Figure 4 is a cross section of the spinal cord. The fibers in the spinal cord are organized in a very specific way. Fibers for feeling and movement run together in nerve highways called tracts. The spinal nerves bring sensory information, such as pain and temperature, in to the spinal cord and send information out to muscles are like on and off-ramps on a major highway. The spinal nerves merge in to the nerve tracts heading up of down in the spinal cord. Two main nerve tracts carry sensation from the body up to the brain: the dorsal columns and the spinothalamic tract. The dorsal columns (DC) located at the back of the spinal cord bring information from the body, arm and legs about the position of the body and vibration. The spinothalamic tract (ST), located at the front of the spinal cord, brings up information about pain and temperature. The corticospinal tracts (CTS) are composed of motor nerves cells that control movement.
Here the story gets a bit more complicated. The feeling fibers coming in from the spinal nerves enter the spinal cord on its back surface. They connect to the spinothalamic fibers that then travel to the front and opposite side of the spinal cord before they enter the spinothalamic highway up to the brain. Thus, the fibers bringing information in about pain and temperature of the right side of the body cross to the left side of the body cross to the right side of the cord. They cross is near the center of the spinal cord. Syringomyelia often starts at the center of the spinal cord. Thus fibers carrying information about pain and temperature are among the first affected in patients with syringomyelia. As the syrinx enlarges, it can affect other tracts and result in weakness and problems walking. Often, one side is more affected than the other. The mix of symptoms depends on which type of fibers and nerve cells are affected.
Discovery of the Syringomyelia & Chiari Malformations
1827, Charles Prosper Ollivier dAngers, published an article about disorders of the spinal cord. In a few spine specimens he found a cavity inside the spinal cord. Since the cavity had not been described before and had no name, he created the name syringomyelia by combining the Greek word syrinx meaning pipe, tube or channel, and the Greek word myelus, meaning marrow. Sixty-four years later, in 1891, Hans von Chiari, Professor of Morbid Anatomy at Charles University in Prague, published a report describing three cases with malformations of the lower part of the cerebellum and brain stem. Each of the three cases was different thus Chiari described three types of malformation: the type I, II, and III. Later, these malformations would be named the Chiari malformations in his honor.
The Chiari malformation Type I (CMI) consists of displacement (herniation) of the cerebellar tonsils into the upper cervical spinal canal. In some cases the lower brain stem also hangs down into the spinal canal. In the Type II malformation (almost always associated with spina bifida), the lower brain stem, cerebellar vermis, and tonsils hang down below the foramen magnum. In the very rare Type III malformation the tissues of the posterior fossa are displaced into a soft sac (meningocele) at the back of the head and upper neck. Dr. Chiari continued his studies and four years later published another report. In this one he described an additional 24 cases and made another important discovery. He found that some of the patients with the Chiari I and II malformations also had a cavity within the spinal cord – syringomyelia – the type of cavity that had been described by Charles Ollivier.
In the article, Dr. Chiari also reported a new type of malformation, Type IV, which consisted of underdevelopment of the cerebellum known as cerebellar hypoplasia. However, today we believe that the Type IV malformation is different from the other three and most specialists no longer include it within the Chiari classification.
Dr. Chiaris studies, performed over one hundred years ago, still serve as the foundation of our modern understanding of these malformations.