IV. Discussion and Conclusions. The object of this investigation was to throw light on the

following questions: first, is i t possible to note a hearing disturbance of specific type in patients suffering from chronic carbon monoxide poisoning? Second, if a hearing deficiency of this type is demonstrated, what factors other than chronic carbon monoxide poisoning should be taken into consideration when determining the etiology of the deficiency. On the basis of examinations made on 700 patients the following conclusions may be drawn:

1) Hearing disturbances were noted in altogether 78.3% of the patients suffering from chronic carbon monoxide poisoning. A considerable smaller number of hearing disturbances, viz. 26.7 yo, were found when examining patients who had been exposed to carbon monoxide in their working places, but in whom chronic carbon monoxide poisoning could not be verified. Thus we may conclude that hearing disturbances were present in approximately three times as many patients suffering from chronic carboli monoxide poisoning as in patients not thus affect. This already indicates that chronic carbon monoxide poisoning may play a part in the origin of these hearing disturbances.

2) The majority of the patients had a similar so-called ))typical* hearing deficiency: the threshold of hearing was about normal, as was the beginning of the audiogram up to 1,000 Hz. Between 1,OOO -2,000 Hz. the curve dropped more or less steeply byt evenly. Thus, the decibel figure denoting the corresponding hearing loss was, when the vibration rate increased, larger and the upper limit of the hearing was lower than iisual. A hearing disturbance of this type is bilateral-symmetrical and is noted also by testing bone conduction. A ))typical)) hearing disturbance always causes an equal decrease of the upper limit of both ears. Recognition of such a

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hearing disturbance is thus easiest on audiometric examinat ion, but a distinct decrease in the time the c5 fork was heard is also essential. Hearing for speech and for whisper is usually seldom reduced, and the Schwabach also seldom shortened. A ))typical)) hearing deficiency was noted in 67.7% of the patients suffering from chronic carbon monoxide poisoning and only 14.0% of the patients not so affected. 1 Ience, atypical, hearing disturbances were found nearly five times more often in patients suffering from poisoning than in such in whom n o definite signs of poisoning could be de- monstraied. In the patients affected with poisoning altogether 86.4% of fhe hearing deficiencies were ufypical,, zuhereas the rorresponding percentage for paf ien fs free from a n y definite signs of poisoning was only 52.4%.

3) Generally the patient himself was not aware of the presence of a hearing deficiency. Of the patients suffering from chronic carbon monoxide poisoning 47.9% complained of impairment of hearing during the time they were exposed to the affect of carhon monoxide. The audiogram, however, disclosed changes in altogether 78.3% of the patients suffering from poisoning. By testing the hearing for speech and for whisper impaired hearing was found in 16.0% only. T h u s only one fifth of the hearing disturbances were revealed when the hearing for speech and for whisper was tested.

4) If all the patients are excluded in whom on hasis of the case history or examination some factor other than carbon monoxide was suspected to be the cause of the hearing deficiency, we conic to the following conclusion: affer excluding all the paf ien fs suffering from chronic carbon monoxide poisoning and having a nfypicah hearing loss, there remain S6.Sy0 in which nothing but chronic carbon monoxide poisoning can account for the hearing deficiency. The corresponding percentage in the cases free from symptoms is only 37.5%. After excluding all other etiological factors except carbon monoxide, ))typical)) hearing losses remain in altogether 44.5% of all the patients suffering from chronic carbon monoxide poisoning but in only 5.2% of those in whom no symptoms of poisoning were found. Thus dypicala hearing disiurbances the cause of which was revealed neither by the case history nor by examinaf ion were 8 times as frequent i n patients suffering f rom chronic carbon monoxide poisoning as in fhose free f r o m such symptoms.

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5) The follow-up examinations discloses that ))typical)) hearing losses improved only slightly, or not a t all. An improvement of hearing was found in only 26.7% of the cases, and i t was always slight. Hearing returned to normal in 10.7%, only in cases having a slight deficiency. The hearing of the patients treated with vitamin a, did not improve more than that of the patients not so treated. On the basis of the follow-up examinations it seems that practically speaking only slight hearing deficiencies improve, and even these to a small degree only. An improvement in hearing was nofed in only one fourth of the cases and were even then of slight degree.

6 ) Comparing the hearing deficiency with the observations earlier mentioned in the literature a t least four common features are noticed: ihe deficiency is of inner ear type and bilateral, improves slightly or not af all, and involves chiefly the upper tones.

7 ) Some facts indicate that a &ypical, hearing deficiency may appear already a t the initial stage of chronic carbon monoxide poisoning, when actual vestibular symptoms are not yet present. The recognition of ))typical)) hearing disturbances may thus be helpfill with a view to an early diagnosis.

8 ) On the basis of the above examination it seems that carbon monoxide very often causes hearing disturbances inpatients suffering from chronic carbon monoxide poisoning.

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