Patients with Chiari II malformations present at birth with myelomeningocoele . The outlook for this population is somewhat restricted, and historically, approximately one third of the neonates with symptoms of brainstem dysfunction did not survive beyond infancy. Survival has, however, increased with prompt ventricular shunting, shunt revision when needed and posterior fossa decompression when appropriate (Talamonti et al. 2007). Overall, some 20–30 % of Chiari II patients will become symptomatic from their hindbrain herniation at some point in their lifetime.

Clinical presentation varies in different age groups. Infants and neonates can present with apnoea , inspiratory stridor, dysphagia , bradycardia and opisthotonus. The dysphagia may be severe enough to necessitate placement of a gastrostomy tube. When present at birth, such symptoms suggest hypoplasia or aplasia of brainstem nuclei and often predict a poor prognosis. More commonly, symptoms begin within the first few months of life and when they are seen in a previously well infant attention should first be given to normalisation of the intracranial pressure through shunt placement or shunt revision if necessary (Holinger et al. 1978; Soleau et al. 2008). Feeding and swallowing difficulties are the most common presenting symptoms for infants at risk and were found in 59–71 % of symptomatic Chiari II patients in one series (Pollack et al. 1992b). Infants may present with weak suck and prolonged feeding times, and they may demonstrate weight loss and failure to thrive, as their physical growth outpaces their ability to meet their nutritional needs. Breathing difficulties are the most dangerous symptom in Chiari patients and have been reported to occur in between 29 and 76 % of patients (Rauzzino and Oakes 1995). Inspiratory stridor is the result of bilateral tenth nerve paresis, with weakness of the vocal cord abductors. It may be caused by direct traction on the tenth nerve or by damage to the dorsal motor nuclei, secondary to micro-haemorrhage or compressive ischemia (Benjamin et al. 2009; Linder and Lindholm 1997; Rauzzino and Oakes 1995). One series observed that all infants began with normal swallowing and feeding that progressively deteriorated (Pollack et al. 1992b). In these cases, they found that feeding problems preceded respiratory difficulties and were a recognisable warning sign. In another publication, the same group found that all neonates that underwent surgical decompression, prior to the development of bilateral vocal cord paralysis , demonstrated some improvement in neurological function following surgery (Pollack et al. 1992a).

Older children more commonly have symptoms of spinal cord compromise, and the syringomyelia associated with Chiari II malformation behaves in a manner similar to that seen in Chiari I patients. Classic presentation is with lower motor neuron features in the arms and upper motor neuron findings in the legs. These include atrophy of the small muscles of the hands , together with disassociated sensory loss. Progressive enlargement of the syrinx cavity leads to loss of the ability to perform fine motor tasks, and fasciculation and loss of tendon reflexes may be evident in the upper limbs. Further expansion of the syrinx may impede descending corticospinal tracts, such that patients who were previously ambulatory may report a history of increasing falls, coming on over months or years.

Syringobulbia , or expansion of the syrinx into the brainstem, will manifest itself as dysfunction in multiple brainstem nuclei, which may wrongly be attributed to the underlying Chiari II malformation. Neurogenic athropathies may also be present. Back pain and scoliosis (see below) may also be the initial presenting complaints.

Changes in bowel or bladder function , however, should prompt imaging to exclude a tethered cord (Rauzzino and Oakes 1995). Headache and posterior cervical neck pain are also common and may be due to the distortion of the descending fibres of the spinal trigeminal tract or the upper cervical roots.

Radiographic assessment of patients with Chiari II malformation can begin with plain films of the cervical spine. Flexion and extension views can be used to assess for cervical instability . The posterior arch of C1 is incomplete in up to 70 % of cases and may be replaced by a compressive band of periosteal tissue. Full spine radiographs permit assessment of scoliosis, segmentation errors and other occult dysraphisms. Rarely, split cord malformation , with a bony median septum, may be identified on plain films (Rauzzino and Oakes 1995).

CT images are inadequate for studying the pathology of hindbrain herniation, but head CT imaging is helpful in evaluating ventricular size and can provide an objective measurement of the adequacy of ventricular shunting, although surgical exploration remains the ‘gold standard’ for the determination of the adequacy of shunt function.

MRI is the diagnostic tool of choice in evaluating patients with suspected Chiari malformations. The hallmark of the Chiari II malformation is the elongation and caudal displacement of the cerebellar vermis, medulla and lower brainstem, below the foramen magnum. In up to 70 % of Chiari II patients, a medullary kink may be identified. Elongation of the fourth ventricle into the upper cervical cord is also a common finding. The cerebellum of Chiari II patients is smaller than normal and is contained in a hypoplastic posterior fossa with obliterated retrocerebellar CSF spaces (Fig. 13.5). The tentorium cerebelli has a low insertion and can also be hypoplastic, allowing the cerebellum to tower above it. Other frequently observed cerebellar findings include persistent foetal location with extension around the midbrain into the cerebellopontine cisterns and cerebellar dysplasias and heterotopias (Rauzzino and Oakes 1995).

The surgical management of Chiari II malformation generally consists of bony decompression followed by some form of dural expansion manoeuvre. Prior to any posterior fossa decompression, however, patients with questionable shunt function should first undergo shunt revision. This approach can sometimes avoid the need for posterior fossa surgery because reducing the downward pressure on the cerebellum may lead to resolution of symptoms. Obtaining effective ventricular drainage may also lead to collapse of the syrinx, without recourse to major posterior fossa surgery. On the other hand, ignoring an active hydrocephalus and performing a craniovertebral decompression may sometimes lead to a potentially dangerous exacerbation of a hydrocephalus that was previously in a state of hydrodynamic equilibrium .

Reference to pre-operative MRI assists in the localisation of the vermian herniation, a medullary kink, the fourth ventricle and a low-lying torcular Herophilus and transverse sinus. For patients with a large foramen magnum, occipital craniectomy may not be required. If the torcular is low-lying, care must be taken to avoid inadvertent entry into the major venous sinuses, which could lead to dangerous blood loss. Care must be taken to distinguish the medullary kink from the cerebellar vermis, and intraoperative ultrasonography may be useful in this regard (Tubbs and Oakes 2004). In Chiari II patients, the choroid plexus usually retains its embryonic extraventricular location and marks the caudal end of the fourth ventricle.

Unlike with Chiari I malformations, adequate decompression of the Chiari II hindbrain hernia requires that the decompression should extend to the level of the displaced posterior fossa content. The herniated brainstem, unlike the cerebellar tonsils, cannot be delivered upwards or coagulated (Fig. 13.6). The fourth ventricular outlet should be identified and widely opened, and it is often necessary to develop several tissue planes, before the floor of the fourth ventricle can be clearly identified. Misidentification of anatomical landmarks or excessive tissue manipulation can result in damage to at-risk parts of the medulla or lower pons. Dense adhesions and hypervascularity may be encountered at points of compression or traction, making identification and dissection of these structures more difficult.

Early surgical therapy may prove to be life saving for symptomatic Chiari II patients, when the symptoms are attributable to brainstem dysfunction. For infants and neonates who present with respiratory distress and stridor, the timing of decompression is important. Beyond this age group, approximately 10 % of patients who survive will have late symptoms of brainstem dysfunction (Talamonti et al. 2007). Continued well-child surveillance and family education can therefore help to ensure that late presentations of brainstem compression do not go unrecognised. The importance of having a functioning shunt in this patient population should again be stressed. Obstructive apnoea , which can have a mortality rate as high as 60 %, can be reversed by an optimally functioning shunt (Cochrane et al. 1991; Hesz and Wolraich 1985; Soleau et al. 2008).

The extensive laminectomy required to decompress a Chiari II malformation increases the risk of delayed cervical instability , and the patients should undergo post-operative cervical spine evaluation (Oakes 1991).