Bell’s palsy and DTI 2023

Anatomy, Bell's palsy, Imaging, Research commentaries 3 Minutes

Stimulated by Qin et al 2023.[1]

Figure 2 from Liu et al 2011.[2]

Bell’s palsy – idiopathic facial palsy
DTI – diffusion tensor imaging
IF – impact factor
FA – fractional anisotropy
MD – mean diffusivity
AD – axial diffusivity
RD – radial diffusivity

key to acronyms

I was just doing a review for a big general journal and the authors used a paper on acupuncture for Bell’s palsy to justify how ‘crucial’ (their word) it was to achieve deqi when stimulating acupuncture points. I commented of course, because sometimes gentle acupuncture has outperformed standard acupuncture. This has happened in conditions characterised by increased somatic sensitivity. But unfortunately, I cannot find those 2 references just at the moment.

You can find more on Bell’s palsy and acupuncture here: Idiopathic facial palsy 2022.

Anyway, this paper was on top of my pile of recent papers when I moved on to write this week’s blog. This is a first… or more specifically, the first time anyone has found the abnormality responsible for Bell’s palsy.

I was just talking about DTI at the weekend at a conference, and was marvelling at how fabulous it was to be able to measure the integrity of tracts of neurons in white matter in the brains of real patients, and use these measurements as outcome measures. The topic I was covering is described here: Rewiring the brain with acupuncture. But DTI has also featured more recently on the blog: Acupuncture and DTI 2022. In the latter, there was a brief mention of measuring DTI in large peripheral nerves at the end of the blog.

I don’t think I had considered the possibility of making measurements on the facial nerve inside the temporal bone, but apparently it is possible, and the authors of this paper in Scientific Reports (IF 4.6) have done it.

The narrowest part of the facial canal is the labyrinthine segment – the part that runs from the internal auditory canal to the geniculate ganglion (see image above from Lui et al 2012,[2] and have a look at the first figure in the StatPearls article on facial nerve anatomy).[3] Apparently it gets to as little as an average of <0.7mm at the narrowest portion of this segment.[4]

It has long been proposed that swelling of the facial nerve within part of its bony canal, perhaps secondary to a mild upper respiratory viral infection, could be the cause of Bell’s palsy. So, the authors measured DTI parameters (FA, MD, AD, RD) at 4 positions over the intratemporal facial nerve bilaterally in both healthy subjects (n=18) and patients with recent onset idiopathic facial palsy (n=19).

I think the key measures are AD and RD, ie the degree to which protons (in water molecules) can diffuse along nerve fibres (AD) or outwards from those fibres (RD). If axons are damaged, then AD reduces. If myelin is damaged, then RD is increased. MD is the mean of AD and RD, and FA is one divided by the other (very roughly speaking – the actual equation is much more complex).

So, what did they find? Unsurprisingly, the measures were the same on both sides in healthy subjects. In the patients, FA MD and RD were all significantly different between the affected and unaffected sides, but AD was not. Mean RD was the only parameter that was different when healthy nerves (n=55) were compared with the affected nerves (n=19). This suggests that in Bell’s palsy, myelin sheaths are affected but axons remain intact, at least in the majority of cases, which is consistent with the high percentage of natural resolution of this condition.

References

1          Qin Y, Liu J, Zhang X, et al. To explore the pathogenesis of Bell’s palsy using diffusion tensor image. Sci Rep 2023;13:15298. doi:10.1038/s41598-023-42570-8

2          Liu L, Arnold R, Robinson M. Dissection and exposure of the whole course of deep nerves in human head specimens after decalcification. Int J Otolaryngol 2012;2012:418650. doi:10.1155/2012/418650

3          Seneviratne SO, Patel BC. Facial Nerve Anatomy and Clinical Applications. In: StatPearls. Treasure Island (FL): : StatPearls Publishing 2023. http://www.ncbi.nlm.nih.gov/books/NBK554569/ (accessed 18 Sep 2023).

4          Gupta S, Mends F, Hagiwara M, et al. Imaging the facial nerve: a contemporary review. Radiol Res Pract 2013;2013:248039. doi:10.1155/2013/248039

Declaration of interests MC

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I am the salaried Medical Director of the British Medical Acupuncture Society (BMAS) - a registered charity established to stimulate and promote the use and scientific understanding of acupuncture within medicine for the public benefit.

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