Acta Neurol Scand. 2014 Feb 20. doi: 10.1111/ane.12226. [Epub ahead of print]
Optic and auditory pathway dysfunction in demyelinating neuropathies.
The involvement of optic and auditory pathways has rarely been studied in demyelinating polyneuropathies. We here aimed to study this further in a cohort of patients with acquired and gentic demyelinating neuropathy.
We studied eight patients with hereditary neuropathy with liability to pressure palsies (HNPP), six with Charcot-Marie-Tooth disease type 1A (CMT1A), ten with chronic inflammatory demyelinating polyneuropathy (CIDP) and seven with antimyelin-associated glycoprotein (MAG) neuropathy using visual evoked potentials and brainstem auditory evoked potentials.
Optic pathway dysfunction was detected in 6/7 anti-MAG neuropathy patients, about half of those with CIDP and HNPP, but only in 1/6 patients with CMT1A. Peripheral auditory nerve dysfunction appeared common in all groups except HNPP. Brainstem involvement was exceptional in all groups.
We conclude optic nerve involvement may be frequent in all demyelinating polyneuropathies, particularly anti-MAG neuropathy, except in CMT1A. Peripheral auditory nerves may be spared in HNPP possibly due to absence of local compression. Evidence for central brainstem pathology appeared infrequent in all four studied neuropathies. This study suggests that acquired and genetic demyelinating polyneuropathies may be associated with optic and auditory nerve involvement, which may contribute to neurological disability, and require greater awareness. © 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.
BAEP , CIDP , HNPP , VEP , CMT1A, anti-MAG, auditory, cranial, neuropathy, optic
24571608 [PubMed – as supplied by publisher]
A curious Study, which once again poses more questions than it answers, but that maybe due to only the abstract being available.
Curious in several ways.
The testing methods used, brainstem evoked responses, are generally used to find CNS abnormalities, although it has to be said that they can also show abnormalities along the peripheral portions of the cranial nerves. The optic nerve , one of the cranial nerves which are generally thought to be part of the peripheral nervous system, is thought to be one of the exceptions to this rule. Along with the olfactory nerve it is considered a central nervous system tract rather than a peripheral nerve.
This study appears to suggest that HNPP can show optic nerve dysfunction, but this is not found so readily in cases of CMT1a. A result which would tally with the experience of some members of the HNPP support groups. However, it then goes on to say that the peripheral portion of the auditory nerve tested by Brainstem evoked potentials, is not affected in HNPP but is so in CMT1a. The suggestion being that HNPP is spared because the auditory nerve is not subject to compression.
So why then is the optic nerve not spared? And Compression is but one way in which peripheral nerves can be ‘traumatised’, nerve injury can also be caused by torsion, traction and by infection. Besides one form of compression that can occur in cranial nerves is compression by vascular loops. This is known to happen in auditory, trigeminal and facial nerves, if not other cranial nerves too.
There are also several questions regarding testing and methods used. ie was peripheral end organ testing performed for the auditory nerve, the abstract appears to suggest that it was not.
This study appears to contradict a previous study on sensori-neural hearing loss (SNHL) in cases of PMP22 type HMSN, http://www.ncbi.nlm.nih.gov/pubmed/15891642 , which appeared to show that SNHL was common in CMT1a and perhaps more so in HNPP. It was suggested that HNPP may be more susceptible due to it’s reactivity to extrinsic factors.
This is all very good though, even though it appears to be a contradiction of a previous study. The finding and results cannot be compared without also comparing methods, exclusion criteria, and analysis techniques. This is science, and nothing is written in stone, but there is a stepwise refinement of observation, hypothesis, theory, and conclusion. It’s an ongoing cyclic process. The two studies are very different, so it should not be surprising that the results will differ.
This study would appear to raise questions of CNS involvement in HNPP, due to the optic nerve dysfunction found during testing. And although auditory nerve dysfunction wasn’t found, this appeared to focus on Brainstem testing of the peripheral portion of the auditory nerve, and yet other peripheral end organ tests did not appear to be carried out, ie pure tone hearing tests.