Arnold-chiari malformation is characterized by downward displacement of the through the foramen magnum.
USMLE® Step 1 style questions USMLE
A 23-year-old female comes to clinic with gradual onset of severe headaches, accompanied with occasional neck pain and difficulty walking in a straight line. The patient’s past medical, family, and social history are noncontributory. She currently rates the pain as a 9 on a 10-point scale. Physical examination shows cranial nerves II-XII grossly intact, moderate neck tenderness with a full range of motion, sensation is normal along all extremities, finger-to-nose is positive, Romberg test is negative, and the patient has an unsteady gait. An MRI is obtained and is shown below. Which of the following is the most likely diagnosis?
The cerebellum, or "little brain", is divided into two hemispheres which are separated by a narrow worm-like structure in the middle called the vermis.
Directly beneath the hemispheres are two smaller lobes called cerebellar tonsils.
Normally, the cerebellum and cerebellar tonsils sit right above the foramen magnum which is the opening at the base of the skull through which the spinal cord passes through.
The cerebellum is contained within the posterior fossa or floor of the back of the skull, which also contains the medulla oblongata.
The cerebellum controls balance, posture, and helps to initiate as well as fine-tune voluntary motor activity - think about the fancy finger work of a piano player or the vocal cords of a singer - that’s the cerebellum in action.
The medulla, on the other hand, controls autonomic functions such as breathing and blood pressure, as well as reflexes such as coughing, vomiting, gagging, and swallowing.
Now, there’s also a set of four interconnected cavities in the brain called ventricles, each of which create and circulate cerebrospinal fluid.
Highest up, are two C-shaped lateral ventricles that lie deep in each cerebral hemisphere.
The two lateral ventricles drain their cerebrospinal fluid into the third ventricle, which is a narrow, funnel-shaped, cavity at the center of the brain.
The third ventricle makes a bit more cerebrospinal fluid and then sends all of the cerebrospinal fluid to the fourth ventricle via the cerebral aqueduct.
The fourth ventricle is a tent-shaped cavity located between the brainstem and the cerebellum and is continuous with the central canal.
After the fourth ventricle, the cerebrospinal fluid enters the subarachnoid space, which is the space between the two inner linings of the brain - the arachnoid and pia mater.
Now, below the brain is the spinal cord and within it runs the spinothalamic tract.
The spinothalamic tract is an ascending spinal pathway that carries sensory information from the spinal cord to the brain and it’s actually made up of two distinct tracts.
The lateral spinothalamic tract carries information about pain and temperature, while the anterior spinothalamic tract carries information about touch. These fibers usually decussate, or cross the spinal cord near its center at the anterior white commissure, before finally carrying the signal upwards to the thalamus and brain.
So chiari malformations occur when parts of the cerebellum slip down through the foramen magnum and into the upper spinal canal.
It’s usually caused by underdevelopment of the posterior fossa of the skull during fetal development.
As the cerebellum continues to develop and grow within the confined space of the malformed posterior fossa, it eventually gets pushed down through the foramen magnum.
There are different types of chiari malformations - the two main ones are called a type I Chiari malformation and a type II chiari malformation, and they differ in terms of what structures herniate.
A type I Chiari malformation occurs when only the cerebellar tonsils herniate down into the foramen magnum. As a result, the displaced cerebellum can compress the fourth ventricle and block the normal flow of cerebrospinal fluid from the ventricles into the subarachnoid space.
Consequently, cerebrospinal fluid builds up in the ventricles, which is called hydrocephalus, and leads to increased intracranial pressure.
Over time, cerebrospinal fluid may also buildup in the spinal canal although the exact mechanism of how this occurs remains unknown.
Eventually, however, this fluid can pool in the spinal canal causing a syrinx, or fluid-filled cavity, to form at the center of the spinal cord. This is a condition known as syringomyelia.