Superficial siderosis (SS) is a very uncommon type of deafness, sometimes accompanied by vestibular loss as well, that is caused by iron deposition in the brain. It was first described in 1908. There are two types of SS -- classical infratentorial, and cortical. According to Pichler et al (2017), from the Mayo data the prevalence of SS (both kinds) is 0.21% in those 50-69 years old. The cortical type is more common (a total of 2 infratentorial vs. 13 cortical). We are mainly discussing the infratentorial type here, as this is the one that can cause hearing loss. From Pichler et al, the frequency of infratentorial SS should be about (0.21%)/6, or roughly 0.035%, or 35/100,000, in the 50-69 year old age group. This is not the same as population prevalence overall, as SS is relatively rare in young people, and much more common in those over 69. Still, it is substantial when compared to the prevalence of related conditions, such as bilateral vestibular loss.
Classical SS is hypothesized to result from an insidious, low-volume leak of red blood cells into the subarachnoid space (Wilson et al, 2017). Most individuals with deafness have a source of bleeding in the posterior fossa, but the bleeding may also be remote, such as from a cervical meningocele (Lekgabe et al, 2012), or dural tear (Egawa et al, 2013). Angiography is usually not very helpful, suggesting that most of the bleeding more of the nature of a seepage. Over years, iron accumulates in the brain ,and hearing and the sense of smell is gradually lost. Of course, hearing and smell is often lost with age due to other causes, so there is some ambiguity here. There may also be ataxia and myelopathy.
With the common use of brain MRI scanning, SS is more commonly diagnosed, as there is a characteristic low signal seen on both T1 and T2 sequences. Ordinary fluid is black on T1 and white on T2. Iron is black on both. The posterior fossa is most affected including the cerebellar convexities and superior cerebellar vermis. The olfactory and vestibulocochlear cranial nerves are especially vulnerable, (Wilson et al, 2017), but MRI doesn't help here as they are too small to be visualized.
SS is largely caused by slow bleeding at the sites of dural lesions, mainly in the spine or posterior fossa (Wilson et al, 2017). Often there is no site of bleed identified mainly with MRI -- angiography rarely contributes to detecting a cause (Wilson et al, 2017). As we are seeing more far use of anticoagulants due to their enhanced safety, it seems likely that there will be more SS seen in the future. However, the long delay implies this may not be found for decades.
As mentioned above, there are two types of SS -- classical infratentorial, and cortical. The cortical type is much more common. Recent studies of patients in dementia clinics suggest that superficial siderosis of the cortical type is found in about 3%, associated with microbleeds. There is also an association with cerebral white matter lesions. (Shams et al, 2016). This type of SS is from bleeds within the brain, and is not especially correlated with hearing loss. Cerebral amyloid angiopathy(Profice et al, 2011) is presumed to be the cause of many of these.
According to Takeda et al (2018), and Lee et al (2018), bilateral vestibular impairment also occurs in SS. This may be a contributor to the patient's ataxia. These patients are recognized from their MRI. Aran Yoo and Kattah reported another case in 2017. Both SS and bilateral vestibular loss are very uncommon -- the estimate above based on Pichler's data from Mayo was 40/100K for infratentorial SS, vs. about 24/100K for bilateral vestibular loss. Thus it is concievable that infratentorial SS accounts for a substantial percentage of bilateral vestibular patients. More study is needed here.
As SS causes nerve damage, this type of hearing loss might not be amenable to developing treatments to regenerate hair cells. Unless the source of bleeding can be stopped, only supportive treatment, such as cochlear implant (Sugimoto et al, 2012), is available at this date for SS. The logic of a cochlear implant is difficult to follow, given that SS damages the auditory nerve.
|Audiogram when patient first presented with hearing loss, when patient was in early 20's, about 10 years after her surgery. Hearing is worse on the side of the tumor.||Hearing after 11 years, is worse on side of tumor. There has been considerable deterioration.|
A woman in her 20's complained of progressive hearing loss over the last year. In addition she has tinnitus, and trouble understanding. 10 years prior, a cerebellar astrocytoma was removed from her right cerebellar hemisphere. Several MRI scans done subsequently noted not an area of damage to the R cerebellar hemisphere but also hemosiderin accumulation was noted by one radiologist. Another attributed the same findings to cerebellar atrophy.
An extremely thorough set of tests were done for alternative causes of hearing loss including autoimmune tests. Balance was normal, Reflexes were normal, and the general neurological exam was entirely normal including the cerebellar examination.
She was seen again about 10 years later, with similar findings. Again, hearing was profoundly diminished, cerebellar examination was normal, and a current MRI scan still was still very abnormal due to low signal seen on T2 imaging. Detailed vestibular testing (rotatory chair, VEMP) was normal -- thus in spite of severe hearing loss, vestibular function was entirely unaffected. This suggests that the site of damage is specific to the cochlea or cochlear nucleus. This is against the report of Miwa et al (2013) who found VEMPs to be reduced in 3 patients with SS.
The obvious question in superficial siderosis is where is the lesion ? Is it in the cochlea, 8th nerve, brainstem ? The most prevalent thought is that SS damages the auditory nerve where it is exposed to spinal fluid.
Our patient also had no OAE response at all, suggesting a cochlear origin to her hearing loss. There have been only a few other cases of superficial siderosis where OAE’s were done. Kim(Kim, Song, Park, Kim, & Koo, 2006), reported a young man with SS and no OAE’s. Takahashi(Takasaki et al., 2000) reported another case with no OAE, and a “remarkable elevation of the detection threshold for the cochlear microphonic”. In other words, again saying that at least part of the hearing problem was in the cochlea, as the cochlear microphonic is generated from the hair cells and does not require an 8th nerve.
|T2 image of superficial siderosis. Note the low signal in the folia of the cerebellum, although on T2, fluid images as high signal.||Flair image. Again, note low signal in the folia of the cerebellum.|
There is no known treatment for SS, but the iron chelator deferiprone (DFP) has been proposed as a potentially useful treatment. Cossu et al (2019) reported long term treatment with DFP in 4 patients, with the results that it appeared safe and well tolerated. Kessler et al (2018) also reported stabilization or improvement in 19 patients.
Ryan (Ryan, Piplica, & Zhang, 2014) stated that only 15 cases of SS had been implanted world-wide. Their particular new case had only marginal improvement with CI.
Tyler(Tyler, Martin, & Baguley, 2012) reviewed the world literature at the time, and stated that about half of implant recipients, had “clear sustained benefit”, while 6 showed limited/no benefit, and 2 had temporary improvement that was not sustained. They suggested that when there is “stable” disease, implants are helpful, but when there is progression, implants are not a good idea.
Overall then, it appears that cochlear implants are presently only reasonable when hearing is stable (i.e. not deteriorating).
Electroaudiometry should be done prior to committing to a cochlear implant in patients with superficial siderosis. Electroaudiometry is a method of determining if the 8th nerve is still working, by bypassing sound and using electricity to stimulate the 8th nerve.