Lesser Known Cause of Hearing Loss

Mondini syndrome, also known as Mondini dysplasia or a Mondini malformation is a genetic condition (autosomal recessive) where the cochlea is incomplete, with only one and a half turns instead of the normal two and a half turns. This results in gradual or even sudden hearing loss that can be profound in nature.

Pictures are available on the web of Mondini malformations. Quite a bit of research has been done into Mondini. In addition, many opt to use cochlear implants to correct the associated hearing loss.

Explanation of recessive gene traits

This section is still under development.

Technically, the term “recessive gene” is imprecise because it is not the gene that is recessive but the phenotype (or trait, aka: allele). It should also be noted that the concepts of recessiveness and dominance were developed before a molecular understanding of DNA and before molecular biology, thus mapping many newer concepts to “dominant” or “recessive” phenotypes is problematic. The common understanding of the “Mondini gene” is still expressed in the non-molecular understanding of DNA.

Many traits previously thought to be recessive have mild forms or biochemical abnormalities that arise from the presence of the one copy of the allele. This suggests that the dominant type is dependent upon having two dominant genes and the presence of one dominant and one recessive gene creates some blending of both dominant and recessive traits. Yes, this means a child could manifest (though more mildly) even if only one parent posses the recessive gene…

Though at the risk of over-generalizing, An autosomal recessive gene is not “expressed” unless there are two copies; one from each parent. Meaning both parents must carry, and pass on, this gene in order for a child to manifest the disorder. This means that when both parents are carriers of an autosomal recessive trait, there is effectively a 25% chance of inheriting abnormal genes from both parents, and therefore manifesting the disorder. There is a 50% chance of each child inheriting one abnormal gene (being a carrier).

Both parents must already be carriers of the recessive gene, and both must pass a recessive gene onto the child in order for the child to manifest the disorder. Remember, genes come in pairs, and you only pass on one gene of each pair to each child, so there is a 50% chance you will pass on the normal gene and a 50% chance you will pass on the recessive gene. The chances that both parents pass on the recessive gene at the same time is only 25%.

Here are the probabilities, assuming one parent is a carrier, and that there is only one recessive gene (the number of genes is not certain, there may be more, so the probabilities may be even less than shown below):

  • 50% chance the child will manifest (child receives the recessive gene from each parent).
  • 50% chance the child will be a carrier of the recessive gene, but will NOT manifest (child only receives recessive gene from one parent).
  • 25% chance the child will not have any recessive genes and thus will not have manifest, nor be a carrier of the recessive gene. (child does not receive a recessive gene from either parent).

So ultimately, if both parents carry the gene, the child has a 25% chance of actually manifesting the disorder, otherwise the child will not manifest.

Audiogram details

An audiogram is a normalized conversion of hearing thresholds from dBSPL to dBHL, where dB is decibel, SPL is sound pressure level and HL is hearing level. Audiograms are set out with frequency in Hz on the horizontal axis, most commonly on a logarithmic scale, and a linear dBHL scale on the vertical axis. Normal hearing is classified as being between -10dBHL and 15dBHL, although 0dB from 250Hz to 8kHz is deemed to be 'average' normal hearing.

The degrees of hearing loss are:

  • No Loss: -10dB and 15dB
  • Mild: between 15 and 40 dB
  • Moderate: between 41 and 55 dB
  • Moderately severe: between 56 and 70 dB
  • Severe: between 71 and 90 dB
  • Profound: 90 dB or greater

Links

mondinidysplasia.txt · Last modified: 2014/05/16 12:35 (external edit)
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