Tonomoter

I.  Abstract

Tonometer is an important instrument to measure the intraocular pressure. Following the principle of Goldmann applanation tonometer, our company offer three types:R and T Type are suitable for Haig Streit type slit lamp,while Z type is suitable for Carl Zeiss type slit lamp. Press flat a certain area (3.06mm diameter) of patient’s cornea with different pressure through pressure-measuring probe inlaid with a prism. Under slit lamp microscope, and with the help of illumination of fluorescence and blue rays, two green semi-circles whose inner parts connect each other can be seen. Thus, numerical value of intraocuolar pressure (IOP) can be got from the reading turn drum.

As the cornea area pressed flat by the tonometer is very small, change of IOP caused by outer force is also very small. The eye pressure we get is nearer to the true value of IOP as compared with Schiötz tonometer, with the mean error no more than ±0.066Kpa. It is one of the commonly-used tonometer.

II.     Preparation before measuring

A. Preparation of patients

1.Clean the front part of probe with solitude of mercury benzene borate of 1:32000 or sterilized fluid that will not damage the probe. Then wipe with sterilized cotton pick. And insert it into the metal covering on the top of lever of tonometer.

2. Drop 0.2% novesin (or other available surface anesthetic for ophthalmic) into the eye to do surface anesthesia. Once time every 1-2minutes, twice to three times together. Do anesthesia to both eyes to avoid blinking.

3.  Fluorescence dying the eye

4. Put the fluorescence paper inside the fornix conjunctional flap under the examined eye. This pasts several seconds.

5.A glass soaks with 0.5% fluorescence to the conjunction.

B.  Preparation of tonometer and slit lamp microscope

1.Assemble the tonometer onto the slit lamp microscope. Doctor can watch through the eye-piece on the leftside.

2. The intersection angle between the illumination of slit lamp and microscope should be 40~60°angle. This helps to brighten the head of probe. The slit diaphragm is opened to 8mm round light spot. A piece of cobalt blue filter is inserted into the illumination light way so that the yellow-green fluorescence can be seen clearly. With 10´microscope, and reading turn drum at 1gram of loading.

C.  Preparation of measurement

1. Adjust the direction of probe according to the patient’s cornea. If the cornea is spherical, the easier way is to measure horizontally (i.e. point ‘0’ on the head aligns to white mark line of cover ring). If patient’s cornea astigmatism exceeds 3D, and the applanated area is not round but oval, turn the horizontal axis of up-and-down prisms to the meridian of 43 degree of intersected long axis of oval, so that the applanated area is still 7.354mm2. For example, postulate the cornea astigmatism is 120 degree, curve radius of meridian is 6.5mm (=58D); curve radius of 30 degree meridian is 9.5mm (40D), meridian direction of lowest diopter is 30 degree , long axis of pressed oval is also 30 degree (refer to FIG.1), so align the mark 30 degree of probe to the red mark of 43 degree on the prism cover. The cornea applanated surface of hype-astigmatism is oval. Only at the direction of 43 degree axis, the diameter of oval is the same with that of circle of same square.

2.  Patient’s head is fixed on the forehead rest and chin rest of slit lamp.

3. Require the patient to open eye widely to look forward. Or the fixation lamp is used to control watching direction of patient’s eye. If necessary, doctor can widen the patient’s eyelid with hand, but not to press the eyeball.

4.  Before measurement, require the patient to blink his eyes some times to make cornea be evenly moist with fluorescence and tear.

III.    Measurement

1. Move slit lamp (together with tonometer) forwards slowly to make the probe touch with the center of cornea tip.

2.Watch through leftside eyepiece of microscope with left eye, two small semi-circles which do not connect can be seen. Turn reading turn drum and increase pressure on the cornea (originally at the scale of ‘1’) till two semi-circles connect to each other(refer to Fig. 3) (width of circle is about one tenth of diameter of applanated area, i.e. 0.3mm). Read the numerical value of pressure and turn to ‘0’ again, semi-circles depart again. Measure for three times like this way, if the reading value is almost the same, the measurement is right.

3.Take out tonometer quickly through controlling level. Times the reading with 10, i.e. mmHg of practical intraocular pressure.