Of the 33 vertebrae in the spine, 24 have a central hole, called a foramen. The vertebrae are stacked on top of each other to form a column, and the foramina line up to create the spinal canal running through its centre. This spinal canal houses and protects the spinal cord. The outer layer of the spinal cord is formed of a fibrous tissue called the dura mater. The epidural space lies between the dura mater and the walls of the spinal canal. It contains the dural sac, spinal nerve roots, extradural venous plexus, spinal arteries, lymphatic vessels and fatty tissue (Fyneface‑Ogan 2012).

Medications can be administered into the epidural space. These medications, most commonly anaesthetics, opioids and steroids, provide analgesia or anaesthesia to a specific part of the body, depending on the level of the epidural space into which the drug is administered. Approximately 335,000 epidural procedures are done every year in the UK (Cook et al. 2009).

The most common technique for identifying the epidural space to administer epidural medication relies on the principle that the pressure in the epidural space is lower than that in the surrounding tissues; this is known as the 'loss of resistance' (LOR) principle. The clinician pushes a needle, connected to a syringe filled with air or saline, through the soft tissues between 2 vertebrae while applying constant pressure to the plunger of the syringe. As the needle passes through tissue the air or saline cannot be forced out of the syringe, and is 'felt' as resistance. As it enters the epidural space, the clinician will 'feel' a sudden LOR and the air or saline can easily be pushed out of the syringe and into the epidural space (Wilson 2007).

Gaining access to the epidural space is usually a safe procedure, but side effects and complications can occur (NHS Choices 2013). Temporary nerve damage and localised infection occurs in 1 in 1000 people. Rare complications (1 in 10,000 to 1 in 100,000) include permanent nerve damage, epidural abscess, and epidural haematoma (Royal College of Anaesthetists 2014a). Dural puncture, a common side effect which occurs in 1 in 100 patients, develops when the epidural needle accidentally punctures the dura and arachnoid mater, the membranes that cover the brain and spinal cord and enclose the cerebrospinal fluid. A dural puncture can cause the fluid to leak out of the subarachnoid space, reducing the pressure of the fluid encapsulated within the dura mater. Following a dural puncture, people may have post‑dural puncture headaches (PDPH) (NHS Choices 2013).

A reliable method to positively identify the epidural space may lower the incidence of dural puncture and PDPH, and therefore improve patient outcomes and reduce costs.