What is Asymmetric Nerve Hearing Loss

Given my last blog article regarding an ENT who got sued for not doing an MRI after audiogram showing an asymmetric hearing loss, the question that should be on everybody's mind is what IS asymmetric nerve hearing loss on a hearing test?

I am sad to report that there is no clear consensus on what is asymmetric nerve hearing loss.

First of all, at its most basic definition, asymmetric nerve hearing loss is when hearing is worse in one ear when compared with the opposite ear due to nerve damage.

Technically, hearing should decrease equally on both sides over time, but when one side gets worse faster than the other, it can cause concern because:

1) A tumor can be pressing against the nerve causing it to slowly fail faster than the other ear's nerve which is not compressed.
2) There is a loud noise history with asymmetric exposure to the ears. One example is hunting with a rifle where the left ear would repetitively get noise-induced damage from the gunshot in a right-handed shooter.
3) Bad luck

The conflict is at what point is an asymmetry considered significant enough to warrant a workup which may include an MRI scan of the head and/or auditory brainstem response (ABR)? Also, for a given patient history, a physician/patient may elect to recheck hearing bi-annually to monitor for asymmetric progression rather than pursue workup immediately (further studies being pursued only if asymmetry progresses over time).

Is asymmetry defined as 1 decibel difference between ears? Is it 50 decibel difference?

Even if the decibel difference is agreed upon, than over what frequencies? All of them or just a few of the frequencies?

So for the purposes of a "medically" significant asymmetric nerve hearing loss, the definition must define not only HOW big the difference in hearing loss is present between ears but also at what frequencies.

In one paper, it is 15 decibels at just 3000 Hz.

In another paper, it is 20 decibels at any two adjoining frequencies (or 15 decibels at two adjoining frequencies between 2000 and 8000 Hz).

Get this... even when criteria is defined, one study found substantial disagreement among five expert judges on what hearing tests they considered showed asymmetry.

Even insurance companies are jumping into the fray in what they considered significant enough to warrant covering the MRI scan. At one point when I ordered an MRI scan of the head for what I thought was a significant asymmetry found in one patient's hearing test, the insurance company denied the test citing "that there needed to be other neurological deficits PLUS the asymmetry seen on hearing test before MRI scan would be authorized".

I personally pursue further workup using the 20 decibel difference at two adjoining frequencies OR 30 decibel at one frequency.

Assuming medically significant asymmetric hearing loss has been defined and is present...

What are the chances of something bad going on (i.e., tumor) when an MRI scan of the head is ordered for asymmetry found on hearing test?

Less than 10% (depending on study, can be as low as 1-4%).

Given such a low positive yield on testing, why even pursue a MRI scan?

Because of defensive medicine to avoid being named in a lawsuit if the affected patient just happens to be within that 10% found to have an acoustic neuroma.

Indeed, in one study, 40% of ENTs stated medico-legal concerns factored into their decision to get an MRI scan of the head.

Head CT Scans Increase Risk of Cancer in Kids

By how much?

Brain tumor/cancer risk was 3X greater than the general population in children who received two to three CT scans of the head.

And the chance of developing leukemia was three times as great for children who received five to 10 CT scans of the head.

In the study published in Lancet, the researchers examined 176,587 children who had received a CT scan before age 22 and looked at their medical history for an average of 10 years afterward.

Though the risk sounds "high," the overall risk is still relatively small and CT scans should still be performed if medically warranted.

Phrased another way, the risk corresponds to one additional case of leukemia in the 10 years after the first scan for every 10,000 patients younger than 10 who were scanned or an extra case of brain cancer for every 30,000 children scanned.

Even so, the study does illustrate that CT scans should not be ordered unless there's a very good reason to do so, especially in children given they are more radiosensitive compared to adults.

In the ENT world, the most common reason to order a CT scan is to evaluate for sinusitis.

My own personal preference is to obtain one in children if and only if all other sinusitis interventions have already been performed.

Such "sinusitis" interventions include:

• Allergy intervention (steroid and/or anti-histamine nasal sprays)
• Failure to respond to antibiotics
• Adenoidectomy (if large adenoids present which can be determined on endoscopy)
• Turbinate Reduction (if turbinates are enlarged)
• Check for cystic fibrosis and primary ciliary dyskinesia

If above interventions have been performed, only than would I consider a CT scan of the sinuses.

Some (other ENTs) may argue that CT scan of the sinuses should be performed early as a more directed intervention can than be performed.

My argument against that is, EVEN if the CT sinus comes back abnormal, I STILL would perform the above interventions first because often the above interventions will clear up and sinus abnormality seen on a CT scan.

Only when everything else has been tried would I consider sinus surgery in a child and only than would a CT sinus be justified in my mind.

However, there are certain other situations I may consider getting a CT scan relatively quickly...

• If I find a sino-nasal mass on exam concerning for tumor or nasal polyps
• Congenital hearing loss
• Cholesteatoma (tumor of the ear)
• "Significant" Facial Trauma (ie, not just for a simple broken nose)


Reference:
Radiation exposure from CT scans in childhood and subsequent risk of leukaemia and brain tumours: a retrospective cohort study. The Lancet. 7 June 2012. doi:10.1016/S0140-6736(12)60815-0

Source:
Children's CT Scans Pose Cancer Risk. Wall Street Journal 6/6/12

MRI Scans CAN Cause Vertigo

Researchers at Johns Hopkins published a study that reports that MRI scans CAN cause feelings of dizziness induced by magnetic fields causing motion of the electrically charged fluid of the inner ear.

In both ears, there is a "gyroscope" called the labyrinth. Whenever the head turns or a force like gravity is exerted on this system, fluid moves within which tells the brain that motion has occurred. It's much like looking into a glass of water and based on the way the water tilts in the glass, you can guess which way motion is occurring.

In any case, the study took two groups of people, one with an intact labyrinth system and another without a functioning labyrinth system. Only those individuals with an intact labyrinth system suffered vertigo while getting an MRI. Furthermore, the stronger the MRI's magnetic field, the worse the vertigo.

Beyond the "that's interesting" trivia, these results are VERY important in any type of brain activity research. Why? Well, if a person is feeling dizzy, the brain activity you may be studying using the MRI may be attributable to the dizziness itself and the patient's coping mechanisms to deal with it rather than whatever brain activity research you may be investigating.

On a more science-fiction note, based on these findings, an MRI can theoretically be used to potentially diagnose and treat dizziness due to the inner ear. After all, if it can cause dizziness, it certainly can be used to help it as well!

Reference:
MRI Magnetic Field Stimulates Rotational Sensors of the Brain. Current Biology, 22 September 2011. doi:10.1016/j.cub.2011.08.029