Key points

Symptoms in the adult Chiari patient

Adult patients with Chiari malformation, (Chiari 1 malformation), also termed cerebellar ectopia or hindbrain descent, classically present with strain-related headaches, which are reported to occur in 80% to 100 % of patients. Typically these are brief, intense headaches, lasting a few seconds, located posteriorly at the base of the skull. They are generally reproducible with similar maneuvers, such as coughing (tussive headaches), straining, and lifting. Although they are characteristic, adults not uncommonly also have headaches of other etiologies, such as migraine headaches or neck pain radiating into the suboccipital area, which may coexist or overlap with more typical Chiari headaches. It is important to establish whether a Chiari patient has different types of headaches, if only to be clear that non-Chiari headaches will not be affected by surgery. This is particularly true in patients in whom tonsillar descent has been identified as an incidental finding on a study initially performed for other reasons, who do not present because of suspected Chiari-related symptoms.

Posterior headache and neck pain are also seen in patients with basilar invagination and with ligamentous instability at the craniocervical junction. The posterior fossa volume is characteristically reduced in patients with Chiari malformation. Volume also is reduced with basilar invagination, and true tonsillar descent may also be present in these patients. In some patients with basilar invagination, treatment may require craniocervical stabilization in addition to posterior fossa decompression.

Displacement and compression of the brainstem and stretching of lower cranial nerves can give rise to several other manifestations including nausea and vomiting and swallowing, as well as cough symptoms. Visual symptoms, including blurring of vision, double vision, and occasionally awareness of nystagmoid movements, are generally attributed to traction on cranial nerves III, IV, and VI, and their central connections. Tinnitus is often reported by Chiari patients. Patients also report balance problems and sleep disturbance. Findings on examination may include nystagmus, absence of venous pulsations on fundoscopic examination, and impaired truncal balance. The gag reflex may be absent. Facial pain may result from trigeminal nerve traction. Tongue fasciculations and hemiatrophy are sometimes encountered and are attributed to hypoglossal nerve traction. The anatomic basis of fatigue, memory impairment, and what patients sometimes refer to as brain fog is less clear. Findings related to syringomyelia may include upper extremity weakness and atrophy, sensory disturbances, and long-tract signs indicative of myelopathy. Autonomic symptoms, including postural hypotension, are sometimes seen in patients with Chiari malformation as well as in syringomyelia patients.

One assumes that the basic anatomic features, such as posterior fossa volume and size of the cerebellar hemispheres, have been established and stable by the time an individual has reached adult size. The question therefore arises frequently what it is that leads to the development of clinical symptoms in adult years, such as the headache described previously. One hypothesis is that years of normal activity, including coughing, lifting and straining, result in gradual downward displacement of the tonsils until there is interference with cerebrospinal fluid (CSF) flow at the level of the foramen magnum. The observation made in 1 study, that almost 25% of adult patients presenting with symptoms of tonsillar ectopia had a history of recent trauma, is important in this connection, when one assumes that trauma can also result in critical downward displacement of marginally low cerebellar tonsils. Although symptom progression is gradual in most patients, occasionally symptoms related to Chiari malformation and even symptoms of syringomyelia may develop suddenly in relation to a severe coughing spell. Sudden death due to Chiari malformation is rare and is probably related to critical brainstem compression. Without doubt there are also some individuals with Chiari malformation and even with associated syringomyelia, who remain minimally symptomatic their entire life.

In a typical Chiari patient, the cerebellar tonsils are pointed and often peg-like at the C1 and sometimes the C 2 vertebral levels. Axial images at the foramen magnum will show obliteration of the subarachnoid space between tonsils and brainstem, and often show distortion of the brainstem by one or both tonsils. It must also be made clear that the designation of 5 mm tonsil projection below the foramen magnum as the critical measurement defining Chiari malformation is somewhat arbitrary. Depending on some factors identified and others less clear, individuals may have normally rounded tonsils that project slightly more than 5 mm but cause no obstruction of CSF flow, while in other patients the configuration is such that reduced CSF flow can occur at the foramen magnum with less than 5 mm of tonsillar descent. It is also recognized that the precise measurement of tonsil descent may vary with the degree of the patient’s neck flexion or extension during the imaging study, which is difficult to reproduce exactly on sequential imaging studies. This is a consideration of importance when differences of a few millimeters of tonsil descent are reported on studies obtained at different times. It is also important to note that variations in posterior fossa architecture exist within the general concept of reduced posterior fossa volume in patients with Chiari malformation. Basilar invagination, with encroachment of the dens into the posterior fossa space, has already been mentioned. Other variations include a horizontally oriented occipital bone, low insertion of the tentorium cerebelli, and thickened skull bone.

Pulsatile motion of the brain, including the cerebellar tonsils, allows the tonsils to act as pistons, thereby forcing CSF into the spinal cord. This is postulated to be the mechanism of syringomyelia formation. It has been suggested that persistence of the central canal of the cord, present at birth but not infrequently present in adults, favors the development of syringomyelia. By contrast, involution of the central canal by the time critical descent of the tonsils occurs would explain why many adults have a Chiari malformation without syringomyelia.

Rarely syringomyelia is seen in patients without tonsillar descent who have a different abnormality impairing normal CSF outflow from the cranial cavity. Examples of such conditions are a retained rhombic roof over the fourth ventricle, or a postinflammatory or other obstructive membrane at the level of the foramen magnum. Such membranes may sometimes be identified by Constructive Interference with Steady State (CISS) imaging. This has sometimes been referred to as a Chiari zero malformation.