Background: The dermatomal drawings used in clinical practice are based on questionable experiments performed in the 1930s. Though widely used in the clinical practice to identify the level of nerve root irritation, this may not be a reliable practise.

Purpose: To identify the pain distribution and overlap of single level L4, L5 and S1nerve root irritation.

Methods: 187 patients with radicular pain to or below the knee made a precise drawing of their pain distribution and then underwent a MRI scan. Only patients identified as having a single level disc herniation were included in this study. Using computer software, the pain distribution of all people who had the same level nerve root irritation was made by layering their pain drawings. The darkest parts of these compound drawings was where most patients experienced pain.

Results: 89 patients were excluded due to tumour, depression or disc lesions on several levels. 98 patients with single level disc herniation were included.

There was wide overlap in the pain from each nerve root level, and these areas were much wider than shown on dermatome charts. In general, L4 nerve root pain tended to be anterior on the leg, L5 and S1 nerve root pain was on the posterior leg, with L5 root pain tending to be more lateral.

Conclusion: Pain distribution from the L4, L5, S1 nerve roots is not concordant with the sensory distribution of common dermatome charts. This might be due to the methods by which these dermatomes were constructed, or because sciatica is a complex pain experience.