[Free Full Text] Hearing disturbance in achondroplasia

"Achondroplasia has a variety of head and neck manifestations. The otolaryngologic complications include respiratory manifestations as a result of upper airway obstruction and frequent otitis media which may be accompanied by hearing loss and delayed speech". Bilateral Ossiculoplasty in 1 Case of Achondroplasia, J.Choi et al, 2013, Korean Journal of audiology.

Primary conclusions stated in this article:

1. Otitis media has been long recognized as a complication in achondroplasia, but little data have been published on the rates of otitis media and middle ear dysfunction in the achondroplasia population.

2. High rates of otitis media have been considered secondary to the orientation and size of the Eustachian tube as well as impairment of nasal airflow and temporal bone abnormalities.

3. Although hearing loss in achondroplasia is mostly a conductive hearing loss, sensorineural hearing loss and mixed hearing loss are observable in achondroplasia.

The American Speech-Language-Hearing association shares these important concepts:

• Conductive hearing loss occurs when sound is not conducted efficiently through the  outer ear canal to the eardrum and the tiny bones (ossicles) of the middle ear. Usually involves a reduction in sound level or the ability to hear faint sounds. This type of hearing loss can often be corrected medically or surgically.

• Sensorineural hearing loss (SNHL) occurs when there is damage to the inner ear (cochlea) or to the nerve pathways from the inner ear to the brain. Most of the time, SNHL cannot be medically or surgically corrected. This is the most common type of permanent hearing loss.


• Mixed hearing loss is when conductive hearing loss occurs in combination with sensorineural hearing loss (SNHL). In other words, there may be damage in the  outer or  middle ear and in the  inner ear (cochlea) or auditory nerve.

The primary alteration that occurs in achondroplasia is a disturbance of endochondral ossification and the result is a selective deficiency of bone growth in long bones. In terms of hearing, the alterations occur in the temporal bone of the skull.

During the fetus development in the uterus, there are developmental changes in the three ossicles of the middle ear: the malleus and incus develop as an endosteal bone while the stapes develops from cartilage. And this produces the typical findings of the temporal bone in achondroplasia:

1) towering of the petrous ridge and a high jugular bulb;

2) Narrowing of the skull base. The skull base is distorted due to imbalance between cartilaginous and membranous neurocraniums. The distortion of the temporal bone can cause hearing loss. From "Morphological Change on Skull Base by Achondroplasia" Y.Nakai et al, Department of Radilogy, Kyoto Prefectural University of Medicine

3) relative rotation of the cochlea and other temporal bone structures

Conductive hearing loss in achondroplasia is the most dominant form of hearing loss and the causes are most likely to be middle ear dysfunction or ossicular chain stiffness, either congenital or acquired owing to past chronic middle ear disease (otitis media).

A very interesting and relevant conclusion in this paper was:

"Previously, abnormal structure of the temporal bone was suggested as the cause of Eustachian tube dysfunction" (ETD means that the Eustachian tube is blocked or does not open properly resulting in an inability for air to get into the middle ear. Therefore, the air pressure on the outer side of the eardrum becomes greater than the air pressure in the middle ear. This pushes the eardrum inward. The eardrum becomes tense and does not vibrate so well when hit by sound waves). "But a prior study showed no evidence of poor middle ear drainage, using high resolution temporal bone computed tomography (TBCT).

It was also revealed that there was no correlation between CT findings of the temporal bone and the type or severity of hearing loss. Although an ossicular anomaly could be a possible cause regarding the development of the ossicles (especially the malleus and incus), almost all studies reported the normal appearance of ossicles in achondroplastic patients.

Acquired ossicular stiffness can develop when repeated, persistent middle ear disease accompanies it and it can be the cause of conductive hearing loss."

In this article, the authors suggest that the Malleo-stapedial interposition, an ossiculoplasty, can be an approach to conductive hearing loss in achondroplasia. "The purpose of surgery is correction of the conductive hearing loss, and it was decided to correct the whole possible cause of the hearing loss during the operation. Malleo-stapedial interposition can preserve physiologic sound transmission and has shown a 66-85% success rate in many reports."