# A Machine Learning Approach to Support Treatment Identification for Chari I Malformation
## 1 Introduction  
Chari malformation (CM) is characterized by the descent of one or both cerebellar tonsils by at least 5 mm beyond the McRae's line at magnetic resonance imaging (MRI) scan, that runs from the anterior (basion) to posterior (opisthion) border of the foramen magnum [1]. A descent of less than 3 mm is considered a physiological variant, while a herniation between 3-5 mm is borderline and needs for radiological follow-up in symptomatic cases, associated with syringomyelia (defined below), a pointed tonsillar profile or crowding of the subarachnoid space at the craniocortical junction (CVJ). On the other hand, Chari syndrome corresponds to the clinical findings associated with a radiologically defined CM. They include: headache, usually occipital provoked/worsened by coughing or other Valsalva-like maneuvers; brainstem complaints (nystagmus, dysphagia, sleep apnoea); cerebellar (ataxia) and/or cervical cord complaints (muscular hypotrophy, motor and sensory deficits); and oto-neurological symptoms (dizziness, loss of balance, hearing loss and hyperacusis, nystagmus, oscillposia). CM is often associated with syringomyelia, which is a neurological condition characterized by thepresence of a fluid-filled cavity within the parenchyma of the spinal cord or central canal, due to an alteration of cerebro-spinal fluid (CSF) circulation [2; 3; 4; 5]. CM is currently classified according to its anatomical features. * CM type I (CM-I): herniation of one or both cerebellar tonsils at least 5 mm below the foramen magnum, often asymptomatic. Hydrocephalus and anterior flattening of the midbrain, pons and medulla oblongata may occur. It is further sub-classified into CM I-A, when associated with syringomyelia, and CM I-B, when no syringomyelic cavity is present [6]. * CM type II (CM-II): caudal migration of the brainstem, cerebellum and fourth ventricle through the foramen magnum together with downward displacement of the cervical spinal cord. It is always associated with open spinal dysraphism or cystic spina bifida (myelomeningocele or myelocele) and also with syringomyelia and hydrocephalus. * CM type 0 (CM-0): syringomyelia with no associated tonsilla rhombencephalic herniation or minimal (less than 3 mm long). * CM type 1.5 or \"Chiari Complex\" (CM 1.5): tonsillar prolapse and brainstem kinking in the context of CVJ malformation. Although CM-I and CM 1.5 share morphological and anatomical similarities, an accurate radiological distinction needs to be made because patients with CM 1.5 have more probability to undergo initial decompressive surgery failure and there is usually persistence of syringomyelia [4]. * Acquired Chiari malformation (ACM) (also referred to as acquired tonsillar ectopy): herniation of the cerebellar tonsils secondary to posterior fossa trauma (85%) and space-occupying lesions, such as hydrocephalus, brain tumours (meningioma, 36%) and arachnoid cysts (32%) [2; 7]. CM-I is a rare disease, but the spread of neuroimaging in the area has increased the number of cases, often asymptomatic or paucisymptomatic: the incidental diagnosis today is between 1% and 4% in individuals undergoing MRI of the brain and cervical spine, with a range between 1.9 and 8.4/100,000 [6]. In the pediatric population, European studies have estimated CM-I to be relatively common, with an incidence of 0.24-3.6%; out of them, almost a quarter have syringomyelia at diagnosis, with a slight prevalence in females [3]. The exact etiopathogenesis of CM-I is not entirely clear. A lot of hypotheses have been proposed. Besides the association with craniosynostosis, spinal cord disorders, CSF hypotension, endocranial hypertension and skeletal abnormalities in CVJ [7; 8; 9], the most accredited theory is the alteration of the paraxial mesoderm after neural tube closure. This condition leads to underdevelopment of the occipital somites, resulting finally in a disproportion between cerebellum and a smaller posterior fossa [2].