Anatomy of the normal intervertebral disc
In order to understand the effects of a herniated disc it might be helpful to understand the structure of a normal intervertebral disc. As the name implies an intervertebral disc functions as a joint between two adjacent vertebral bodies. Its major role is mechanical. Eighty percent of the body weight in the standing posture is transmitted through the vertebral bodies and their intervening discs. The flexibility of the spinal column provided by the intervening discs allows for the complex movements allowing bending, flexion and torsion. The average lumbar disc is approximately 7–10 mm in height and 40 mm across in the front to back plane.
What are the possibilities following acute herniated lumbar disc? An acute herniated lumbar disc can irritate the exiting nerve roots resulting in lumbago with or without sciatica. In majority of patients the pain if it has to resolve does so within six weeks. This happens because of three possibilities:
- The degenerative disc fragments become more dehydrated and shrink resulting in some loss of volume and reduced compression
- The compressed nerve root narrows at the site of compression providing some space
- The pain is relieved despite no perceivable changes on Magnetic Resonance images over time. It is believed to be due to the reduction in the levels of inflammatory molecules released at the time of acute disc herniation
Central disc herniation (Purple): Seen in the elderly or young athletes. More likely to present with acute cauda equina syndrome- a neurosurgical emergency! This presents with severe pain, inequal deficits on the two sides, urinary retention and numbness of genitalia. Posterolateral lumbar disc herniation (Pink): The typical clinical picture includes initial lumbalgia, followed a week later by progressive sciatica in a patient around 40 years of age. Extreme lateral lumbar disc herniations (Green): ELLDH occur more frequently among elderly patients, with a peak incidence in the sixth decade. These occur more frequently at upper lumbar levels and as these extrusions compress the exiting root with its dorsal ganglion, the clinical presentation often involves lancinating leg pain, whereas low back pain is often mild to moderate. ELLDHs are characterized by the compression of the nerve root which exits at the same level and this is in contrast to classic posterolateral disc compression, which affects the nerve root leaving at the level below.
Extraforaminal lumbar disc herniations (Yellow): EFLDH feature cephalad migration of fragments away from their disc space of origin beyond the dorsal root ganglion of the exiting nerve root. EFLDH occur beneath or lateral to the facet joints and are characterized by the compression of the nerve root which exits at the same level. These herniations are uncommon and maybe missed by routine MR Imaging protocols.
- if pain control is unsuccessful despite these measures outlined above,
- if there is a motor deficit greater than grade 3,
- if there is radicular pain associated with foraminal stenosis, or
- if cauda equina syndrome is present. The latter represents a medical emergency.