Preventing Post-dural Puncture Headache
Post-dural puncture headache (PDPH) is an excruciating and debilitating condition typically lasting 3 to 7 days, but some anecdotal reports indicate that it can last up to two or more months. Some cases are so severe that even opioids have little or no effect. It is generally accepted that the physical phenomena that causes the headache is related to CSF leaking from the dura with resultant pressure reduction of the fluid remaining in the dura. In this scenario, the membrane at the base of the skull which separates spinal and brain fluid is normally at equilibrium, but when the pressure drops in the dura this membrane is stretched and applies tension to surrounding nerves.
The dura which encases the subarachnoid space is filled with cerebrospinal fluid (CSF) and has higher fluid pressure than that in the surrounding epidural space. While spinal tap and spinal anesthesia are intentional punctures (to sample the spinal fluid or deliver anesthesia to the subarachnoid space, respectively), it is not uncommon that accidental punctures occur due to over-penetration of the needle during an epidural anesthesia procedure.
The occurrence of PDPH following a spinal tap or spinal anesthesia is typically in the range of a few percentage points, but for obstetrical cases can be as high as 20 to 30% of the cases. Men have a substantially lower incidence of PDPH than women, and the elderly have the lowest rate of all. This variation can be explained by today’s prevalence of thinner needles and resulting deflections proportional to tissue density (more about this later).
A Brief History
Over a period of twenty years in anesthesiology practice and as an assistant anesthesiology professor, the author administered or supervised 4,465 spinal anesthetics (based on a retrospective survey of reports and records) with no occurrences of post-dural puncture headache.1 The technique used, utilizing a lateral approach with incidence angle near 35% and a 20-gauge Becton Dickinson (B-D) Quincke point needle, had been first suggested by an instructor during the author’s unsuccessful attempts at the midline approach in administering his first spinal anesthetic. This approach, recommended as useful in “difficult” situations by that instructor, appeared easier to administer and was thus adopted as the exclusive methodology during the author’s practice. The retrospective survey of reports and records, which specifically recorded incidence of headache by a mandatory next-day visit and the anesthesia service’s discharge note, did not find a single instance of PDPH in any of the 4,465 consecutive spinal anesthesia cases reviewed.
It is clear to the author, both from experience and laboratory testing, that four variables in particular can be used to prevent PDPH occurrence: the shape of the needle point (beveled is necessary), the needle diameter (20 gauge is preferred), the angle of entry to the dura (about 35%), and the orientation of the bevel face relative to the dura. Laboratory experiments on a cadaver dura which was submerged in clear fluid and filled with pressurized dyed fluid, found that a perpendicular puncture resulted in significant leakage, while a lateral puncture with bevel down resulted in no leakage.1
A model of needle punctures helps to illustrate the mechanism of puncture closure for three different needle orientations (see Figure 1). The first puncture (A) creates a flap that can open in either direction (like a saloon door). Since the cerebrospinal fluid in the subarachnoid space is at higher pressure, CSF would flow out of the dura in this situation. In the second puncture (B), the bevel is facing away from the dura, creating a flap that is kept open (outward from the dura) by CSF pressure. The last puncture (C), created by a needle inserted laterally and with the bevel cutting the dura inward, creates an ideal flap which will close behind the needle as it is withdrawn by the physician. In other words, this last puncture is self-sealing due to the differential pressure of CSF keeping the flap closed. Without CSF leakage, PDPH is prevented.
Thorsen wrote in 1947, “When the direction of puncture was as tangential as possible, leakage often failed to appear…in spite of considerable pressure.”2 In hindsight, this clue was profound.
Questionable Logic of Thin Needles
The development and use of fine needles followed Moore’s theory in the 1950’s that the dura fibers separated when the bevel’s face was parallel to the dural fibers.3 At that time, finer needles failed to reduce the incidence of PDPH. This failure was attributed to the needles still being too large and thus damaging the dural fibers rather than parting them. At the time, the conventional wisdom was to use smaller needles so as to separate the fibers by a lesser amount. In hindsight, this logic appears flawed since Moore’s conclusions had been contradicted years earlier by Thorsen’s serial sections of dural punctures (punctured one to eleven days before the expiration of moribund patients).2 In all cases, the dural fibers were cut rather than separated (see Figure 2).
Ironically, the trend to thinner needles in an attempt to reduce puncture damage to the dura has introduced an additional, uncontrollable variable-needle deflection while failing to eliminate PDPH. Laboratory tests have confirmed that pushing a thin needle through materials having varying densities (to simulate variation in human tissue)1,5,6 result in varying amounts of needle deflection. Needle deflection (in the case of beveled needles) is in the direction away from the bevel and thus introduces an angle of incidence when penetrating the dura, although not near the optimum 35%. It is presumed that the reason that such a large percentage of dural punctures are not complicated with PDPH6 is the inevitable bending and deflection of the customary thin spinal needles used.
The significant variation in the amount of deflection due to tissue density and resistance helps to explain why a midline approach, performed on children and women (having a shorter distance between the skin and dura), would be expected to have diminished needle bending with a more nearly perpendicular dural entry. Further, during pregnancy, the softening of female tissues is accentuated and thus would be expected to further reduce needle deflection. Indeed this is the population that has the highest incidence of PDPH. Men and the aged, on the other hand, have a reduced incidence of PDPH. For men, the larger body mass offers thin needles more resistance and thus greater deflection before entering the dura. Likewise, it appears that for the aged, the effects of aging, dehydration, arthritis, etc. contribute to increased tissue density and similar needle deflection.
Recommended Needle Insertion Procedure
The following is the author’s preferred lateral approach to needle insertion and exclusive use of a 20-gauge needle. The use of this technique has remarkably had zero incidences of post-dural puncture headache over the 20 years and 4,465 procedures performed. The procedure is as follows and is performed on a relaxed patient positioned by an assistant in a sitting or lying position: