Selective Nerve Root Blocks: Results - Part A

Results

The value of selective nerve root blocks has been defended largely by assertion. Proponents have claimed that nerve root blocks are invaluable in refining a diagnosis of nerve root compression when clinical examination and imaging studies are equivocal (1, 2, 6, 9). Root blocks are not indicated in every patient with suspected radicular pain. They are indicated only in those cases where doubt exists about exactly which root is the source of pain. Examples include patients in whom myelograms are indeterminate, patients with abnormal segmentation, and patients in whom CT scans or myelograms suggest that more than one root is affected by disc herniation or foraminal stenosis (9, 10). According to some studies, such doubt pertains in up to 20 per cent of patients presenting with apparent radicular pain.

Other illuminating features revealed by the use of selective nerve root blocks are that in the majority of patients, single roots are responsible for their symptoms regardless of pathology. Even in patients with intermittent claudication, symptoms are abolished in 90 per cent of cases by blocking a single nerve (5).

Occasionally one can encounter prolonged 'therapeutic' effects following selective nerve root blocks in that the patient remains relieved of their pain for more than 6 months. In such cases, surgery can be avoided or deferred.

Why some patients should obtain prolonged relief is an enigma. It could be related to effects of local anaesthetic such as prolonged dampening of C-fibre activity; 12 it could be a physical effect such as clearing 'adhesions' or inflammatory exudates from around the nerve root sleeve; but no data are available by which these possibilities might be distinguished or validated.

When corticosteroids are used two possibilities arise. First, corticosteroids have a prolonged local anaesthetic effect (13), so that pain relief could be due to this effect rather than an anti-inflammatory action. On the other hand, there is considerable, circumstantial evidence that many cases of lumbar radiculopathy involve inflammation of the nerve root or its adnexae (14, 15, 16). Under these circumstances, the injection of corticosteroids could be perceived as offering a prolonged anti-inflammatory effect to suppress the actual cause of pain from a nerve root.

The implicit value of selective nerve root blocks is that by enabling more accurate diagnosis they lead to better treatment. This contention, however, is largely axiomatic or a matter of faith, for it has never been formally validated. To do so would require comparing the therapeutic outcome of similar cohorts of patients who underwent the same therapy either with or without the benefit of selective nerve root blocks. Such studies have not been performed and, indeed, might be difficult to justify on ethical grounds. The best available data are simply reports that the majority of patients with positive root blocks fare well after surgery, provided the pathology is a disc herniation, bony entrapment of the nerve or periradicular adhesions (1, 6, 7, 9). Patients with intraneural fibrosis, such as postoperative arachnoiditis, may respond to blocks but respond poorly to those surgical interventions that have been tried (7, 9).