Defining Posterior Fossa:

Posterior means in the back and fossa comes from the Greek "dish". As a medical term this indicates a space in the back of the head.

Defining Arachnoid mater:

The arachnoid mater is part of the meninges. It is named for its woven, spiderweb-like appearance. The meninges are connective tissue membranes that form the outermost covering of the brain (on the inside part of the skull) and continue down the spinal cord. The arachnoid mater occupies the middle of the meningeal layer, between the dura and pia maters. (note: Mater means mother)

Defining Cyst:

For the purpose of this discussion a cyst is an enclosed sac filled with CSF (cerebrospinal fluid). CSF is the water that cushions the brain inside the skull.

Putting it together:

Sierra has a large fluid filled sac (cyst) which has formed within the spiderweb-like layer covering the brain (arachnoid mater) in the back (posterior) part of her head.

Effects:

The effects of such a cyst will vary depending on the amount of pressure exerted and on what part the brain. i.e. If there is a lot of pressure on the cerebellum (as in Sierra's case) the structure appears distorted and misshapen on an MRI. If untreated, the pressure caused by the cyst could cause stress and eventually damage to the tissue. The cerebellum is responsible for coordinating the body's movements. Damage usually results in clumsy, disorganized movement and a decreased sense of balance. People often feel dizzy, having a poor sense of balance, they are often perceived to have visual problems because of decreased coordination. For example, a person with a damaged cerebellum may see a pencil on a table and wish to pick it up but find it difficult to locate the pencil in relation to their own hand and then to command the appropriate muscles of the hand to close around the pencil in order to complete the task.

Summary & Discussion:

Having been diagnosed with a posterior fossa arachnoid cyst before birth Sierra's MRIs show a distorted cerebellum but with no signs of stress or damage to the tissues at this time. At eight days of age the condition was treated with shunt placement from the cyst to the peritoneal cavity. This shunt will provide pressure regulation also for hydrocephalus for the rest of her life. Follow up CT scans will monitor the progress of the cyst as it drains out of the area. As the cyst decreases in size, the cerebellum and the rest of the brain will be free to grow normally and occupy the space. At this time, thanks to early detection and proper treatment, Sierra has no symptoms of brain damage and is developing normally.

Written by: Rachael Scott

Sources:

discussions with Drs Green, Glass & MacPherson at Children's Hospital, Seattle, WA

Essentials of Human Anatomy & Physiology, 7th Ed. Marieb, Elaine, Library of Congress ISBN 0-906-5385-2

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