Chiari Malformation

Chiari malformations are a spectrum of hindbrain herniations. Type I, the form most often treated by neurosurgeons, is caudal displacement of the cerebellar tonsils through the foramen magnum, conventionally 5 mm or more below the basion–opisthion line in adults^[1]. Type II (the Arnold–Chiari malformation) involves descent of the vermis, brainstem and fourth ventricle and is almost always associated with myelomeningocele. Types III and IV are rare. Many Chiari I patients have a small, crowded posterior fossa, and some have craniocervical instability or ventral compression from basilar invagination, which changes the operation required.

In a series of 364 symptomatic adults and children, the most frequent complaint was a suboccipital headache brought on by coughing, sneezing or straining (Valsalva), often with neck pain; other symptoms included dizziness, ocular disturbances, otoneurological symptoms, dysphagia, sleep apnoea and sensory or motor findings^[1]. A syrinx produces dissociated (pain and temperature) sensory loss, weakness and wasting of the hands, and scoliosis, particularly in children. Children may present atypically, with irritability, failure to thrive or sleep-disordered breathing^[2].

Sagittal MRI shows the degree of tonsillar descent and the shape of the tonsils (pointed, “peg-like” tonsils suggest true crowding). MRI of the whole spine screens for a syrinx and a tethered cord. Cine phase-contrast MRI can show reduced CSF flow at the foramen magnum, which supports a symptomatic diagnosis in borderline cases. Measurements of the clivo-axial angle, the pB–C2 line and the position of the dens identify ventral compression and instability; flexion–extension imaging is used when instability is suspected. Imaging of the head should exclude secondary causes of tonsillar descent, such as a mass, hydrocephalus or intracranial hypotension, because decompression in these settings is harmful.

Indications are typical Valsalva-induced headaches or other attributable symptoms, a syrinx, or progressive neurological findings. Asymptomatic tonsillar ectopia without a syrinx is usually observed. Patients with significant ventral compression or instability may need transoral or endonasal odontoidectomy, or occipitocervical fusion, in addition to or instead of decompression.

• Prone positioning with the neck gently flexed; a midline incision to the occiput and C1.
• Suboccipital craniectomy of modest size (about 3 cm wide and 3 cm high) with removal of the posterior arch of C1; a large craniectomy risks cerebellar ptosis.
• Options for the dura: bone-only decompression with release of the outer dural layer and atlanto-occipital band, or opening the dura with an expansile duraplasty. Intradural steps may include arachnoid dissection, opening of the foramen of Magendie and reduction (coagulation or subpial resection) of the tonsils.
• Intraoperative ultrasound can show whether CSF pulsation is restored after bony decompression.
• A watertight duraplasty reduces the risk of pseudomeningocele and CSF leak.

In a series of 130 children, decompression with duraplasty improved symptoms in most patients and the syrinx reduced in the large majority of those who had one^[2].

Complications include pseudomeningocele, CSF leak, aseptic (chemical) meningitis, wound infection and hydrocephalus. Opening the dura increases these risks but may lower the chance of persistent symptoms or syrinx, so the choice is tailored to the patient. Headache relief is generally better than recovery of established myelopathy or sensory loss; a syrinx that does not shrink should prompt a search for residual obstruction at the foramen magnum.