For all Chiari I patients, treatment of symptomatic hydrocephalus should be prioritised. Endoscopic third ventriculostomy is now preferred to traditional ventriculoperitoneal shunting as the first option for CSF diversion whenever patients present with clinical signs of raised intracranial pressure (Massimi et al. 2011).

Management options for arachnoid cysts include resection or fenestration (Fig. 10.5). Insertion of cyst shunts is not considered by the author due to their high failure rate. Treatment for Chiari I malformations due to solid masses requires tumour resection with foramen magnum duraplasty (Fig. 10.6).

For Chiari I hindbrain hernias, there is general agreement that surgical treatment should be reserved for symptomatic patients. In children, however, it may be unclear which complaints are linked to the malformation. It can also be a challenge to differentiate between physiological and pathological tonsillar descent, as revealed by MRI. In the absence of neurological symptoms or progressive scoliosis , the author does not recommend decompression for children with a Chiari I malformation, unless they demonstrate the typical occipital headache provoked by Valsalva-like manoeuvres, such as sneezing or coughing.

With all patients, it should be borne in mind that not every headache is due to tonsillar herniation, even when such an abnormality is present. Nor does the presence of syringomyelia constitute an indication for surgery in its own right, in children or adults alike. When neurological symptoms are present, however, surgery should be recommended. As a general rule, progression of neurological symptoms occurs more rapidly than does enlargement of an underlying syrinx. The author has not observed enlargement of a syrinx in an asymptomatic patient with a Chiari I malformation.

Even though foramen magnum decompression is widely accepted as the treatment of choice for Chiari I malformation, there is no general agreement on how this operation should be performed (Schijman and Steinbok 2004; Haroun et al. 2000). Gardner’s original operation consisted of a wide craniectomy of the posterior fossa and opening of the 4th ventricle in order to plug the obex with a piece of muscle. The dura was left open. He reported 5 mortalities after 74 such procedures (Gardner 1965). Similar mortality rates, as well as significant morbidity caused by manipulations at the obex (Williams 1978), led other surgeons to modify this operation. Guided by modern imaging techniques such as MRI in the 1980s and modern CT scanners, the anatomy of patients with Chiari I malformation could be studied in much more detail than was previously possible. This led to less invasive procedures, such as leaving the arachnoid intact after dural opening (Logue and Edwards 1981), incising only the outer dural layer (Gambardella et al. 1998) or even restricting the operation to a purely bony decompression (James and Brant 2002). It should, of course, be the intention of every neurosurgeon to restrict any operation to its essential requirements in order to limit surgical morbidity, complications and discomfort for the patient but, at the same time, to do so without compromising the beneficial effects of the procedure.

The following account of a surgical technique describes the author’s preferred method, as do subsequent accounts of operative techniques in this chapter. It should be noted, however, that there are many variations, and it is true to say that the only manoeuvre that all methods have in common is removal of bone from the occipital squama.

Surgery is performed in prone position. The decompression is limited to a foramen magnum decompression of 3–4 cm, together with removal of the posterior arch of the atlas. The atlanto-occipital membrane is coagulated and dissected off the dura, which is then incised in a Y-shape, under the microscope, and held open by sutures. Care should be taken to leave the arachnoid intact in order not to pull on and tear underlying bridging veins or small blood vessels supplying the spinal cord, brainstem or cerebellum. Leaving the arachnoid intact at this stage also avoids contamination of the subarachnoid cisterns with blood. Venous sinuses may be encountered upon opening the dura, in the midline or at the foramen magnum, and these will require suturing. The arachnoid should be examined for evidence of scar formation or adhesions to the cerebellum, brainstem or spinal cord. The arachnoid is then incised, starting below the cerebellar tonsils and continuing to an extent that allows these structures to be spread apart for inspection of the foramen of Magendie. For this purpose, the cerebellar tonsils are coagulated at their tips and medially. Resection of tonsils is advised against as this may risk injury to important blood vessels such as the posterior inferior cerebellar artery (PICA). If Magendie is patent and no arachnoid adhesions are detectable elsewhere, then no further intradural dissections need be performed. If the foramen is obstructed, then it should be opened by sharp dissection. In patients with severe arachnoid scarring, dissection will need to create at least a communication between the cranial and the spinal subarachnoid channels. In such instances, it may not always be possible to open Magendie without risking injury to important structures, such as the PICA. The dissection should not, in these circumstances, be carried out laterally, to avoid injury to perforating vessels of brainstem or spinal cord. A duraplasty is then inserted using alloplastic material. To avoid formation of adhesions between nervous tissue and the duraplasty or suture line, the graft is lifted off the cord by tenting sutures, which are fixed to muscle attachments laterally. Finally, the wound is closed, paying particular attention to the muscular layer in order to avoid CSF fistulas . Postoperatively, all patients should be supervised on the intensive care unit for at least 24 h before returning to the normal ward.

In patients with additional basilar invagination , a combination of ventral and dorsal compression may be associated with instability of the craniocervical junction. These pathophysiological components may not, however, be relevant in all affected patients. Of a group of 53 patients with basilar invagination in the author’s series, 35 were managed surgically. In 16 patients, there was neither a ventral compression by the odontoid nor craniocervical instability. These patients were managed with foramen magnum decompression for their Chiari malformation as the only procedure. In another 10 of the 35 patients, who demonstrated no clinical signs of ventral compression by the odontoid such as caudal cranial nerve deficits but either radiological evidence of craniocervical instability or assimilation of the atlas to the occiput, foramen magnum decompression was combined with craniocervical stabilisation. In the remaining 9 patients, ventral compression of the medulla oblongata had caused caudal cranial nerve dysfunctions. These patients underwent transoral resection of the odontoid , followed by posterior decompression and craniocervical fusion (Fig. 10.2) (Klekamp and Samii 2001).

Whenever the position of the odontoid leads to brainstem compression, the key elements of surgical treatment are distraction of the C1/2 intervertebral joints and C1/2 fusion. This distraction may reverse the ventral compression to a degree that no additional transoral resection of the odontoid is required. In the author’s series, the decision for a transoral decompression was based on clinical signs of caudal cranial nerve deficits in the presence of compression of the medulla by the odontoid. Whether a transoral resection of the odontoid is obsolete (Goel and Shah 2009) or still required for patients with substantial and irreducible ventral compression (Smith et al. 2010) remains a controversial issue.

For craniocervical stabilisations, the implants need to be adjusted carefully to the abnormal anatomy. Precise planning is required to allow their safe fixation at the occipital bone as well as the allocation of the bone graft despite of the craniectomy required for foramen magnum decompression. All implants need to be covered completely by the muscular layer during closure, without too much strain being placed on soft tissues; otherwise, local discomfort or even CSF fistulas may result.

If clinical signs of hydrocephalus are present in Chiari II patients, treatment of the increased intracranial pressure has always the first priority. Endoscopic third ventriculostomy is again an optional alternative to ventriculoperitoneal shunts (Elgamal et al. 2011). In the literature, it has been stated in two large series that if the hydrocephalus has been managed successfully, then only a small minority of Chiari II patients require a decompression (Talamonti and Zella 2011; Rauzzino and Oakes 1995).

If one considers the pathophysiological considerations put forward by McLone (McLone and Knepper 1989) and the positive results obtained following intrauterine operations on foetuses (Danzer et al. 2011; Tulipan et al. 1999), Chiari II malformations are potentially preventable or reversible, if the decompression is carried out early enough in symptomatic patients with a sufficiently treated hydrocephalus. A number of studies have reported a benefit if decompression for Chiari II is performed as soon as neurological symptoms begin (Rauzzino and Oakes 1995; Teo et al. 1997; Pollack et al. 1992, 1996). Yet, there appears to be very little scientific evidence to support routine use of this approach for symptomatic infants (Tubbs and Oakes 2004). This may reflect the fact that, once severe brainstem dysfunctions are present, a decompression may not reverse established neurological deficits (Kirsch et al. 1968).

In the author’s series, 42 patients presented with a Chiari II malformation. Surgical management was only recommended in 13 of these patients, when there was no evidence for ventricular shunt malfunction but a clear history of progressive brainstem or cervical cord dysfunction (Charney et al. 1987; Kirsch et al. 1968). Six were under 2 years of age, presenting with signs of central dysregulation. Three patients presented between 5 and 14 years of age (Fig. 10.3) and the remainder in adulthood, with progressive upper extremity dysfunction.

When operating for Chiari II malformation, the decompression must be undertaken at the spinal levels corresponding to the tonsillar descent, rather than at the foramen magnum, which is enlarged in this entity. Patients are operated in prone position. The exposure extends from the foramen magnum to the lowest lamina covering the herniated tonsils. After laminectomy of these segments, the atlanto-occipital membrane should be coagulated and dissected off the dura. The dural incision then starts at the level of the foramen magnum and extends over all levels involved. Then the arachnoid is opened. Dissection should be limited strictly to the midline in order to avoid injury to perforating vessels or caudal cranial nerves. It should be borne in mind that the brainstem is displaced caudally in these patients, taking with it important structures such as the PICA or caudal cranial nerves. As outlined for Chiari I operations, it is also desirable to open the foramen of Magendie in Chiari II patients, although this should be undertaken only if it can be performed safely. Coagulation of tonsils , which is a safe technique in Chiari I to gain access to Magendie , is not recommended in Chiari II as these are very often tightly adhered to the underlying brainstem and any such manoeuvre carries considerable risks . Arachnoid dissection should concentrate on creating a passage between the intracranial and spinal subarachnoid channels. Once that is achieved, a duraplasty should be inserted. As described for Chiari I surgery (above), the duraplasty is lifted off the cord by tenting sutures.

Laminectomies in this patient group carry a significant risk of producing postoperative kyphosis or swan-neck deformities (Lam et al. 2009) (Fig. 10.3). They should therefore be combined with posterior fusion , which can be achieved elegantly with lateral mass screws (Fig. 10.7).

In the author’s series, ventriculomegaly was more common in foramen magnum arachnoiditis compared to Chiari I malformations (52 % vs. 8.6 %). This implies that if a borderline tonsillar herniation is associated with ventricular dilatation, then arachnoiditis at the foramen magnum may well be present. Depending on the extent and severity of the arachnoiditis, surgical management may require CSF diversion, in addition to foramen magnum decompression.

When foramen magnum arachnoiditis is not diagnosed preoperatively, its presence, extent and severity must be determined after dural opening. Dural opening has to respect the arachnoid layer in such cases in order to avoid surgical morbidity related to vascular injuries in particular. Thereafter, the aim of surgery is to create a free CSF passage between the intracranial and spinal subarachnoid spaces. It is not advisable to dissect all arachnoid scarring off the spinal cord, medulla oblongata and cerebellar tonsils. On the contrary, such attempts are risky and simply lead to new adhesions and scar tissue formation. In foramen magnum arachnoiditis, the foramen of Magendie is always obstructed. A decision must therefore be made, intraoperatively, as to whether or not it can be opened safely. If important structures such as the PICAs are embedded in arachnoid scar tissue, then the risk may be too high. Some authors recommend placement of small catheters to provide an outflow for the 4th ventricle (Abe et al. 1995). However, even with ultrasound guidance, this remains a very risky manoeuvre, and the author has encountered patients with severe neurological deficits as a consequence of malpositioned catheters in this region. A safer strategy is to leave the foramen Magendie closed and place a supraventricular shunt. Indeed, because of such concerns, some surgeons have previously recommended limiting the management of foramen magnum arachnoiditis to ventricular shunting (Appleby et al. 1969).