The axonal damage of sacral roots, the bladder dysfunction and constipation that slowly progress in severe cases of spinal stenosis, are not assessed in literature of high scientific evidence. The decision-making process to choose interventions lacks of guidance to confirm the presence of these three conditions, their natural evolution, impact on quality of life and functionality, and its management. The symptom of pain and the wait for an advanced loss of motor function seem to be the criteria that determine the follow up in spinal stenosis, although the guidelines and systematic reviews show a limited effect of exercise or medication. The usefulness of electrodiagnostic studies is compared with that of MRI to diagnose spinal stenosis, limiting their true diagnosis power. This article suggests an alternate use of these two diagnostic tests, using the Bayes Theorem, the Fagan nomogram and the pre test and post test likelihood ratios to confirm the presence of axonal destruction by paraspinal electromyography and the tibial nerve H reflex, as a complement to a positive MRI for lumbar stenosis. With the same method, the bladder ultrasound and urodynamics are evaluated to confirm bladder dysfunction. A case of spinal stenosis exemplify the guidelines follow up, the functional decline and the proposal to use tools of scientific evidence to broaden the evidence in decision making for best judgment on spinal stenosis