The incision is made by means of a paracentesis knife, which is shaped like a tiny bistoury set at an angle to its handle (Fig. 187). The double-edged spear-shaped knife is now seldom used, as with it there is a tendency to puncture rather than to incise the membrane.
The tympanic membrane is pierced by the paracentesis knife at its inferior posterior margin. With a quick movement the drum is incised freely, the incision being carried in an upward direction midway between the malleus and the circumference of the membrane posteriorly, until it reaches Shrapnell’s membrane (Fig. 188). In making this incision the inclination of the membrane must not be forgotten. Owing to its lower margin being more deeply placed than the upper, there is a tendency for those who have not had much practice in doing a paracentesis to begin their incision too high up, as they fail to realize the greater depth of the canal at this point. The soft tissues of the upper posterior wall of the external meatus close to the membrane, if much congested, may be incised also in the act of withdrawing the knife. In doing this the chorda tympani nerve may perhaps also be cut, resulting in loss of taste on the affected side for a time; this is a matter of no importance. As a result of this free incision, drainage is given to the contents of the tympanic cavity, attic, and antrum.
In order to prevent rapid closure of the perforation and to give better drainage, some authorities advise making a flap-shaped incision. To do this, the membrane is incised upwards, nearly to its upper border; the knife is then carried backwards and downwards before it is withdrawn from the wound.
| Fig. 188. Tympanic Membrane showing Incision in Acute Suppuration of the Middle Ear. Usual line of incision; dotted line shows continuance of incision to make a flap opening for drainage. | Fig. 189. Line of Incision in Acute Suppuration of the Attic. |
Occasionally the acute inflammation is limited to the attic, Shrapnell’s membrane appearing deeply congested and bulging outwards so as to cover the processus brevis, whilst the rest of the membrane may be only slightly injected. In such cases it is sufficient to incise the bulging area, beginning the incision just above the region of the processus brevis and carrying it horizontally backwards to its posterior extremity (Fig. 189).
After-treatment. In acute middle-ear inflammation, after the first rush of blood and discharge has been mopped away, a small drain of sterilized gauze should be inserted into the auditory canal and the ear protected with a pad of sterilized gauze. The dressing and gauze drain should be changed as often as may be necessary, depending on the amount of discharge. The ear should not be syringed out unless the discharge becomes very profuse and thick.
In acute middle-ear catarrh with exudation, a Siegle’s speculum ([Fig. 194]) should be inserted into the meatus after free incision of the membrane, and as much fluid as possible extracted by suction. In addition, gentle inflation by means of Politzer’s method will help to expel from the middle ear the fluid, which should then be mopped out of the external meatus. This should be repeated daily.
Difficulties and dangers. The usual fault is to mistake the congested posterior wall of the external meatus for the membrane.
If the patient is not under an anæsthetic, the incision may be made too timidly, the membrane being only scratched. The pain thus inflicted will cause the patient to jerk away the head and probably prevent the membrane from being incised freely. The incision, therefore, must be made in a bold and rapid manner. It is better to make the incision too free than too small.
Care must be taken not to plunge in the knife too deeply for fear of wounding the mucous membrane of the inner wall of the tympanic cavity. This may result in adhesions between it and the membrane.