Arnold-Chiari type I malformation
Incidence
Autosomal dominant. An Arnold Chiari malformation can occur in all ethnic groups and in both females and males. For unknown reasons, there is a slightly higher rate of occurrence among females. Rarely, this has been found in several members of one family, but it is not known if any genes are involved.
Clinical Characteristics
An Arnold-Chiari Malformation (ACM) occurs when the brain forms incorrectly. This abnormality occurs very early in pregnancy, when the brain is still forming. This malformation involves the base, or bottom, of the brain, where the skull and the spine meet. This malformation results in some of the brain's structures being moved downward into the spinal canal. This pushing down of the brain into the spinal canal can lead to pressure on the lower parts of the brain and spinal cord. The symptoms found with this malformation are related to the damage done to the lower parts of the brain and spinal cord. ACM 1 symptoms include: Pain: headache, neck pain, arm pain, leg pain (the most common symptom). Weakness in the arms and legs (more in the arms), change of feeling in the arms and legs, difficulty with balance and coordination, double vision, slurred speech, difficulty swallowing, vomiting, ringing in the ears (tinnitus), abnormally high muscle tightness. ACM 1 involves the cerebellar tonsils. No other brain structures are displaced in ACM 1. The protrusion of brain tissue may also interfere with the flow of CSF to and from the brain. There is frequently an associated accumulation of fluid within the spinal cord (syringomyelia). Arnold-Chiari type I malformation usually causes symptoms in young adults and is often associated with syringomyelia, in which a tubular cavity develops within the spinal cord. Adults and adolescents who are unaware they have Arnold-Chiari I type malformation may develop headache that is predominantly located in the back of the head and is increased by coughing or straining. Symptoms of progressive brain impairment may include dizziness, an impaired ability to coordinate movement, double vision, and involuntary, rapid, downward eye movements. Due to the vague nature of the symptoms and the variable course, misdiagnosis is common.
Precipitants
It is very rare for trauma to "cause" Chiari, but trauma can "trigger" symptoms in a Chiari patient even if they never had symptoms before.
Provocation Tests
Arnold-Chiari I type malformation may develop headache that is predominantly located in the back of the head and is increased by coughing or straining.
Diagnostic Procedures
When the diagnosis is suspected the study of choice is an MRI scan. These malformations are very difficult to see on CT scans and impossible to see on plain x-rays.