Biomedical subjects
Publications and source records attributed to A Gentine.
When it is well known that the electronystagmographic examination cannot diagnose the Meniere's disease, it is very useful to ensure that no other pathology is involved and for the follow up of the patients. The vestibulo-oculogram, on the other hand, suggests the presence of hydrops, according to the mechanical model.
The angular accelerometer of vertebrates, the semi circular canal, is a pressure gage. The transformation, acceleration to pressure, is due to the inertia of the endolymph. In the 0 to 1 Hz frequency interval, the endolymph-cupula system can be described by a second order, localised parameter model with two real time constants. This mechanical model explains the occurr]nces of the vestibular-ocular reflex during rotatory stimulations. However, some features of the response to caloric stimulations cannot be explained from this unique point of view. To correspond to these features, the SCC has to be considered as an inflatable structure, sensitive to weak pressure variations between the endolymph and perilymph. Different parameters of the mechanical model, the internal radius of the membranous canal, the dimension of the cupula and its elasticity coefficient depend on the inflating pressure. Ménière's disease is caracterised by hydrops, i.e. an increase of the pressure in the membranous SCC. The different mechanisms related to this hydrops, in particular introducing the inflating pressure as a second input to the ampular system, associated to a new concept of the mechanoneural transduction can explain the classical vestibular symptoms of Ménière's disease. Therefore, this overall mechanical model of the SCC indicates that hydrops is the mechanical cause of Ménière's disease and the primary cause is more likely to be a defect in the regulation loop of the inflating pressure.
The labyrinth is a sensor of vibration. Therefore, multifrequency vestibular stimulation proves indispensable in obtaining a more accurate study of the VOR. We chose to use a pseudo-random binary sequence, generated on a mini computer. The angular velocity instructions are made up of eight repetitive sequences, the spectrum of energy in which is adapted to the frequency range of VOR. A process eliminating the quick phases, the blinking and other artefacts is applied to the nystagmic response, in order to observe only the slow phase velocity. The velocity of eye and chair are compared and the transfer function of VOR is performed. Gain, phase, coherence function and predominance of the mean slow phase velocity between left and right responses are the four indices which define VOR function. The multifrequency rotatory test is interesting for topographic diagnosis, and makes out three diagnostic ranges: a mechanical peripheral, a neurosensory peripheral and a central one. Two practical measures could help the propagation of this method: a simpler vestibular stimulation in order to suppress the rotatory chair, a simplified method of data processing using a personal computer.
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In this first article of four the problem is displayed using experiments on humans for two kinds of gravity related positions. In each case the nystagmus depends sinusoidally on a particular orientation of the canal to the vertical. In each case, too, a dissymmetry of the response occurs. This kind of behavior confirms that the caloric stimulation induces both a gravity dependent and a gravity independent effect. A simple mechanical model gives account of the gravity dependent effect. It does not imply the otolithic system. Since the cupula adheres to the ampullar wall and to the crista, this is a hydrostatic model in contrast to Bárány's model, which is of hydrodynamic type.
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