Comparative otopathology: aging, noise, and ototoxic drugs

Cochlear vascular and electrophysiological effects in the guinea pig to 4 kHz pure tones of different durations and intensities

Round window cochlear microphonic recordings from 33 guinea pigs were obtained prior to and following exposure to a 4 kHz pure tone at levels ranging from 124 dB to 140 dB for 5 to 80 minutes. When electro-physiological evaluation was complete, the animals were killed and tje cochlear tissues prepared using a soft-surface preparation technique. Experimental and control animals were mixed and randomly assessed without prior knowledge of the groups to which the specimens belonged. Histopathological observations were quantified and computer analysed. Statistical analyses suggest that noise exposure results in an overall decrease in blood flow to the cochlea with slight intracochlear increases in flow, perhaps compensatory in nature, to locations corresponding to the 4 kHz exposed region. Results implied that the initial localized hair cell damage known to occur as a result of overexposure to a discrete pure tone is not caused by a decreased blood flow. Relationships between electro-physiological and vascular changes and noise 'dose' are discussed.

Diagnosis and surgery of acoustic tumours

In the diagnosis of acoustic neuromata the stapedial reflex test, the forward vs reverse Békésy test, the loudness balance test, and the adaptation test have all been found superior among the special tests used in predicting a tumour diagnosis. Conventional X-ray studies of the internal acoustic meati are always performed, followed by tomograms and in suspected cases by meatocisternography. In the present series translabyrinthine total removal was performed in 21 cases, a planned decompression in 5, while in 2 the removal was subtotal. Only two permanent facial paralyses arose, one corrected by masseteric plasty and the other by facial-hypoglossal lateral perceptive hearing impairment as being caused by a tumour until proven otherwise.

A prospective study of gentamicin ototoxicity

Twenty patients were included in a prospective otoneurological study performed to assess the ototoxicity in gentamicin therapy. Gentamicin was administered intravenously, and the serum level was currently determined. Audiographic and electronystagmographic studies were carried out at the institution and discontinuation of the treatment and again a few weeks later. Ten patients exhibited ototoxic actions, predominantly cochlear, 4 of the cases being fully reversible. Two patients developed severe hearing loss, associated in one with bilateral extinction of vestibular function. Low serum levels of gentamicin did not rule out the possiblity of ototoxicity. These results urge the continuing of prospective studies and indicate that gentamicin should be used only as a link in the primary treatment of severe infection or in cases in which other, less toxic agents have failed.

[The vessels of the inner ear (author's transl)]

The inner ear as an example of a highly specialized sensory organ also possesses a highly specialized vascularisation. This represents an impressive example for a reasonable adaption of the terminal blood vessels to a specific function of the organ fulfilling more than only the nutrition. In this paper the microvascular bed of the cochlea is examined using both the injection method of the vessels and the biomicroscopic observation in vivo. The combination of these technics supported by histologic and stereoscan microscopic examinations has made it possible to give an account of the functional morphology of the inner ear vessels. As a detailed structural analysis of the vessels morphology with the help of dyes that fill the whole of the vessels space (i.e. Berlin blue) is not possible, perfusion experiments with silver nitrate were performed on the inner ear. After the perfusion the vessels are cleaned again, the silver however imbibes the intercellular reticular substances and after exposure produces a continous and sharp framework of the endothelium and--when present--muscular cells, thus showing the angioarchitectural contours. There is a very clear division of the cochlear vessels in a three dimensional space: The arterial and venous vessels are vividly separated from one another, forming two systems of microvascular units in the lateral wall and the spiral lamina. Each unit begins with special blood vessel convoluts in the modiolus, consisting of loops of arterioles. They are weakly muscularized whereas no muscle structures are seen elsewhere in the other parts of the inner ear vessels. There are no a.-v. anastomoses or sphinkters at all. The function of the vessel loops in the modiolus is to flatten the pulse wave as well as to regulate the blood flow in the microvascular bed by vasomotion. This was proved by statistical examinations of 1200 measurements of the widths of the vessels at several points of the cochlea in a blind study with and without vasoactive drugs. The terminology of the vessels is not standardized. The nomenclature in this paper has regard to the classification of the vessels, the course and the topographic localisation. Silver staining reveals changes in the form of the endothelium cells from the arterial towards the venous end. While the arteries show a long stretched spinle or lancet like form they change over blunt, oval, triangular or rhomboid forms into polygonal cells with spiked border lines at the venules. All experiments together give an account that the blood supply of the inner ear is in close correlation with the blood supply of the brain and too possesses autoregulative mechanisms, which must be localized in the convoluts at the beginning of every microvascular unit of the cochlear vessels.

[Pharmacokinetical, histological, and histochemical investigation on the ototoxicity of gentamicin, tobramycin, and amikacin (author's transl)]

The pharmacokinetics of gentamicin, tobramycin, and amikacin in the inner ear fluids and serum of the guinea pig were studied. The concentrations of these antibiotics were determined by a microbiological method and could be confirmed by the use of 14C-labeled gentamicin. Retention was clearly demonstrated in perilymph and endolymph, whereas there was no retention in the cerebrospinal fluid and the compartments of the eye. A linear relation between concentrations in the perilymph and dosage of gentamicin was ascertained. There was no difference between the concentration of drug in endolymph and that in perilymph. The concentrations of these antibiotics in the perilymph were symmetrical and many times higher than those in the brain. Long-term treatment did not influence the pharmacokinetics of the three antibiotics in the inner ear. However, increased levels of drug in the inner ears in animals with uremia and in some animals with otitis media explained the increased ototoxicity that was observed in histological investigations of these two conditions. Cisternal puncture and diuretics did not change the concentrations of aminoglycoside antibiotics in the inner ear. In large experimental series the hair cell degeneration pattern of the new aminoglycoside antibiotics was determined by the surface preparation technique as well as the influence of the different factors upon this pattern. A prophylactic effect on the ototoxicity of the aminoglycoside antibiotics could not be found in the 2,3-dimercaptopropanol, but by dividing the daily dosage administered. Young guinea pigs were generally not very sensitive to gentamicin, in some cases however much more. Late ototoxicity could not be found after administration of gentamicin. The pharmacokinetical and especially the histological investigations allowed an evaluation of the ototoxicity of the new aminoglycoside antibiotics. By histochemical investigations no influence of the new aminoglycoside antibiotics upon the amount of unspecific esterases and alkaline phosphatase in the inner ear could be detected, but an increase of the amount of acid phosphatase in slightly damaged outer hair cells.

Hereditary deafness in the dalmatian dog

The temporal bones from 18 deaf Dalmatian dogs were examined with the light microscope, and the cochleo-saccular degeneration process studied. End organ degeneration is already advanced by the age of 4 weeks, whilst loss of neurons in the spiral ganglion was found only in adult animals. Both processes are first evident in the central portion of the cochlea, and increase in extent with advancing age, The implications of these findings for the management of non-conductive hearing loss in man are discussed.