The inner ear is a delicate and complex structure, so it is critical that it be formed properly. Abnormalities in development arising prior to birth, called congenital malformations, can be associated with a complete absence of vestibular function from birth. Milder malformations can at first be silent, only to cause progressive or sudden losses of function in childhood or later in life. These malformations can involve the entire inner ear and affect both hearing and balance, or can be limited to just a portion of the inner ear. They can arise because of an inherited gene defect, or can happen if the fetus is exposed to infections or toxins during the development of the ear during the first few months of pregnancy. Often the development of the ear ceases when the infection or toxic exposure occurs, leaving the inner ear in a relatively unformed or “arrested” state.
Malformations are visible on CT scans of the skull. This is usually ordered after hearing or balance testing suggests defects in the inner ears. The outer ear can look completely normal even when the inner ear is malformed.
The Scheibe dysplasia is the most common cause of congenital deafness. It is a malformation of the cochlea and saccule, but spares the rest of the labyrinth. Although the saccule does not function, patients generally do not have recurrent vertigo or imbalance because they still have a gravity sensor that functions in each ear, the utricle.
The Mondini dysplasia affects the cochlea and the labyrinth, so hearing and balance function are both affected. Some patients have no vestibular function at all, which can cause delays in motor skills requiring balance, such as sitting up and walking. In other cases, balance function is still present at birth but can deteriorate over time, often taking the form of repeated vertigo spells. A perilymph fistula may be associated with this deformation and can cause vertigo spells; surgical correction of the leak may be needed to control symptoms. Endolymphatic hydrops has been found in many cases at autopsy.
The most common inner ear abnormality seen on CT scanning is enlargement of the vestibular aqueducts. These tubes normally transmit cerebrospinal fluid from around the brain to the inner ear. When the tubes are enlarged, pressure changes in the fluids can damage the ear. We’ll discuss this in our next posting.