you will improve.
It is the symptoms, and particularly neurological deficits: weakness in the legs, poor bladder control, numbness in the genital region.
As for the size itself, larger herniations actually have a good chance of reabsorbing completely, precisely because of that immune response.
As I say often: we treat the patient, not the scan.
