The epidural potato − and beyond

Abstract

Dr Tuckey's potato model for teaching epidural puncture deserves high praise. It is simple, portable and convincing to students at all stages, and in any language [1]. The model is also adaptable to demonstrating the loss-of-resistance-to-negative-pressure test (or ‘suck-and-see’ technique) that I have used exclusively as a safety measure for lumbar or thoracic subarachnoid puncture in humans, and for cisternal puncture in experimental animals [2]. This old and much neglected technique prevents accidental sequestration of any spinal injectate in the subdural space between the pia arachnoid and the dura mater − a nuisance that otherwise occurs in 5–10% of spinal injections [3, 4]. To adapt Dr Tuckey's model for subarachnoid puncture: 1Take a large, washed potato free of wrinkles. Slice in half, taking care to avoid cutting through an ‘eye’ dimple near the circumference of the cut surface. 2Dry the cut surface of the chosen half. 3Wrap a sheet of kitchen plastic film loosely over the cut and dried surface, and secure a gas-tight seal around the circumference with two or three tightly stretched rubber bands, leaving a very slight air pocket between the plastic film (representing the pia-arachnoid membrane) and the cut surface of the potato (representing the inner surface of the dura mater). 4Take a 4-cm 18-gauge hypodermic needle as an introducer and insert it into the butt end of the potato, aiming for the cut surface and advance a 25-gauge Whitacre needle through the introducer (clinically, a 27-gauge Whitacre needle is ideal but, for this artificial model, the stiffer 25-gauge is needed to overcome the more resistant texture of a raw potato). 5Remove the stilette and firmly attach an empty 2-ml syringe to ensure a gas-tight fit. 6Immerse the potato slant-wise in a glass of water and observe the cut surface of the potato. 7While advancing the Whitacre needle, distract the plunger of the attached syringe with middle finger and thumb while exerting counter pressure on the barrel rim with the extended forefinger, and retract the plunger to a volume of 1.75–2.0 ml. This will create a negative pressure of approximately − 600 mmHg, transmitted to the needle orifice [2]. 8Continue to advance the 25-gauge needle very carefully while applying vacuum and an assistant steadies the ‘dura’ until the needle orifice appears. At that instance, the transmitted vacuum ‘snaps’ the ‘pia-arachnoid’ onto the needle point, penetrating the ‘pia’ and collapsing the artefactual subdural space, as water spurts back into the vacuum syringe. 9The needle orifice has evaded the subdural space and is safely within the subarachnoid space. The specialty has waited a long time for a simple bench model to demonstrate the advantages of the ‘suck-and-see’ technique for subarachnoid puncture and its three intrinsic virtues of improved safety, efficacy and parsimony at all spinal levels from the cisterna magna downwards. Dr Tuckey's simple model makes a significant contribution to that educational goal.