**2.4 Stickler syndrome**

Stickler syndrome affects about 1 in every 7.500/9.000 newborns. Mutations in the *COL2A1, COL9A1, COL11A1*, and *COL11A2* genes impairing collagen production have been identified as the cause of this disorder. Except for *COL9A1* mutations which are transmitted in an autosomal recessive fashion, the syndrome is autosomal dominantly inherited. The affected patients show a typical long and flat face with malar and mandibular hypoplasia (midface hypoplasia). The nose is small with a depressed nasal bridge and anteverted nares. The flatness of the face gives the appearance of large eyes. Vision is altered: myopia, cataracts, glaucoma and retinal detachment can be some of the associated findings. Cleft palate, bifid uvula and macroglossia may also occur.

Joint problems are presented by the patients from an early age. This involves arthritis which causes joint pain and stiffness. Flattened vertebrae and spine deformity such as scoliosis or kyphosis vertebral may also be present. Additionally, the prevalence of mitral valve prolapse in this syndrome has been reported to be higher than that in the general population.

**Hearing loss** can be both sensorineural and conductive. The conductive hearing loss in Stickler syndrome type I (*COL2A1*) can be due to the stapedial fixation. It can therefore be improved by stapes surgery. (Baijens et al., 2004)

Mutations in the fibrillar collagen genes *COL11A1* and *COL11A2* can cause sensorineural hearing loss probably due to the essential role these two genes have in the function of the basilar or tectorial membranes. There seems to be a correlation of hearing loss severity, onset, progression and affected frequencies with the underlying mutated collagen gene (Shpargel et al, 2004). In the study by Admiraal et al the mean sensorineural hearing threshold in Stickler patients with *COL11A2* mutation was about 40 dB HL and was liable to increase at the highest frequencies. (Admiraal et al., 2000)

In the study conducted by Szymko-Bennett (Szymko-Bennett et al., 2001) most of the 46 adults with Stickler syndrome had a sensorineural hearing loss, affecting high frequencies. Additionally, hearing loss was not more progressive as compared to age-related hearing loss.
