The most frequent type I is usually asymptomatic into adulthood. First, unspecific symptoms like headaches and neck pain occur. The headaches can be amplified by the Valsalva manoeuvre. In the further course of disease, several neurological deficits appear:

In the diagnosis of Chiari malformation, the first radiological sign in sagittal MR images is the inferior displacement of cerebellar tonsils through the foramen magnum. They can reach the lamina of the third cervical vertebra. The medulla oblongata, the fourth ventricle and the pons can be elongated and displaced caudally. A typical finding is a syringomyelia of the cervicothoracic spinal cord (◘ Fig. 7.1).

In axial MR images, further dysmorphic features can be detected: a small posterior fossa, a narrowing of the cisterna magna and a displacement of cranial nerves. The cerebellar tonsils are merged with the dorsal and lateral medulla oblongata by meningeal adhesions. Further findings are spina bifida and hydrocephalus [4].

Funduscopy by an ophthalmologist can show papilledema as a sign of raised intracranial pressure in cases of hydrocephalus.

In cases of symptomatic Chiari malformation , especially when neurological deficits occur and radiological signs are typical, surgical treatment should be indicated. Surgery includes suboccipital craniectomy with resection of the C1 lamina for bony decompression and an expansile duraplasty with a dural substitute or autologous tissue. Sometimes the laminae of C2 and C3 as well as the cerebellar tonsils have to be resected. The surgical technique of suboccipital decompression is described in detail in ► Sect. 7.1.5.

If symptomatic hydrocephalus occurs, then this has to be treated when persisting after the decompressive surgery. There are two options for the surgical treatment of hydrocephalus: a ventriculoperitoneal shunt or a third ventriculostomy.

The patient is in a prone position, and the inclined head is fixed with a Mayfield clamp. By using a suboccipital midline approach, the suboccipital external table of the skull and the lamina of C1 are exposed (◘ Figs. 7.2, 7.3, 7.4, and 7.5). Only in cases of very deep cerebellar tonsils the laminae of further cervical vertebrae have to be exposed.

Over the posterior fossa, two paramedian and further caudally two lateral burr holes are placed (◘ Fig. 7.6). A curved dissector is used to separate the dural from the internal table of the skull via the burr holes (◘ Fig. 7.7). This has to be done consequently down to the foramen magnum to avoid an early rupture of the dura during craniotomy. The osteoclastic craniotomy (craniectomy) is done with a Kerrison punch or a craniotome starting from the burr holes (◘ Fig. 7.8). When crossing the midline with the craniotome, the surgeon must be aware of a potential occipital dural venous sinus. A rupture of this dural venous sinus has to be closed with sutures.

After elevating the bone flap, a laminectomy of C1 is mandatory (◘ Figs. 7.9 and 7.10). This is a precondition for an adequate dural enlargement and reconstruction. The incision of the dura starts at the C0/C1 level and goes downwards to the upper edge of the C2 lamina. The cranial part of the incision is led over both cerebellar hemispheres resulting in a Y-shaped dural opening (◘ Figs. 7.11 and 7.12). If this surgical step leads to an opening of a possible occipital venous sinus, the latter has to be closed with Vicryl 4-0 sutures (Johnson & Johnson Medical, Norderstedt, Germany). Further space for the cerebellar tonsils and the medulla oblongata can be gained by separating arachnoid adhesions (◘ Figs. 7.13, 7.14, 7.15, and 7.16).

Afterwards, an expansile duraplasty is recommended. We use the dural substitute Tutopatch (Bess Medizintechnik GmbH, Berlin, Germany). After finishing the sutures, fibrin glue is applied for adequate watertight closure (◘ Figs. 7.17, 7.18, and 7.19). The postoperative result is documented by MRI 3–6 months after surgery (◘ Fig. 7.20).