Submicrocephalic tonsillar herniation malformation (Chiari malformation) Submicrocephalic tonsillar herniation malformation is a congenital joint malformation of the cerebellar tonsils, first reported by Austrian pathologist Hans Chiari in 1891, hence the name Chiari malformation, which is a congenital malformation of the hindbrain. The underlying cause of Chiari malformation is the herniation of the lower part of the cerebellar tonsils into the foramen magnum due to congenital hypoplasia and small volume of the posterior cranial fossa. The pathology is characterized by herniation of the inferior cerebellar tonsils into the spinal canal, with extension, distortion, and displacement of the pontine, medulla, and fourth ventricle into the spinal canal. Approximately 56% of this malformation is associated with spinal cavernous malformation. The typology and diagnostic criteria of chiari malformation have not been fully standardized, but are generally divided into 4 types according to their pathological changes: Chiari malformation type I, Chiari malformation type II, Chiari malformation type III, and Chiari malformation type IV. The existence of types I, II, and III is well established, while the existence of type IV is more controversial. For the typing of chiari malformation, the following criteria are mostly used abroad. Chiari malformation type I: The main pathological change is the herniation of the cerebellar tonsils and the inferior cerebellar earthworm down into the spinal canal. The primary cerebellar tonsils herniate below the foramen magnum without cerebrospinal bulge, and the tonsils are more than 5 mm below the foramen magnum (including 5 mm), or the tonsils are 3-5 mm below the foramen magnum, with signs associated with chiari malformation, such as spinal cord cavitation, cervical medullary junction distortion into an angle, and mild subluxation of the medulla oblongata and the 4th ventricle. Chiari malformation type II: primary herniation of the cerebellar tonsils with at least one of the following: (1) herniation with the earthworm; (2) herniation of the cerebrospinal meninges; (3) large part or all of the medulla oblongata and ventricle 4 into the spinal canal. Chiari malformation type III: rare, mainly manifesting as herniation of the cerebellar tonsils, medulla oblongata, and ventricle 4 into the cerebrospinal bulge. Chiari malformation type IV Chiari malformation type IV is rare, although unrecognized, and is currently thought to present primarily with cerebellar hypoplasia without downward bulging. The mechanism of Chiari malformation is not well understood, and although there are many theories, none of them are accepted. The pulsatile transmission theory of cerebrospinal fluid: Gardner established the pulsatile transmission theory of cerebrospinal fluid based on the embryogenesis theory of Weed’s nervous system, which suggests that embryonic blockage of the outlet of the fourth ventricle results in enlargement of the ventricles and hypoplasia of the subarachnoid space, leading to the formation of traffic and obstructive hydrocephalus and spinal hydrocephalus. The pulsation causes the impact of the water to fail to enter the subarachnoid space and disappear, instead impacting the opening of the central canal of the spinal cord, making it more enlarged and the fibers around the lower neck and the following central canal equally aligned and easily cleaved, causing the ventricular fluid to enter the spinal cord and forming a trafficked spinal cavitation. This is evidenced by the fact that the composition of the cavity fluid is more similar to ventricular fluid than to cerebrospinal fluid, and that compression of the lower brainstem by Arnold-Chiari malformation, spinal cord trauma, vascular malformation, hemorrhage, infection, and tumor can all cause spinal cord cavitation according to a similar kinetic theory. Compression theory: Williams believes that the above factors lead to crowding in the posterior cranial fossa and occipital foramen to compress the lower brainstem and upper cervical medulla causing separation of the spinal fluid, due to the intracranial pressure forming a ball valve effect, which causes the flow of cerebrospinal fluid to the cranial side and prevents it from flowing in the opposite direction, during sitting up or Valsalva action (forceful breath holding, nasal puffing), the transient pressure increases and suction (suck Sherman’s observation of multiple axial and eccentric cavities using MRI demonstrates the existence of this mechanism. Adhesion theory: Dall Dayan believes that during the Valsalva’s maneuver, the venous pressure increases and is transmitted to the peridural plexus of the spinal cord, but because the obstruction at the foramen magnum does not allow the cerebrospinal fluid to flow cranially, but enters the spinal cord parenchyma through the Virochow-Robin gap, the cavity may not communicate with the fourth ventricle or the central canal, and the water-soluble contrast agent Amipaque delays The water-soluble contrast agent Amipaque can delay the entry into the cavity lumen, which supports the conclusion that the cavity is connected to the subarachnoid space, and the eccentric cavity can be caused by some undetermined anatomical factors such as adhesions between spinal cord components. Domestic scholars have further verified Gardner’s theory by establishing the CM-SM animal model. Therefore, in recent years, most scholars at home and abroad prefer to treat CM-SM with posterior cranial fossa decompression and occipital greater pool reconstruction, and have achieved certain efficacy, and the literature confirms that this procedure improves the patency of CSF circulation in the occipital greater pool area. Puncture, incision, and shunt of the cavity are unnecessary and prone to side effects, and are usually considered when posterior cranial fossa decompression is not effective. Diagnosis of Chiari malformation This disease occurs between the ages of 13 and 68, with a predominance of middle-aged adults. It is not difficult to diagnose the disease, and a neurosurgeon should be consulted when there is unexplained pain in the occipital neck and upper arms, or when there is also vertigo, tinnitus, diplopia, unsteadiness and weakness in walking, as well as separation of deep and superficial sensation, limb motor dysfunction and muscle atrophy. An MRI of the skull and spinal cord will clarify the diagnosis. Currently, the generally accepted diagnostic criterion is that the MRI scan in the sagittal position of the cerebellar subungual herniation on one or both sides exceeds the edge of the foramen magnum by 5 mm or less, and it is not difficult to make a diagnosis based on imaging combined with clinical manifestations (headache, pressure on posterior brain structures, spinal cord symptoms, hydrocephalus, etc.). However, with the widespread use of MRI, many asymptomatic subungual herniations of the cerebellum (exceeding the normal diagnostic criteria by 5 mm) have been identified, and these patients need to be managed with caution and may be given observation and regular imaging. Although many studies have confirmed that posterior cranial fossa as well as occipital foramen volume is significantly smaller than normal in patients with this disease, the role of posterior cranial fossa volume in the diagnosis of this disease is not yet standardized because the disease is often combined with other cranial base deformities, and it has been suggested that patients with occipital foramen diameter less than 19 mm almost invariably present with clinical symptoms. Treatment of Chiari malformation Once diagnosed with Chiari malformation combined with spinal cord cavitation, the patient should be hospitalized promptly for surgery to control the serious consequences of further progression of the disease. Since 1958, when Gardner reported the hydrodynamic theory and surgical treatment of ACM combined with spinal cord cavitation (SM), the surgical treatment of CM-SM has evolved considerably and has received more satisfactory results. The main surgical procedures currently used are: (1) posterior cranial fossa decompression; (2) posterior cranial fossa decompression (PFD) occipital pool reconstruction; and (3) cavity-subarachnoid shunt. However, the two methods of posterior cranial fossa decompression (PFD) occipital pool reconstruction and cavity-subarachnoid shunt are the most common methods in China and abroad, and the efficacy of surgery is mostly in the range of 60% to 80% as reported by various groups of literature. Posterior cranial fossa decompression (PFD) occipital pool reconstruction The principle of surgical treatment is to relieve the compression of the hindbrain and spinal cord and to reconstruct the unobstructed cerebrospinal fluid circulation channels so that the physiological circulation of cerebrospinal fluid can be restored. During surgery, it is necessary to achieve: (1) release the pressure separation at the craniocervical junction and decompress the compressed hindbrain and spinal cord; (2) reconstruct the occipital pool and restore the subarachnoid space. The surgical approach is a suboccipital median approach, occluding the posterior border of the foramen magnum up to the inferior collar line and both sides to the 4 and 8 o’clock positions of the foramen magnum, about 2.5 to 4.0 cm. The posterior arch of the atlas is occluded about 1.5 cm wide, and the dura mater is incised in a Y-shape. The artificial dura was taken to perform an enlarged repair of the dural spine to form a new occipital pool. The literature reports that most of the recent results of spinal cavity-subarachnoid shunts are satisfactory, and there is no uniform standard for its indication. However, the following disadvantages exist in placing the drainage: (1) it is bound to cause damage to the spinal cord; (2) there is a possibility of blockage (some studies have suggested that half of the patients will have blockage within 4 years); (3) the shunt can aggravate the lower herniation of the brain, resulting in cerebellar and medullary symptoms; (4) a small number of patients have postoperative reduction or disappearance of the cavity but the symptoms of neurological impairment continue to worsen. Coupled with the large number of cases reporting significant cavity reduction with posterior cranial fossa decompression and occipital pool reconstruction, it is still debated whether cavity shunting should be used as a routine step, and there is no single procedure with perfect results.