This is a very common problem seen through GP’s and Neurosurgeons. There are some important elements to do this. One is the concept of recognising what is serious (i.e cauda equina syndrome), the concept of exiting and traversing nerve roots, and different types of prolapse.
We need to look at the typical lumbar vertebrae first to identify this.
Surgical Anatomy
The junction between the vertebral body and the transverse processs is called the pedicle, and the junction between the spinous process and transverse process is called the lamina
Clinically removal of the lamina is called a laminectomy and is often used to access the lumbar disc and for lumbar canal decompression. The superior articular facet runs directly from the pedicle.
The superior articular facet has clinical relevance. Through disc degeneration, this increases the pressure on the superior articular facets, leading to facet hypertrophy and nerve root compression.
In comparison to the cervical spine the exiting nerve roots run inferior to the rather than superior the pedicle.
The disc consists of the outer layer called annulus fibrosis and inner centre called the nucleus pulposus (remnant of the notochord). The herniation of the nucleus pulposus onto the nerve roots (posterolateral disc) or centrally can cause cauda equina syndrome.
The importance of the exiting and traversing nerve root can be described here. The corresponding nerve root for that vertebrae runs directly under the pedicle which is lateral as you will see above. The traversing nerve root runs directly in the midline within the thecal sac. It therefore follows that the majority of disc prolapses unless they are far lateral will compress the traversing nerve root rather than the exiting nerve root.
An L4/L5 disc herniation will compress the L5 nerve root, unless it is far lateral which will than affect L4. Clearly a large central disc prolapse will cause Cauda Equina syndrome that is an emergency.
Similarly an l5/S1 disc prolapse will most likely compress the S1 nerve root unless it is far lateral that may compress the L5 disc root. In far lateral compressions a foraminotomy maybe performed to release the nerve root as it runs under the pedicle in the inter-vertebral foramen.
In a nutshell to summarise this key point:
- Severe central prolapses will be cause compression of the cauda equina
- Majority of prolapses are posterolateral and will compress the traversing nerve root unless it is far lateral for which the exiting root will be affected.
Patient Presentation
In general back pain, that is aggravated by movements, and coughing/straining, They will present with ‘shooting’ radicular pain affecting the characteristic nerve root.
Signs
Lasegue’s sign- straight leg raise. Often used to test lesions at L5/S1. The angle of a positive straight leg raise should be <60. Dorsiflexion of the ankle will exacerbate the pain by compression of the L5 nerve root.
Femoral Stretch Test- this is the reversal of the straight leg raise used to test higher lesions within the lumbar spine namely L2-L4
Pain and Progressive Neurology over the affected limb:
Knowing your dermatomes and myotomes can help estimate the possible pathology.
L4 Nerve Root Compression
Power: Provides motor to quadriceps muscles. Reduced knee extension
Reflexes: Reduced knee jerk reflex
Sensation: Medial foot and medial malleoulus
L5 Nerve Root Compression
Power- reduced dorsiflexion and big toe extension. Patient may present with a footdrop
Reflexes- reduced ankle jerk reflex
Sensation- lateral calf and dorsum of the foot
S1 Nerve Root Compression
Power- reduced plantar flexion
Reflexies- reduced ankle jerk reflex
Sensation- lateral aspect and sole of the foot.
Management
Conservative Management- the majority of disc prolapses can be managed conservatively, and expect resolution around 6 weeks
Surgical Management- if obvious cauda equina syndrome or progressive neurological deficit eg a foot drop, or if refractory to medical management. See cauda equina syndrome for surgical complications
MRI Scan saggital- T2 weighted image (CSF White) with bilateral disc herniations at L4/L5 and at L5/S1
When interpreting MRI scans always look at the axial section. You can’t tell from the saggital section the degree of effacement of the cauda equina. It is therefore essential to view. Here a disc at L4/L5 with compression of the right nerve root with effacement of the cauda equina.
