The most recent scientific evidence indicates that tonsillar herniation in CMI is the result of an underdeveloped posterior cranial fossa (PCF) and hindbrain overcrowding.1 Although all cases of CMI appear to be associated with an underdeveloped PCF, it cannot be said that all cases of CMI are associated with tonsillar descent below the opening of the foramen magnum.1
It stands to reason that hindbrain overcrowding has already occurred before tonsillar descent begins. In other words, tonsillar descent begins to occur because the cerebellar tonsils have already overcrowded the PCF and, for lack of space, are subsequently forced downwards through the foramen magnum. In some cases, this descent can occur rather quickly due to trauma. However, in many cases, this downward descent can take years to occur. In some cases, the tonsils may remain at the foramen magnum indefinitely, causing major symptoms in patients that go undiagnosed for years.
Cases of CMI can be almost impossible to diagnose - and cases with less than 5mm herniation, even more so. The presence of non-trauma induced syrinx is indicative of CMI and helps in the diagnosis when other causes are ruled out. However, cases of minimal CMI with syrinx are extremely rare due to the fact that non-trauma induced syrinx may take years to form - and, by this time, herniation has usually exceeded that considered to be minimal.
But, there ARE documented cases of minimal herniation CMI, without syrinx, in which people have been diagnosed and successfully treated. People who have this condition are almost always overlooked or misdiagnosed, and may suffer years of pain before finally finding the cause. Symptoms of CMI vary significantly and can include almost any neurological symptom. While headache is the number one symptom of CMI, there have been cases in which headache was not a symptom at all. These types of inconsistencies can make the condition even harder to diagnose. See the symptoms list for more details.
Awareness of recent advances made in the diagnosis of CMI are paramount in preventing permanent neurological damage to the patient. As CMI remains undetected in the patient, syrinx is more likely to develop. Many doctors consider syrinx the single most dangerous condition associated with CMI and recommend immediate surgery to resolve it - even if the patient is asymptomatic. Unfortunately, the damage that syrinx inflicts upon the spinal cord is irreversible. Early detection can prevent most, if not all, spinal cord damage in patients with CMI. This, again, stresses the importance of early diagnosis and treatment of minimal CMI.
Today, unfortunately, the diagnosis of minimal CMI (especially 0mm cases) is difficult to obtain. Most doctors (even CMI experts) do not use volumetric measurements to diagnose the condition. Instead, a line is drawn from the basion to the opisthion and the amount of cerebellar tonsillar descent below this line is measured (click here to see how the line is drawn). In addition, some doctors will not diagnose the condition because of a lack of hindbrain malformation, which is largely subjective. Again, it stands to reason that hindbrain malformation would more likely be the result of a longstanding, untreated CMI and, therefore, less likely to occur in cases of minimal CMI. Again, it must be stressed that CMI is not a malformation of the hindbrain but, one of the PCF.1
Dr. Thomas Milhorat's Chiari Redefined study of 364 patients concluded that the cause of CMI is a volumetrically underdeveloped posterior cranial fossa.1 In this study, Dr. Milhorat calculated the volume of the skull for comparison with the volume of enclosed tissues and fluids. Patients were diagnosed based on known volumetric standards in control patients. Volumetric abnormalities are consistently found in patients with CMI. However, the majority of doctors who are involved in diagnosing this condition still use the traditional means of measuring tonsillar herniation - by drawing a line from the basion to the opisthion. Downward descent of the cerebellar tonsils is a symptom, not a cause, of CMI. It makes little sense to rely so heavily upon a symptom as the primary means for diagnosing such a complex condition when the cause is known and proven.
Additionally, 100% of the patients in Dr. Milhorat's study exhibited compression of the CSF spaces posterior to the cerebellum, while significantly less (92%) exhibited tonsillar descent of more than 5mm. This is quite definitive in demonstrating that a measurement of tonsillar descent alone cannot, and should not, be used as a means of diagnosing CMI.
The single most significant risk in misdiagnosing CMI is the subsequent formation of syringomyelia which often results in irreversible damage to the spinal cord.
Most people with minimal CMI suffer years, if not a lifetime, without a diagnosis. CMI cases were rarely diagnosed before the early 80's when use of MRI technology became more prevalent in the medical field. Unfortunately, despite advances made due to the use of MRI, minimal cases of CMI are almost always overlooked. It is not uncommon for an MRI to clearly show a lack of CSF flow space posterior to the cerebellum and, yet, be reported as normal. Various neurological deficits can indicate a medical problem exists. But, these indicators may introduce an entirely new problem - the possibility of misdiagnosis.
CMI is commonly mistaken for Multiple Sclerosis because its symptoms are so similar. As with MS, symptoms of CMI usually appear in women between the ages of 20 - 40. Both conditions include neck pain, pressure in the neck, frequent headaches, dizziness, blurred vision, difficulty with motor skills, numbness, balance problems and muscle weakness. Patients are often diagnosed with MS, based on symptoms alone, even though testing of bodily fluids have provided no definitive diagnosis. Therefore, minimal CMI should be considered a possibility in MS patients when a diagnosis has been made based solely on symptoms.
It is not uncommon for CMI headaches to be mistaken for rebound headaches because of their chronic, recurring nature and the fact that they are usually relieved by caffeine. The blockage(s) caused by CMI introduces complex problems to the balance of blood and cerebral spinal fluid inside the cranial cavity. A buildup of fluids inside the cranium due to blockage, or lack of flow, causes an increase in pressure. Caffeine has very strong vaso-constrictive effects, reducing blood volume inside the skull, and thereby relieving pressure and headache.
Perhaps the most common misdiagnosis is that of Chronic Fatigue Syndrome. The nature of CMI is to present symptoms for which no cause can be found. For most doctors, CMI is a very puzzling condition. Headache, sore throat, and fatigue are just a few of the symptoms found in both conditions. Most CMI sufferers report having been previously diagnosed with CFS or some other variation of this disease (CFIDS, Fibromyalgia).
Spinal taps can also be very useful in diagnosing CMI by revealing elevated intracranial pressure (ICP). This provides real medical evidence that there is a legitimate medical concern. However, there is still the potential for misdiagnosis. Elevated ICP can be indicative of CMI or, another well-known condition called PTC (pseudotumor cerebri). PTC is described as an increase in intracranial pressure for which there is no known cause. Although little is understood of the exact cause(s) of PTC, it is much more likely to be diagnosed than minimal CMI when raised ICP is the primary symptom. Ironically, it is proven that a blockage of cerebral spinal fluid, such as CMI, can cause the increase of ICP often thought to be indicative of PTC3.
The concern of mistaking CMI for PTC lies in the choice of treatment. Lumbar shunts are the treatment of choice for PTC but, can lead to acquired CMI in patients (meaning herniation of the cerebellar tonsils).5 A lumbar shunt (or valve) is permanently installed in the lumbar region of the spine, and is used to drain cerebral spinal fluid when ICP levels become higher than normal. While lumbar shunts are good solutions for persons suffering from PTC alone, they may present the same risks as spinal taps in persons with CMI - but, occurring over a longer, more extended period of time (read more on spinal taps). There are documented cases in which patients have been diagnosed with PTC, had a lumbar shunt placed, and years later been found to have CMI with herniation that was not present at the time of the original PTC diagnosis. The circumstantial evidence seems to indicate there may be an unknown number of patients diagnosed with PTC who actually have minimal CMI. Whether those numbers are signifcant or not, remains to be seen. A person having CMI and receiving a lumbar shunt as treatment would likely have years of relief before worsening CMI symptoms would occur, and a proper diagnosis made. There are alternative treatments for PTC (other than lumbar shunts) that would pose less risk should the patient actually have minimal CMI. This is something that should be considered. Again, the evidence is circumstantial but, does seem to indicate the possibility of misdiagnosis in some patients.
Minimal cases of CMI (those with herniations less than 5mm) are more at risk for misdiagnosis than those with significant herniations. It should also be pointed out that MS and PTC are both equally serious conditions that should not be dismissed easily. The bottom line is, these conditions are extremely difficult to differentiate, and much care should be taken in their diagnosis and treatment.
Treatment of CMI can vary according to the severity of symptoms and whether or not syrinx exists. When syrinx exists, surgery becomes necessary. Pockets of fluid formed by syrinx can lead to splitting and tearing of the spinal cord, resulting in permanent neurological damage. Surgery may also become necessary when syrinx is not present if the patient's condition continues to deteriorate. In these cases, surgery may offer much needed relief.
For patient's whose symptoms are manageable, medications can be given to reduce pain. Prescription diuretics are also used to reduce the amount of cerebral spinal fluid produced by the body - thereby reducing intracranial pressure and providing relief for headaches.
Prior to surgery, some hair must be shaved. Some surgeons only shave a small strip along the path of the incision to be made while others shave the back of the head entirely. Surgery begins by making an incision in the middle of the back of the skull, extending midway down the back of the neck. The muscles are separated from each other in the midline and peeled off the base of the skull and the back of the first cervical verebra. Next, a craniectomy is performed to enlarge the opening of the foramen magnum and create additional CSF flow space posterior to the cerebellum. A piece of bone about the size of a silver dollar is removed by grinding away the bone with a surgical instrument. A laminectomy of the first cervical vertebra is performed as well (the back of the vertebra is ground down). In some cases, a laminectomy of additional cervical vertebra may be necessary. Some surgeons believe that the operation can be concluded at this point but, the tonsils of the cerebellum are still down in the cervical spinal canal, compressed and deformed by the dura, which does not necessarily relax after the overlying bone has been cut away. The dura should be opened to expose and decompress the herniated cerebellar tonsils. Cauterization of the cerebellar tonsils with a small amount of electrical current causes them to shrink and retract into the skull cavity. Once this is done, there is ample space for CSF flow around the tonsils. Between the tonsils is an importnat outlet for CSF flowing out of the ventricular system of the brain, and in some cases of CMI, this outlet is obstructed by an abnormal membrane. The surgeon examines the ventricular outlet and relieves any obstructions. The dura is now reconstructed with a patch. A variety of natural and artifical materials have been used for patching the dura. The surgeon may use the patients own tissue to create a patch. Next, an artificial plate is placed over the portion of bone that was removed to add protection for the cerebellum. Finally, the neck incision is closed using stitches, staples, or glue. A bandage is placed over the wound temporarily to prevent infection.8
The presence of syrinx and herniations exceeding 5mm can certainly help in the diagnosis of CMI but, these indicators alone should not be used as a means of making a diagnosis. It is imperative that the medical community gain a better understanding of the most recent advances and techniques used in the diagnosis and treatment of minimal CMI without syrinx - specifically, those outlined in Dr. Thomas Milhorat's Chiari Redefined study. Early diagnosis can lead to treatment before the formation of syrinx. If allowed to progress, the formation of a syrinx can lead to serious and irreparable nerve damage to the spinal cord.
