Primary or congenital CM, which may be due to genetic mutation or nutrient deficiency, is characterized by space compression that puts pressure on the brain and spinal cord.
A less common form, acquired or secondary CM, emerges in adults if cerebrospinal fluid (CSF ) is drained from lumbar or thoracic areas of the spine due to injury, infection or other causes.
With CM, the cerebellar tonsils extend into the foramen magnum. Because the spinal cord is typically the only matter in this orifice, the presence of cerebellar tonsils can block CSF flow. Syringomyelia (CSF -filled cyst or “syrinx” within the spinal cord) may result. Hydrocephalus (excess CSF in the brain) is another potential related condition.
CM is estimated to occur in one of 1,000 births, although advances in diagnostic imaging suggest the incidence may be higher due to the delay of some symptoms until adulthood. Spinal curvature, whether scoliosis or kyphosis, is common in those in with CM. Myelomeningocele, which can occur in those with Type II CM, increases risk of developing tethered cord syndrome later in life.
Diagnosis and Treatment
Diagnosis of Type I CM typically follows a patient presenting with motor skill deficiencies, such as lack of hand coordination. Muscle weakness, dysphagia, hearing loss, vomiting, insomnia, tinnitus or dizziness may also occur. Refractory, usually occipital-predominant, headaches accentuated by Valsalva maneuvers such as straining, coughing, or sneezing are a hallmark.
Clinicians should flag refractory headaches combined with neurological symptoms and be aware that skull and spine anomalies will be apparent in an estimated 30 to 50 percent of patients with Type I CM. Arriving at a conclusive diagnosis for this condition can be complicated and a rigorous evaluation before recommending a treatment pathway and collaboration with a neurologist can enhance diagnosis.
Computed tomography (CT) scan can aid in visualizing structure but often overlooks both Chiari malformation and syringomyelia. Magnetic resonance imaging (MRI ) scan is considered the definitive diagnostic tool and should be utilized to view structural abnormalities and determine the cerebellum’s extension into the spinal canal. MRI sagittal views well demonstrate the extent of descent of the cerebellar tonsils. If any suggestion of a syrinx is evident, a cervical and/or thoracic MRI may also be indicated.
In this, the most common form of Chiari malformation (CM), the cerebellar tonsils extend into the foramen magnum without disturbing the brain stem. The only CM that can be acquired through injury, illness or other causes, Type I may not cause symptoms and is typically diagnosed in adolescents or adults, often during examination for another condition.
In Type II CM, cerebellar and brain stem tissue both extend into the foramen magnum. This congenital condition also may be accompanied by myelomeningocele, a form of spinal bifida that occurs when the spinal canal and backbone do not close before birth, resulting in spinal protrusion and possible paralysis. This formation is known as Arnold-Chiari malformation, named after the researchers who researched the condition.
Cerebrospinal fluid flow dynamic studies also can be used and are evaluated at four regions: peribulbar cistern, foramen magnum, and ventral and dorsal spinal subarachnoid spaces.
In the event of asymptomatic diagnosis, watchful waiting combined with patient education for symptoms may be indicated. If patient headaches respond to anti-inflammatory medications, surgery may be delayed. Prophylactic surgery, which requires significant recovery time, is not indicated for Type I CM.
The treatment approach generally followed at Santa Barbara Neuroscience Institute at Cottage Health System is conservative. When patient symptoms are chronic and CSF flow is severely restricted by syringomyelia, surgery may be indicated to repair these functional disturbances and stop progressive central nervous system damage.
The most common surgical approach is posterior fossa craniectomy or posterior fossa decompression. This is done in conjunction with a cervical laminectomy and involves exposing and enlarging the dura, using synthetic material graft or harvested tissue, to relieve pressure on the cerebellum and establish CSF flow. Brain tissue is not typically resected except when reoperations are needed.
“The typical operative procedure involves removal of the suboccipital bone, as well as the C1 and possibly C2 lamina, depending on the downward extent of the tonsils,” says Richard Y. Chung, MD , neurosurgeon at Santa Barbara Neuroscience Institute at Cottage Health System. “The dura is expanded with a graft to enlarge the area. I will occasionally bipolar coagulate the lower tips of the tonsils to have them retract to improve CSF flow, but this is not always needed.”
Outcomes are generally good in terms of resolution of headaches. When significant neurologic symptoms or deficits are present, recovery of those issues can be more variable and may take weeks or even months to resolve with no guarantee of the degree of resolution. Patients rarely require reoperation, except in the event of postoperative complication such as infection or CSF leak.
“Patients who present with persistent headaches unresponsive to treatment or with features atypical of common conditions such as migraines merit imaging and referral to a neurologist for evaluation,” says Dr. Chung. “For patients’ benefit, I recommend primary care physicians keep some of these less common conditions in mind.”
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