Arnold-Chiari Malformation 1.5

A 29-year-old male presented with complaints of difficulty in walking along with unsteadiness for the past 6 months. On clinical examination, mild ataxia was present; there was no definite motor weakness or sensory loss. Magnetic resonance imaging (MRI) brain revealed peg-shaped cerebellar tonsils herniating below the level of foramen magnum by ~15 mm along with caudal displacement of the brainstem and obex of the fourth ventricle with consequent crowding of the foramen magnum, compression of medulla, and obstructive hydrocephalus. Basilar invagination was seen with tip of dens ~10–11 mm above McGregor line. Dorsal medullary bump and focal edema/early syrinx was seen at C2 level [Figure 1]. Diagnosis of Arnold-Chiari malformation 1.5 was made.Figure 1: Sagittal T1 (a) MRI image shows peg-shaped cere bellar tonsils herniating below the level of foramen magnum (white arrow) and basilar invagination (black arrow). Sagittal T2 MRI image (b) shows inferiorly displaced obex (white arrow) and focal edema in the dorsal medullary bump at C2 level (white arrowhead). Curved arrows in (a) and (b) demonstrate hydrocephalusChiari 1.5 refers to tonsillar herniation seen in Chiari 1 malformation in addition to caudal displacement of brainstem and fourth ventricle.[1,2] Caudal migration of the obex beyond the foramen magnum and elongation of the medulla and fourth ventricle and dorsal medullary bump are described.[3] Basilar invagination, atlanto-occipital fusion, retroflexed odontoid, abnormal clivus-spinal canal angle, and scoliosis are common bony abnormalities seen in patients with Chairi 1.5 malformation.[1] Neural tube defects in the spine associated with Chiari 2 malformation are not seen. Syringomyelia is usually present. Differentiating Chiari 1.5 from Chiari I malformation is important as those with Chiari 1.5 malformation need sufficient foramen magnum decompression and fenestration of intradural arachnoid adhesions to achieve resolution of syringomyelia and avoid repeated surgeries.[1,2] Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.