May 4, 2017
The modern era of Chiari surgery was ushered in by Bernard Williams, the first neurosurgeon to intensively deal with the pathology of Chiari I malformation.
At the end of his prematurely interrupted life, he wrote an editorial advocating for leaving the dura open at the end of Chiari decompressions.
In his opinion, leaving behind an iatrogenic pseudomeningocele was more acceptable than dealing with the mass effect of a pseudomeningocele caused by the ball valve mechanisms of an imperfectly closed duraplasty.
This article caused a schism in the Chiari surgical technique which still endures today, between the American lore of closing the dura with a duraplasty and the British tradition of deference to the parting words of Mr. Williams.
In the following years, Ulrich Batzdorf M.D., Arnold Menezes M.D., and Thomas Milhorat M.D., demonstrated that through repetition, improvement, and impeccable technique, the incidence of pseudomeningocele could be contained to values below 5%.
Years later, the pseudomeningocele is still the defining complication in the field of Chiari surgery. [2,5]
Pseudomeningoceles can negatively affect the patient via a number of different mechanisms: 1) mass effect on the duraplasty and the dural contents; 2) aseptic meningitis; 3) negative effects on CSF flow and pressure; and 4) transcutaneous fistulas (with consequent bacterial meningitis).
Patients with a history of large postoperative pseudomeningoceles are often the recipients of subsequent ventriculo-peritoneal shunts, in the face of elevated CSF pressures.
The real overall incidence of pseudomeningoceles is probably underestimated, since many Neurosurgeons spring to corrective surgical action only in the case of transcutaneous CSF fistulae. Moreover, a number of Neurosurgeons tend to “accept” the chronic presence of non-expanding pseudomeningoceles, as a part of the normal spectrum of postoperative surgical results.
The concepts of “failed Chiari surgery” and of “posterior fossa revision” are quite recent and have gained traction thanks to the birth of Centers dedicated to the diagnosis and management of Chiari I Malformation. Persistent pseudomeningoceles are increasingly recognized as one of the causes for “failed Chiari surgery” in quite a large number of patients.
Running locked, running unlocked, and interrupted stitching configurations have been used in Chiari surgery, with similar results in the best of hands. Flawless execution is the only determining factor for successfully avoiding pseudomeningoceles, in the face of different needles and stitching materials used.
The dura of Chiari patients has several unique, challenging features. It is extremely thin over the cerebellar hemispheres (at the very top of the Y-shaped dural incision), because of the combined effects of a small posterior fossa and a pulsatile, crowded cerebellum. It is also very adherent to the periosteum at the level of the foramen magnum. Dural rents or shredding of the outer dural layer during the craniectomy can increase the chance of a postoperative pseudomeningocele.
The mismatch between a thin, fragile dura and a stiff, thick duraplasty (i.e. bovine pericardium, GoreTex) can create undue tension at the dural edge of the durarraphy (= dural suture line), with leaking stitch-holes.
Watertight closure is to be tested with Valsalva maneuvers. As a rule, one or two Valsalva maneuvers are not enough. Attention should be paid to the air-fluid level (often visible through the thinned and transparent dura), being sure that the entire height of the durarraphy is below such level, to guarantee reliable Valsalva testing. We routinely fill the subdural space with 20-40cc of injectable saline before passing the last dural stitch, to improve the odds.
Dural sealants have been engineered to prevent pseudomeningoceles. While these compounds have demonstrated themselves quite valuable in the supratentorial compartment, their record in the posterior fossa (and especially with Chiari surgeries) is not as impressive.
In the posterior fossa, gravity tends to bring the CSF through the dural stitching (and not away from it like in the supratentorial compartment).
Dural sealants cannot and should not be regarded as a magical remedy or a compensation to less than meticulous dural suturing.
Extradural techniques of Chiari decompression have been advocated to try to avoid pseudomeningoceles altogether.  Since the intrinsic limitations of the extradural Chiari techniques are not the focus of this Editorial, we will only say that a downside of this strategy has been the decreased number of dural openings and duraplasties.
Our preoccupation is that the new generations of Residents, tempted by the availability of sealants and extradural techniques, could end up becoming less proficient in the fine art of a watertight dural closure of the posterior fossa.
Dr. Liu and his colleagues have recently combined a subpial tonsillar resection along with a primary durarraphy (i.e. without a duraplasty). Using a variant of this technique, we were able to perform more than 250 cases without a single instance of pseudomeningocele, exploiting the finding that the Chiari dura is at its thickest along the midline.