occipito-cervical instability and pseudomeningocele. rare post-operative complications include hydrocephalus and brain stem compression by retroflexion of odontoid. also, an extended cvd created by a wide opening and big duroplasty can cause a cerebellar "slump". this complication needs to be corrected by cranioplasty. in certain cases, irreducible compression of the brainstem occurs from in front (anteriorly or ventral) resulting in a smaller posterior fossa and associated chiari malformation. in these cases, an anterior decompression is required. the most commonly used approach is to operate through the mouth (transoral) to remove the bone compressing the brainstem, typically the odontoid. this results in decompressing the brainstem and therefore gives more room for the cerebellum, thus decompressing the chiari malformation. arnold menzes, md, is the neurosurgeon who pioneered this approach in the 1970s at the university of iowa. between 1984 and 2008 (the mr imaging era), 298 patients with irreducible ventral compression of the brainstem and chiari type 1 malformation underwent a transoral approach for ventral cervicomedullary decompression at the university of iowa. the results have been excellent resulting in improved brainstem function and resolution of the chiari malformation in the majority of patients. epidemiology congenital chiari i malformation, defined as tonsilar herniations of 3 to 5 mm or greater, was previously believed to be in the range of one per 1000 births, but is likely much higher. women are three times more likely than men to have a congenital chiari malformation. type ii malformations are more prevalent in people of celtic descent. a study using upright mri found cerebellar tonsillar ectopia in 23% of adults with headache from motor-vehicle-accident head trauma. upright mri was more than twice as sensitive as standard mri, likely because gravity affects cerebellar position. cases of congenital chiari malformation may be explained by evolutionary and genetic factors. typically, an infant's brain weighs around 400g at birth and triples to 1100-1400g by age 11. at the same time the cranium triples in volume from 500 cm3 to 1500 cm3 to accommodate the growing brain. during human evolution, the skull underwent numerous changes to accommodate the growing brain. the evolutionary changes included increased size and shape of the skull, decreased basal angle and basicranial length. these modifications resulted in significant reduction of the size of the posterior fossa in modern humans. in normal adults, the posterior fossa comprises 27% of the total intracranial space, while in adults with chiari type i, it is only 21%. h. neanderthalensis had platycephalic (flattened) skulls. some cases of chiari are associated with platybasia (flattening of the skull base). history the history of chiari malformation is described below and categorized by the year: 1883: cleland was the first to describe chiari ii or arnold–chiari malformation on his report of a child with spina bifida, hydrocephalus, and anatomical alterations of the cerebellum and brainstem. 1891: hans chiari, a viennese pathologist, described the case of a 17-year-old female with elongation of the tonsils into cone shaped projections which accompany the medulla and are crammed into the spinal canal. 1907: schwalbe and gredig, pupils of german pathologist julius arnold, described four cases of meningomyelocele and alterations in the brainstem and cerebellum, and gave the name "arnold–chiari" to these malformations. 1932: van houweninge graftdijk was the first to report the surgical treatment of chiari malformations. all patients died from surgery or postoperative complications. 1935: russell and donald suggested that decompression of the spinal cord at the foramen magnum might facilitate the csf circulation. 1940: gustafson and oldberg diagnosed chiari malformation with syringomyelia. 1974: bloch et al. described the tonsils position to be classified between 7 mm and 8 mm below cerebellum. 1985: aboulezz used mri for discovery of extension society and culture the condition was brought to the mainstream on the series csi: crime scene investigation in the tenth-season episode "internal combustion" on february 4, 2010. chiari malformation was briefly mentioned on the medical drama house m.d. in the fifth-season episode "house divided", it was the focus of the sixth-season episode "the choice". it is also the focus of private practice season 4 episode 4, where a pregnant woman is diagnosed with it. it was the a cause of death on the reality television series dr. g: medical examiner in the sixth-season episode "bruised and battered". it was also mentioned in the medical drama a gifted man, in the first-season episode "in case of separation anxiety". it is also featured in the 3rd and 4th episode of the 7th season of the series rizzoli & isles where dr. maura isles is diagnosed with the condition. notable people rosanne cash – u.s. singer-songwriter; daughter of johnny cash julia clukey – u.s. luge competitor for team usa in 2010 vancouver winter olympics joanna david – british television and stage actress j. b. holmes – u.s. professional golfer marissa irwin – u.s. fashion model with chiari secondary to ehlers–danlos syndrome bobby jones – u.s. world golf hall of fame golfer and founder of the augusta national golf club allysa seely – u.s. gold medalist at the 2016 summer paralympics in the paratriathlon leah shapiro – u.s. drummer for the band black rebel motorcycle club michelle stilwell – canadian wheelchair racer and politician rachid taha – algerian singer sabre norris – australian skateboarder and surfer