Unlike the “classic” Chiari Malformation type 1, Acquired Chiari Malformation (ACM) is defined by the herniation of cerebellar tonsils subsequent normal hindbrain in prior brain imaging, and it is not related to craniovertebral junction malformations.
Chiari malformation could be also “iatrogenic” and then called “acquired” Chiari I malformation. 2) 3) In fact, the term acquired Chiari malformation (ACM) was used by Payner et al. in the presence of the herniation of cerebellar tonsils subsequent to the documentation of a normal hindbrain in prior brain imaging 4). It was mainly described after cerebrospinal fluid shunting, first after lumboperitoneal shunting, and lately recognized after supratentorial shunting 5) 6) 7) 8).
Chumas et al. reviewed a series of 143 patients to determine the incidence of ACM and found that the incidence of hindbrain herniation may be as high as 70% in asymptomatic patients with LP shunts. Five of these patients became symptomatic and required Chiari decompression. During their follow-up period, which is about 5.7 years, there was one shunt-related death due to unsuspected tonsillar herniation 9).
According to Johnston et al., the pathogenesis of syrinx formation may be explained by the hydrodynamic theory, as the lumbar shunt can be responsible of the disorders of CSF circulation.Johnston et al. also reported one patient having syrinx formation before ACM suggesting that “Chiari malformation may follow rather precede syrinx formation” in some cases 10)
A sneeze caused acute left arm pain in a 36-year-old woman with a lumboperitoneal (LP) shunt that had been placed 3 years earlier for relief of headaches caused by pseudotumor cerebri. Numbness progressed up the left arm, neck, and back of the head and finally into the left face along with weakness of the hand and arm. Magnetic resonance imaging (MRI) and computed tomography revealed new tonsillar herniation and a large eccentric syrinx extending from C2 to T6. The functioning LP shunt was clamped, and a ventriculoatrial shunt was placed. Pain lessened and motor function improved slightly. MRI revealed complete resolution of the syrinx and resolution of the tonsillar herniation 11).