Four Year Study of Chiari I Malformation Redefining the Chiari I Malformation- a study of 364 patients. Highlights of ASAP conference presentation by Dr. Thomas Milhorat, Professor and Chairman of Neurosurgery University of NY, Brooklyn

Before MRI, Chiari Malformations were based on a set of pathological observations made in a limited number of patients more than 100 years ago. The current understanding of the Chiari I is a condition that involves herniation of cerebellar tonsils into the spinal canal. The condition can be associated with Syringomyelia and malformations at the base of the skull. Currently it has a narrow definition by radiologist- the herniation must be at least 3 millimeters beneath the opening of the skull; otherwise it is not regarded as a Chiari. Radiologists further disagree whether it should be 3 or 5 millimeters. This radiographic criteria is considered the beginning and the end of diagnosis. Since MRI there has been dramatic rise of cases and with it a great number of unanswered questions.

  • What is the cause of the Chiari Malformation?
  • What do patients have?
  • What does it produce? What is the clinical syndrome?
  • What is the risk of inheritance?

To answer these questions, a prospective study was begun with 364 patients. A database was created with family history; complete neurological exams and all radiographic findings were presented. Each patient had MRI’s of the head and spine.

Measurements were made of all the structures of the posterior fossa. A quantitative measurement was done and then using a sophisticated technique of volume measurement they measured the volume of the posterior compartment, the volume of the cerebellar and brainstem tissues in the posterior compartment and the volume of the cerebrospinal fluid.

The patient study included 126 patients with Chiari Malformations only, and 238 Chiari patients with a syrinx. There was a 3 to 1 preference female over male. Many had lifelong complaints, but the age of onset was defined as the age when the patient first sought medical attention. The mean age was 25 years. Diagnosis was made at a mean age of 30 years.

89 patients or of the study population could identify a precipitating event that brought on symptoms. The most common happening was an accident of some sort -such as motor vehicle, sports injury, bump etc. Some experienced Chiari Symptoms after childbirth or spinal taps. Coughing and sneezing brought on symptoms in some. The most common complaint of patients with Chiari with or without Syringomyelia was headache. This was a specific headache that began in the suboccipital region as pressure and pain and in some it radiated to the vertex of the head and behind the eyes, less often it radiated posteriorly down the shoulders. Often it was accentuated by postural changes or valsalva maneuvers (coughing, sneezing, straining) and in females it tended to be worse preceding menses.

Visual phenomena were common such as blurred vision, seeing stars or wavy lines, uncomfortable feeling in bright lights; less common was double vision. Females were often diagnosed with MS based on the ocular symptoms. Otoneurological symptoms were the leading cause of seeking medical attention. 74% of the patient population had a Meniere’s like syndrome with two or more of the following complaints: dizziness, disequalibrium, pressure in the ears, tinnitus (ranging from high pitched sounds to sounds of oceans flowing), decreased hearing, true vertigo, images jumping in from of the eyes. Nystagmus were the only major finding upon clinic examination despite all these symptoms. All types of nystagmus were found.

66% of patients had two or more cranial nerve, brainstem or cerebellar disturbances. They included apnea (episodes of irregular breathing during sleep), hoarseness, tremors of the hands, poor circulation, pain in throat, pain in the face, true syncope, shortness of breath, hypertension; 25% described palpitations. Two people can have the exact Chiari Malformation and one can be totally asymptomatic. The presence of a Chiari does not impute a set of symptoms.

We have come to understand the Chiari Malformation in the following way. It is a volumetrically too small posterior compartment of the skull. The posterior compartment is formed by the occipital bone in the back; this particular part of the bone is called the supraocciput. It’s formed by a roof, which is made up of a membrane called the tentorium. It has a front, which is the clivus. Then there is a mystical opening at the top of the tentorium and there’s a mystical opening at the bottom known as the foramen magnum. Now in the Chiari I malformation, the volume of this compartment is too small. The tentorium is sloped like the steeple of a church, not the roof of a house, where the normal tentorium comes out like this, the Chiari tentorium tends to be high rising. The back bone, as a consequence, tends to be short. In many patients with Chiari the clivus is short. Some patients with Chiari have basilar impression where the tip of the second vertebral body, the odontoid, tilts back. We refer to that as retroflexion of the odontoid. These make a small posterior compartment. Oh, and did he leave it out? There can also be herniation of the cerebellar tonsils through the foramen magnum. Milhorat has come to understand this as one of the consequences of a too small posterior fossa- not the actual phenomena. This is not the whole problem, but just a reflection of the too small posterior compartment.

n Milhorat’s study population, 34 patients or 9% had typical Chiari symptoms- typical headaches, visual disturbances, Meniere’s like syndrome, lower cranial nerve disturbances, 2/3 had syrinxes. They had a small posterior fossa, they had a high steeped tentorium, they had a short supraocciput, and they had a small clivus. But you know what? Their tonsils were not down enough to be called a Chiari Malformation so these patients were kicked out of the group. But if you look here at the T-2 weighted images where spinal fluid is white behind the cerebellum you can see this is a compressed posterior fossa with an absence of CSF posteriorally and lateral to the cerebellum. This is a Chiari Malformation with the tonsils at the level of the foramen magnum, and not grossly below it. So we are redefining the Chiari Malformation as not simply how far the tonsils come down, but it is a diagnosis, which includes 7 specific signs. They include: the volume of the posterior fossa being small, CSF volume being reduced, compression owing to smallness of the bones in the back and most of them have varying degrees of tonsillar herniation. The failure to have greater than 3 mm to 5 mm herniation does not exclude the diagnosis of Chiari I in our new definition.

In all 364 patients the most common and reliable finding was compression of the CSF spaces posterior and lateral the cerebellum. 332 of these patients had tonsillar herniation greater than 5 mm but 32 did not and we did not exclude them. These are patients, who had syrinxes, had headaches, they knew they had Chiari but they were excluded from having a Chiari and it was “idiopathic Syringomyelia” because they didn’t have the 5 mm tonsillar descent. These patients are now redefined as Chiari I Malformations.

In a desire to be more precise about the small posterior fossa, a control study of 50 normal people and 50 patients with presumptuous diagnosis of Chiari I were age and sexed matched with the following measured: the length or tentorium, angle or slope of tentorium, length of supraocciput, length of clivus, degree of tonsillar herniation.

The Cavalieri Method was used to calculate total volume of the posterior fossa and total volume of CSF spaces around the cerebellum. In patients with Chiari I, the size of the supraocciput was significantly smaller, clivus was significantly smaller, tentorium angle was increased, and there was an average of 9.8 mm tonsillar descent in this group. Normal tonsils should lie 2mm ABOVE the foramen magnum.

Professor Chiari and others throughout time have regarded the Chiari I Malformation as the least severe form of a series of hindbrain malformations. But could it be just something wrong with the bones of the skull and the consequences of compressing spinal fluid and compressing local structures contribute to the symptomotology and it has nothing to do directly with anything being wrong with the central nervous system?

Based on this study there is strong evidence to suggest it is a defect of mesodermal origin. It is a disorder of bones, and structures that support bone- muscles and ligaments and not nervous tissue. None of the 364 patients had evidence of neural defects such as missing lobes or abnormalities of the central nervous system. All patients had evidence of skull based dysplagia. Interestingly, the location of these bony changes- the back of the skull, the tentorium, the front of the posterior fossa, the anomalies at the cervical-medullary junction- are consistent with a defect of a very particular part of mesoderm. All these structures form together in the embryo and all are linked to one another in development.

This is powerful evidence that the Chiari I Malformation, although it shares the name of Chiari, is not a malformation of the central nervous system, it’s not a developmental anomaly of the hindbrain of the cerebellum and brainstem. But it is a too small posterior cranial fossa, owing to a maldevelopment of the skull.

How do symptoms occur? CSF that is compressed has to go somewhere. In man, under normal conditions, we secrete spinal fluid. We can increase that secretion but we cannot decrease it. It happens at .37 cc every minute of our lives. When we’re upright, sleeping, running, resting. We produce 500 cc a day. It’s formed in the ventricles, circulates down through the 3rd ventricle into the aqueduct of Sylvius, into the 4th ventricle. It exits and then it surrounds all the structures of the nervous system. There is a watery bath between brain, spinal cord, cerebellum and the inner table of the skull. In Chiari patients there is reduced volume. Where does it go? Some of it is distributed up into the head, squeezed up and some is squeezed down and this may very well contribute to the formation of SM in some patients. We believe this displacement of spinal fluid into the cranial cavity may be a suitable explanation for the typical Chiari Symptoms. Distribution of CSF out of the posterior fossa into the semi-circular canals can produce in some a sea-sickness syndrome- dizziness, disequilibrium, pressure in the ears, tinnitus, and so forth. It appears to all begin with a too small posterior fossa that compresses the spinal fluid at the back of the head.

Direct bony pressure can also produce complaints. If the compression of the skull is too great, if the retroflexed odontoid (basilar impression) is too great you can have direct compression of cranial nerves or even brainstem. That can produce problems with swallowing, sleep apnea, hoarseness, tremors, and palpitations. Symptoms of Chiari I appear to be due to two basic phenomena that share in common a small back of the skull, a bony dysplagia of the base of the skull and cervical-medullary junction. These are increase of CSF pressure both in the head and in the spinal canal and direct compression of nervous structures. It is not a malformation of the brain itself.

This anomaly can occur sporadically, and probably occurs most often in this fashion. It can also be associated with genetic transmission in some families. The precise mode of that transmission is under intense investigation thanks to a grant from ASAP to Duke University. 12% of the 364 study population had two or more affected family members.

In closing, Milhorat emphasized once again that an MR finding of a Chiari I does not impute a set of symptoms. Nor does it impute a prognosis except we have learned if you have a Chiari I Malformation you will do well to wear your seatbelt and stay out of harms way, and a Chiari I Malformation certainly does not impute surgical treatment.