I have now read the Vetter article relating to an animal model of a progressive degenerating condition. Hence at a critical stage of early development the vestibular system may have been quite normal. I am looking for cases where we can be certain none of the vestibular organs were ever normal.

Total absence of vestibular function in a fetus will make him (her) later on partially handicapped regarding space orientation or equilibrium and this will not affect his motor or autonomic reflex which are under control of central nervous system & not under control of vestibular system. Vital organs like circulatory or respiratory systems are not under control of vestibular system. Hence, a fetus with total absence of vestibular function would not die prenatally. When this fetus comes to life after birth, he (she) grows & develops his equilibrium in a different manner depending on his (her) visual & proprioceptive systems but he (she) will suffer much during dumpy or dark situation. This patient will have a wide range restriction of catching particular works which require maximum space orientation. Finally such patient will not taste motion.

Kind regards.

"I think the importance of the vestibular system is greatly underestimated, and that it drives the development of the other senses."

How would one determine that the development of one sense is *driving* the development of another sense? The CNS is integrating all of the sensory input in parallel; even the poorly developed senses are providing cues early in development.

Are you familiar with Feldenkrais' conjectures as to the primacy of the vestibular system? Based on his speculations, I would expect this to be true in arboreal species where an inborn falling-induced bracing reflex would be highly adaptive.

So are you asking only about the context of primates? I think it's well-established (yes, I'll find some references if you insist!) that a newborn human can sense accelerations of its head and sounds well-before the other senses are developed. Still, it's hard to argue that the development of the vestibular apparatus is driving the development of other senses -- the mechanoreceptors on the skin give useful signals (ones that inform motor signals) well before the vestibular apparatus, because the infant needs to develop quite a bit before it can direct the movement of its own head. Even the visual system is making large changes on similar timescales to the development of volitional head movement.

I asked myself, at what age is the vestibular-ocular reflex apparent in humans, and could not find an answer. I did find this article that seemed relevant to the topic: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3393735/

My question was not so much directed to the role of the vestibular system in sensory development, which I did not think was too controversial based on animal work. It arose from the observation that I could not find any example of a newborn mammal with total absence of vestibular organs, so I suspected that such fetuses died in utero from lack of vestibulomotor reflexes. If this is correct, this undercuts a lot of theoretical argument and might explain a lot of miscarriages.

Many children, like patients with Usher's type 1, are born with vestibular areflexia, but the end organs are typically present (are present in Usher's). I am not sure what difference it makes, if hair cell transduction is absent.

In normal infants, the VOR is present at birth and can be elicited easily in newborns.

Actually, the autonomic nervous system does respond pretty robustly to vestibular inputs, and hence is partially under vestibular control.

" I am not sure what difference it makes, if hair cell transduction is absent."

There is an enormous difference between having a very small residual vestibular system which still works to some extent. and one that is completely destroyed. It is not as easy as one might think to destroy an ear completely. I worked in a hospital where many surgical ear destructions had been done for Meniere's disease in adults, but in all of them some residual auditory or vestibular function could be found when carefully tested.

None of the vestibular organs were ever normal? If i look at embryology this would be a strange situation, having this as a single defect from birth on. So I think indeed in case of a complete lack of all the vestibular parts, the foetus has other severe congenital malformations too. But consider the theoretical situation there is absolutely no vestibular organ, and all other parts of the body are normal, than you still have prprioceptive and eye function, and than you have the situation going to an astronaut in wheightless situation in space. He lives. And if you have this situation prenatally, and from your birth on, plasticity of the neural system will be there to find some kind of, be it very partial, compensation. Of course that theoretical person will not have the same reflexes as a normal subject has.

"If i look at embryology this would be a strange situation, having this as a single defect from birth on."

I think there are plenty of cases of inner ear malformation as a single defect from before birth, eg Mondini malformation.

"And if you have this situation prenatally, and from your birth on, plasticity of the neural system will be there to find some kind of, be it very partial, compensation."

So one might suppose. So why can't I find a case of total absence of the vestibular system? If true, this is telling us something really important about the fundamental and unique importance of the vestibular system for fetal viability.

"that theoretical person will not have the same reflexes as a normal subject"

And if this happens in the womb, I suspect it is fatal. What we need now is some more definitive pathological evidence, in the first instance from a thorough literature review, or from someone drawing my attention to reports I may well have missed, especially in the non-English literature.

There is no reason to doubt the theoretical existence of temporal bones with total absence bilaterally of any functioning vestibular end organs, unless, of course, any such fetus would never have developed very far in the first place.  A good place to start would be with CLA (Michel anomaly), "complete absence of inner ear structures due to a developmental arrest of the otic placode before the third gestational week":

AJNR Am J Neuroradiol. 2009 Apr;30(4):774-80. doi: 10.3174/ajnr.A1426. Epub 2009 Jan 15.  Complete labyrinthine aplasia: clinical and radiologic findings with review of the literature.  Ozgen B1, Oguz KK, Atas A, Sennaroglu L.

Abstract

BACKGROUND AND PURPOSE:

Complete labyrinthine aplasia (CLA), also referred to as Michel aplasia, is a severe congenital anomaly of the inner ear, defined by the complete absence of inner ear structures. The purpose of this study was to document the imaging findings in a series of patients with CLA, with review of the literature, to better understand this anomaly.

MATERIALS AND METHODS:

The CT and MR imaging findings of 9 patients (14 ears with CLA) were retrospectively evaluated. The audiologic tests and patient charts were also retrospectively reviewed.

RESULTS:

CLA was bilateral in 5 and unilateral in 4 patients. The petrous bone was hypoplastic in all 14 ears, but the otic capsule was aplastic in only 5. The middle ear and mastoid volumes were decreased in most of the ears. The stapes was aplastic in 1 ear and was dysplastic in 5 ears. The internal acoustic canal was aplastic in 4 ears and markedly narrowed in 10 ears. The facial nerve canal showed a variety of anomalies and aberrant courses in 11/14 ears. The bony covering of the jugular bulb was defective in 9 ears. Tegmen tympani defects were seen in 3 patients, and there were several accompanying skull base and posterior fossa anomalies.

CONCLUSIONS:

Although CLA is a rare developmental anomaly, its accurate diagnosis and its differential diagnosis from labyrinthine ossificans is crucial. Proper guidance of these patients for brain stem implantation in the critical period of brain development depends on the recognition of the characteristic imaging findings of CLA.

                                                                   *****************

I conclude:

May 23, 2024

Vestibular Impairment and Postural Development in Children With Bilateral Profound Hearing Loss

Sylvette R. Wiener-Vacher, MD, PhD1,2; Marta Campi, PhD1; Simona Caldani, PhD1,2; et alHung Thai-Van,

JAMA Netw Open. 2024;7(5):e2412846. doi:10.1001/jamanetworkopen.2024.12846

This study, plus many older ones that they are unaware of, is consistent with the above thesis that there are no definite examples of complete vestibular absence at birth. In particular, none of the 60 with profound genetic nonsyndromal deafness had bilateral absent vestibular function. Once allowance is made for persistent insensitivity of clinical vestibular testing and for cases of progressive or postnatally acquired deafness, where vestibular function can be totally destroyed, I still think there are no total cases in their sample, nor indeed in the many similar previous surveys, that have survived after early gestation in the womb.