Criteria For Defining Asymmetrical Hearing Loss
Asymmetrical hearing loss is not always taken seriously by healthcare professionals outside of audiology –
“It can be a tumour”!
Asymmetrical hearing loss is of important diagnostic interest as it has implications on the treatment, surgery and rehabilitation of the patient. Most clinicians have had a patient showing an asymmetric hearing loss on their audiogram (check out our previous blog post on How to Read an Audiogram), bringing forward questions such as: is this worth a referral? Asymmetrical hearing loss is defined as a hearing loss that is significantly worse in one ear compared to the other. But how much worse is significantly worse?
Although there is a lot of reported literature on asymmetric hearing loss (AHL), there is no standard criteria for defining asymmetry. The definition of AHL is relevant because AHL is an important risk factor for pathologies such as auditory nerve tumors. Before we dive into the criteria to use for defining asymmetrical hearing loss, it is crucial that we are aware that AHL may occur as a result of common causes of bilateral hearing loss such as noise induced hearing loss and age, as well as cerumen impaction or otitis media. With that being said, it is still necessary for the clinician to conduct a thorough and comprehensive assessment (including a detailed medical history) to determine the aetiology of the AHL. When acoustic neuroma is suspected, audiologists usually consider criteria for identifying asymmetrical hearing loss (in combination with unilateral hearing loss and decreased word recognition) in order to determine whether a patient should be referred to an ENT for consideration for a Magnetic Resonance Imaging (MRI).
For the purpose of this article, a subset of AHL, namely Asymmetrical SensoriNeural Hearing Loss (ASNHL), will be considered.
Criteria for identifying Asymmetrical Hearing Loss
Many of us have been taught one, two or three of the following rules when determining asymmetry for bone conduction thresholds:
- interaural threshold difference ≥10 dB at 3 octave frequencies (across the frequency spectrum),
- a difference of ≥15 dB at 1 octave frequency and
- ≥15 dB difference at two octave frequencies.
Some authors have defined ASNHL as the difference between bone conduction thresholds of >10 dB at two consecutive frequencies or a difference of >15 dB at one bone conduction frequency . One can see that the above definition is slightly different to the three sets of rules shown above.
As mentioned previously, asymmetrical hearing loss may occur as a result of common causes of bilateral sensorineural hearing loss. However, it may also be as a result of vestibular schwannoma, a serious underlying pathological process such as demyelinating disease, Ménière’s disease or idiopathic sudden sensorineural hearing loss. Vestibular schwannoma very often occurs unilaterally and causes asymmetric symptoms. A list of criteria for identifying candidates for MRI when suspecting vestibular schwannoma includes the following:
- Asymmetrical sensorineural hearing loss thresholds
- Difference of 25 dB or more at two consecutive frequencies .
- Average difference in thresholds between ears of 15 dB or more at frequencies of 500, 1000, 2000 and 3000 Hz (American Academy of Otolaryngology – Head and Neck Surgery criteria).
- Unilateral or asymmetrical tinnitus
- A 15% difference for word recognition in the maximum score.
- Rule 3,000
Rule 3,000 for screening vestibular schwannoma
Saliba and colleagues  found that a criteria of 15 dB or more at 3kHz is more reliable in selecting patients with ASNHL who would have an acoustic neuroma detected with an MRI. In fact, they have suggested that this would reduce the number of false positive MRI referrals. A study by Ahsan et al  consisting of 451 patients found that a majority of patients had a 10 dB difference at 3 adjacent frequencies (n=426) and/or a 15 dB difference between 2 adjacent frequencies (n=410) – that is over 90% of the patients! In addition, they found that only 290 and 176 patients had a 15 dB difference at 3 kHz and 15% word recognition score, respectively. Overall, out of the 451 patients, 89,4% had normal (51%) or incidental (38%) findings on the MRI that did not explain the ASNHL. Only 10.6% of the patients had abnormal MRI findings that explained the presented ASNHL.
These studies confirmed each other’s findings. In all ASNHL criterias that they evaluated, a significant increase in MRI results was noted with patients with a 15 dB or more difference between the ears at 3 kHz. However, it is important to understand that the 15 dB difference at 3 kHz was not an isolated loss, but that there was involvement at other frequencies. It’s just that an asymmetry at 3 kHz raises greater clinical suspicion that should warrant an MRI.
So, what is Unilateral hearing loss?
If asymmetrical hearing loss is present and the one ear has normal hearing, then we will call it unilateral hearing loss. In essence it has the same clinical value as asymmetrical hearing loss and must also be taken seriously.
Although there aren’t any standardised criteria for identifying asymmetrical hearing loss, clinicians should ensure that they keep up to date with peer reviewed clinical research related to the topic. For example, ‘Rule 3,000’ shows great promise in the detection of Asymmetrical Sensorineural hearing Loss as a result of acoustic neuroma, and this will lead to direct referrals and cause clinicians to be more cautious with resources. To help to better identify the aetiology of the asymmetrical hearing loss, a comprehensive assessment should be conducted by the clinician.
The KUDUwave can help you
The KUDUwave audiometer software can assist you with interpretations. One of these interpretations are for asymmetrical hearing loss. The following is an example: