Hans Chiari was born in Vienna in 1851, the son of the gynaecologist Johann Chiari (1817-1854). He studied medicine in Vienna and in 1874-1875 served as assistant to the famous Karl Freiherr von Rokitansky (1804-1878). Following graduation in Vienna he served as assistant to Richard Ladislaus Heschl (1824-1881) until 1879. In 1878 he was habilitated in pathological anatomy in Vienna, and four years later he became extraordinarius in Prague. The following year he was appointed ordinarius and superintendent of the pathological-anatomical museum in Prague. In 1906 he was appointed ordinarius of pathological anatomy in Strassburg, where he remained until his death. 

During the years from 1876 to 1916 Chiari published more than 177 writings. His publications are distinguished by his exhaustive knowledge of medical literature, being extremely careful to give credit to the discoveries of others. The majority of his works concern systematic post-mortem examinations, a field to which he devoted a separate monograph. In connection with autopsies he was also careful to take care of interesting material, partly for his own teaching, partly for the museum.


In 1891 he described a brain malformation that is characterized by abnormalities in the region where the brain and spinal cord meet, and it causes part of the cerebellum to protrude through the foramen magnum (bottom of the skull) into the spinal canal. This was to be called the Arnold-Chiari malformation, named after Chiari and German pathologist, Julius Arnold (1835 − 1915). The malformation was given its name in 1907 by two of Dr. Arnold's students.

Other classifications include the Chiari Malformation Types II and III, each with more rhomboencephalic derivative involvement (i.e.- cerebellar vermis, brainstem, fourth ventricle). A common thread to all three types of malformations include the loss of normal free movement of the cerebrospinal fluid (CSF) out of the normal outlet channels of the fourth ventricle.

The pathophysiology leading to the Chiari I Malformation has not been completely elucidated to date, although several theories include: restricted posterior fossa volume compared to posterior fossa neural elements, possibly due to an underdeveloped occipital somite or to an overdeveloped suptratentorial component and shallow posterior fossa, causing cerebellar herniation; CSF flow across the foramen magnum is abnormal, leading to herniation by the mismatch of volume and pulsations of the CSF forcing open the central spinal canal; or the normal CSF flow is prevented in the compartment of the spinal cord. However, the best evidence thus far seems to support that there is a craniospinal pressure gradient across the foramen magnum that causes the brainstem herniation. 

A large majority of cases of Chiari I malformations are congenital, but there are some reports of acquired malformations secondary to CSF shunts. The true incidence of CIM is unknown; however, in a report by Friede, out of 7,400 brain dissections, 2 cases of C1M as well as 46 additional examples of ‘chronic tonsillar herniation’ were reported. 

Some patients may be completely asymptomatic. However, the most common presenting symptom of patients with CIM is pain. The patient may present with headache or neck pain, exacerbated by physical activity or neck movement. The pain may also be exacerbated by activities that use the Valsalva maneuver (coughing, sneezing, laughing); infants may present with irritability or grabbing at the neck. Other presentations include clumsiness, dysphagia, dysarthria, and symptom complexes caused by cord compression. Symptom complexes present based on the location of the impingement on the spinal cord (i.e.- brainstem dysfunction if impinging more cranial, sensory loss if impinging more caudal, etc.). The differential diagnosis of a Chiari I Malformation includes congenital vs. acquired CIM’s, intracranial mass lesion causing hindbrain herniation, and hydrocephalus. 

Initial workup includes analysis of supratentorial structures to ensure the absence of a mass lesion or hydrocephalus. The mainstay of diagnosis of CIM is via MRI. Although hindbrain herniation 5 mm below the foramen magnum has been used as a rough ‘cut-off’ point for the diagnosis of a CIM, the extent of hindbrain herniation as well as the clinical picture of the patient should be used to diagnose CIM. Patients may be seen with herniations of 2-3 mm but still have the diagnosis of CIM due to their symptom complex. Other associated radiological anomalies may occur, including atlantooccipital assimilation, basilar invagination, and fused cervical vertebrae. Also, cine cardiac-gated MRI to assess CSF flow across the foramen magnum can he helpful. 

Treatment for CIM’s is surgical as no effective nonsurgical alternative exists for symptomatic patients. There is immediate relief of symptoms and the objective decrease in the size of syrinxes following surgical intervention. There are multiple surgical techniques employed, ranging from simple decompression, osseous decompression, osseous decompression with dural grafting, osseous decompression with dural grafting and intradural dissection of adhesions, and osseous decompression with dural grafting, intradural dissection, tonsilar manipulation and dissection. The success of decompression should be confirmed with intraoperative ultrasound to assess CSF flow.