Sacral Dermatomes
It is essential to do a rectal examination and specifically test the sacral dermatomes sensation to assess for herniation at the L4-S1 nerve roots.
Consent the patient for a rectal examination, examine for rectal tone and the ability to contract the anal sphincter. Whilst performing the exam note any constipation or prostate enlargement (carcinoma of the prostate frequently metastasises to bone and may be a cause of back pain.)
Enquire about urinary symptoms as retention of urine or incontinence may indicate a central disc lesion.