Fainting in the Chiari I Malformation (Part 2)

In Part 1 of this series, I summarized 10 medical reports on a total of 13 patients with fainting episodes due to the Chiari I malformation. The syncopal episodes, sometimes referred to as blackouts, were transient events except in two persons who suffered serious consequences: cardiac arrest or sudden death.

Syncope occurred with coughing or sneezing in 3 patients and following neck extension or head turning in another three. In one man, syncope occurred while straining at urination. Some patients noted a warning headache before fainting, while others had no warning prior to sudden loss of consciousness. EEG performed in four patients was negative for seizure.

While the number of cases of Chiari syncope reported in the literature is small, in 2004, Diane Mueller, ND, RN, FNP-BC, and I published a report on the symptoms occurring in 265 patients with the Chiari I malformation. Blackout spells were reported by 39 (6.8%) of our patients.

Why does the fainting occur?
In their 1976 report, Corbett et al. proposed “pressure transmission to the area of intracranial pathology” as the mechanism of syncope in CM-I. Subsequent authors supported brainstem compression as the likely cause of fainting.  These two mechanisms, of course, can co-exist.

Spinal fluid pressures were measured in the cerebral ventricles and in the lumbar spinal canal in three patients in the Hampton et al. study. There was a difference in these pressures in two of the patients. Their observations supported the idea that a cough or sneeze increases the pressure in the cranial cavity and cause a “jamming” of the cerebellar tonsils into the foramen magnum. This likely interferes with the brainstem baroreceptors reflex or causes “dysfunction of the midbrain reticular activating system” Both can result in syncope.

Among the patients described in Part 1 who underwent posterior fossa decompression for the Chiari I malformation, fainting resolved in each case except one who experienced a single fainting spell postoperatively.



MRI of a woman in her early 30’s presenting with headache, fatigue, double vision, dizziness and blackout spells. The cerebellar tonsils (T) are herniated into the upper cervical canal, and the lower part of the brainstem (B) is elongated. A syrinx (S) can be seen inside the cervical spinal cord.

In summary, unexplained fainting in persons with the Chiari I malformation is due brainstem compression resulting in malfunction of the brainstem centers that control autonomic function of the cardiovascular system, and, possibly, the reticular activating system which controls consciousness. When the impact on these centers is significant, sudden loss of consciousness with little or no warning occurs.

In some persons, fainting is triggered by the upright posture and the inability of the brainstem baroreceptors centers to adequately respond. More often, fainting in persons with the Chiari I malformation is due to increased impaction of the cerebellar tonsils that may occur with coughing or sneezing, or to compression and/or distortion of the brainstem that may occur with head extension or neck turning. Fortunately, decompression of the posterior fossa is an effective treatment for Chiari syncope.