|Year : 1969 | Volume
| Issue : 4 | Page : 139-144
Corneal sensations in acute glaucoma
P Awasthi, VK Goel
Department of Ophthalmology, Sarojini Naidu, Medical College, Agra, India
|Date of Web Publication||10-Jan-2008|
Department of Ophthalmology, Sarojini Naidu, Medical College, Agra
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Awasthi P, Goel V K. Corneal sensations in acute glaucoma. Indian J Ophthalmol 1969;17:139-44
Glaucoma is known to decrease the corneal sensations in its acute stage (Boberg-Ans  ). In-spite of the efforts of various workers, none has as yet evaluated a definite relationship between the height of intraocular pressure and the extent of loss of corneal sensations. The present study entails the quantitative evaluation of the loss of corneal sensations in acute congestive glaucoma, its relationship with the height of intraocular pressure and the manner of its recovery.
| Material and Method|| |
The study was conducted on 31 patients with acute congestive glaucoma in one eye, the other eye being nonglaucomatous served as control. The corneal sensations were tested by an instrument, especially devised for the purpose.
It consists of an ebonite cylinder 15.0 cm in length with an outer diameter of 1.85 cm, inner diameter 1.45 cm, a piston 4.5 cm in length, plays within the hollow of the cylinder, at the lower end of which a nylon thread of 0.112 mm diameter and 8.5 cm in length is fixed. A pointer is attached to the piston which measures the amount of movement of the piston on a scale which is engraved in millimeters on the cylinder. As this piston moves, the length of exposed hair changes and this change of length is recorded directly on the scale. The pressure applied by the various lengths of hair can be calculated' as follows. If this thread is considered as a column under thrust until it just bends, the elastic power exerted by the thread depends on the fixation of the circular homogenous thread. If the end of the thread is freely movable the whole length of the thread will take part in the bending. The shorter and thicker the thread, a greater force is required to bend it.
The force required for bending depends on the elasticity of the material, If the thread is fixed at both ends and these are brought nearer to each other, only a part of the length of the unread, the so called free length will take part in bending. It has been proved that the free length (1) is equivalent to half the length of the whole thread (L)
In the instrument the thread is restricted at one end only. When pressed against a surface (cornea), the freely movable thread will make an elastic bending and can be comparable to an elastic column where the free length (1) equals L/2, where L is the total length. (According to Euler Power PE, which by central application of force acts at the end of an elastic column will equal the bending power. This can be expressed by the equation
length of the hair. In the cross section of the thread the moment of
is the diameter of the column. If d = 0.112 mm, I equals 0.72 x 10 CM. The coefficient of elasticity for Nylon is already known. The power of elasticity PE for a nylon thread with length L, diameter 0.112 can under varying conditions be calculated as follows.
For the climate of Agra the average humidity has been taken to be 50% as the humidity here ranges from 25% to 80% throughout the year so with 50% relative humidity E being 3.16 X 10 4 .
Thus the pressure exerted by different lengths of hair has been calculated and represented in [Table - 1]
| Method|| |
The patient is asked to lie down on a table before he is subjected to this investigation, he is made familiar with the touch of the hair on the back of his hand. The instrument is fixed to a stand in such a way that it could be moved up and down above the cornea. The lids are restricted by a thumb and index finger of the examiner, taking precaution that no pressure is exerted on the globe.
The hair with maximum free length is brought on the cornea and touched at right angles in such a way that it just bends at the point of contact with the cornea. The length of the hair is Gradually reduced until the patient just feels the touch of the hair and the free length of the hair is noted. The sensations in milligrams are obtained against the length of hair from [Table - 1].
The patients were examined for their corneal sensations in both eyes as soon as they were admitted to the hospital before any antiglaucoma therapy was instituted. Their intraocular pressure was recorded by a standardised Schiotz tonometer. These patients were given a dose of oral glycerol, 1.5 gm/kgm body weight as a 50% solution in normal saline, cooled to 3° to 5° C. Their corneal sensations and intraocular pressure were again tested after 1 hour and 2 hours of the administration of glycerol. Awasthi and Srivastava  observed that the maximum effect of oral Glycerol is within 1-2 hours of its administration.
This lowered intraocular pressure was later maintained by routine antiglaucoma therapy which included Diamox tablet 250 mg and Pilocarpine 2% drops, the frequency depending upon the height of intraocular pressure. The corneal sensations and intraocular pressure were recorded again in both eyes 24 hours, 48 hours and 72 hours of the commencement of the therapy.
| Observations|| |
The 31 patients investigated were between the ages of 40 and. 65 years, the majority belonging to the age group 41 to 58 years. 23 were males and 8 females.
The above table shows that if the tension is high, there is proportionate loss of sensation.
Effect of lowering the tension with Glycerol in the recovery of sensation is shown in [Table - 3]
The abrupt lowering of the tension with oral glycerol did not improve the sensations of the cornea.
The recovery effect of the corneal sensation on lowering the tension with oral glycerol and then maintaining it with Diamox and 2% pilocarpine drops is shown in [Table - 4].
It is evident from the above tables that after maintaining tension at low level there is 60-70;o recovery of sensations in a period of 72 hours.
| Discussion|| |
In the present study it has been confirmed that the corneal sensations vary with intraocular pressure [Table - 2]. The sensations are diminished with the rise of intraocular pressure above 28 mm Hg., in glaucomatous eyes. Higher the intraocular tension greater the reduction in the sensation of the cornea. Gradual recovery of sensation takes place after lowering of the intraocular pressure. The sudden lowering of the intraocular pressure with glycerol does not help much in the recovery of sensations as cornea does not get time to recover so quickly. If the tension is kept at a low level, over seventy two hours, the recovery of sensation does take place to an extent of 60 to 70 percent.
It has been observed that after the administration of glycerol, although improvement in intraocular tension was evident one hour later, one could improve the sensation from an average of 160 mgm to one of 153.5 mgm only, i.e. only by 6.5 mgm or 4%. If the lowered intraocular pressure was maintained by Diamox and miotics, the recovery of sensation was 20-35 after 24 hours.
The recovery of sensation was much less in those cases who had raised intraocular pressure for longer period before an effective antiglaucoma therapy was instituted. As for example the patients who were brought to the hospital within 48 hours of the attack, recovery of the sensation was 60-70%, patients who came after 48 hours but before 72 hours of the attack, the recovery of the sensation was only 35-60%, while other patients who came for the treatment after 72 hours the recovery of the sensation was very poor. The sensation did not recover beyond 20-30%,. This study shows that the loss of corneal sensation was greater when the intraocular tension was high. The recovery of the sensations was depending on 2 factors-if the duration of attack was for a longer time over 48 hours, the recovery was poor, another factor which is important for complete recovery was that the tension should be kept within normal limits for over 72 hours. A sudden lowering of the tension within first 2 hours did not improve corneal sensation to a great extent.
Corneal sensation can be a fair indication for determining the normal intraocular pressure. It is suggested that instead of repeated tonometery recording of corneal sensation every hour will give a fair indication towards normalization of the intraocular pressure.
| Summary and Conclusions|| |
An instrument is devised for measuring corneal sensation. 31 patients with acute attack of glaucoma have been studied in order to determine the effect of high intraocular pressure on corneal sensations and evaluate the amount of recovery in corneal sensation after lowering the intraocular pressure abruptly and gradually.
The following conclusions were made :-
In acute glaucoma corneal sensation diminishes proportionately with raised intraocular pressure.
Abrupt lowering of intraocular pressure on corneal sensation is not appreciable, but lowering maintained for at least 72 hours produces gradual and considerable recovery in sensations.
Longer duration of attack effects recovery of sensation adversely.
Earlier the tension is reduced by medical or surgical treatment the chances of recovery of sensations are more and thus the disturbances of vision will be the least.
Loss of sensation could be attributed to corneal edema and secondly, damage to the corneal nerves.
There is no effect of lowering of intraocular tension with glycerol on corneal sensation in normal eyes.
| References|| |
Awasthi, P., Srivastava, S. N.: Role of oral glycerol in glaucoma, Brit. J. Ophth. 49, 660 (1965).
Boberg-Ans : Experience with corneal sensitivity. Brit. J. Ophth. 39,
[Figure - 1], [Figure - 2]
[Table - 1], [Table - 2], [Table - 3], [Table - 4], [Table - 5]