|Year : 1977 | Volume
| Issue : 4 | Page : 26-29
Quantitative measurement of corneal sensitivity by asthesiometer
HS Chundawat1, KL Chaplot2
1 S.M.S. Medical College & Hospital, Jaipur, India
2 Department of Ophthalmology, R.N.T. Medical College, Udaipur, India
H S Chundawat
S.M.S. Medical College & Hospital, Jaipur
|How to cite this article:|
Chundawat H S, Chaplot K L. Quantitative measurement of corneal sensitivity by asthesiometer. Indian J Ophthalmol 1977;25:26-9
|How to cite this URL:|
Chundawat H S, Chaplot K L. Quantitative measurement of corneal sensitivity by asthesiometer. Indian J Ophthalmol [serial online] 1977 [cited 2013 May 20];25:26-9. Available from: http://www.ijo.in/text.asp?1977/25/4/26/34609
Clinicians from various disciplines of medicine, from the begining of the 19th century have given much importance to the corneal sensation as a diagnostic sign of many diseases. Exact method of its quantitative measurement was not clear till the work of Boberg  appeared.
Although arbitrary, but presently as a routine, clinical assessment of corneal sensitivity is commonly done by touching a wisp of fine cotton to the cornea avoiding the optical axis as the direction of touch. Objective response is noted by knowing the blinking, and subjective by asking the patient whether he felt increased, decreased or just the normal sensation.
Thus, the definite method for quantitative measurement of corneal sensitivity is still lacking. Changes in corneal sensation with age is not clear. Hence this study was undertaken for the measurement of corneal sensation, (a) to know the efficiency of the instrument, (b) change in corneal sensation in various age groups, and (c) change in corneal sensation in different quadrants of the same cornea.
| Materials and Methods|| |
In this study, the sensitivity is measured in different areas of the cornea by an instrument named as 'ASTHESIOMETER' manufactured by us.
Study was undertaken on 188 eyes of various age groups who attended the eye out patient department of R.N.T. Medical College & Hospital, Udaipur, for correction of their refractive errors.
This instrument constructed by us works exactly on the principles of Chochet and Bonnets' commercially available asthesiometer. This instrument consists of a brass cylinder which is threaded throughout its length. This cylinder holds a graduated metallic rod the length of which can be varied by forward and backward movement of it. This metallic rod has a fine bore in the centre throughout its length, through which nylon thread of 0.12 mm. is passed. The rod is graduated in millimeters and the marking are at every 5 mm. interval up to 60 mm. starting from free end to the attached end of the rod. The distal cylinder along with the metallic rod is covered by a metallic cap. The cross-sectional area of the nylon thread is 0.0113 mm 2 . Free length of the nylon thread, when metallic rod is totally inside the distal cylinder is 60 mm.
Like Norn  we also feel that there is no difference between the two instruments, regarding their use and result.
Technique Of Measuring The Corneal Sensation
Observer sits in front of the seated patient, who is asked to look at a point behind and above the observer. At first free length of nylon thread is kept at 60 mm. then asthesiometer is approached slowly towards cornea of the patient's eye, which is to be tested. The thread kept at right angle of the anterior corneal surface is pressed delicately against the cornea with just enough force that the thread becomes just visibly curved. At this stage patient is asked to say whether he/she feels the touch sensation or not. If the patient does not feel it, then the length of the thread is reduced by 5 mm. every time and procedure is repeated, till the answer is positive (patient feels the touch sensation). The length of this remaining thread is read on the scale and converted in milligrammes pressure per cross-sectional area of the nylon thread. Standard conversion chart of Chochet and Bonnet was used for this purpose.
After recording the central corneal sensitivity, patient is asked to look up, to record the sensitivity of the lower cornea. Similarly all the quandrants were tested with the eye ball rotated downwards, nasally and also laterally. In addition to the central and peripheral quandrants, sensitivity of mid-peripheral areas of the cornea was also measured. Thus in all in each case not less than 9 quadrants were tested.
| Observations|| |
188 eyes belonging to different age groups were subjected to this study. Results are illustrated in [Table - 2].
It is evident from the above table that sensations are highest in the centre of the cornea (area No. 1). The upper mid-periphery (area No. 2 & 3) is less sensitive than lower mid-periphery (area No. 3 & 4). Upper peripheral zone (area No. 6 & 7) shows lowered sensitivity in comparison to lower periphery (area No. 8 & 9). This table also speaks that as age advances corneal sensations are decreased.
Diagrammatic presentation of the cornea shows average sensitivity as pressure in mgs. per cross-sectional area of the nylon thread in the region concerned.
It is obvious from [Figure - 2] and [Table - 3] that the central cornea is most sensitive and as we approach the periphery the sensitivity decreases. The upper part of the cornea which constantly remains covered by the upper lid is the least sensitive as compared to the uncovered lower peripheral part. Further no difference is found in corneal sensation in inner and outer half.
| Discussion|| |
Cornea is most sensitive part of the eye and its centre is highly sensitive as compared to peripheral areas. This high sensitivity in the centre is probably due to the fact that the corneal nerves which enter at the limbus are more deeply situated in the stroma and as they approach the centre they become more superficial. In addition to this all nerves entering from the periphery are more compactly placed in the centre. Further, central cornea is also occupied by the visual axis of the eye which may add to increased sensitivity. Results of the present study are strongly suggestive of increased sensitivity in the centre. Similar findings have been reported by Dixon, J.M.  But he observed that central cornea felt touch sensation on 10 mgm pressure, which is 1.8 mgm less than that of our findings. This difference may be because of manual error.
Forsious,  has shown that corneal sensitivity is reduced in the presence of arcus senilis, also when it is present in fairly young people. It is very likely that arcus senilis changes are because of low corneal sensation at the periphery or corneal sensation are changed at the periphery due to these degenerative processes. We also know that corneal sensations are reduced to a greater extent as age advances and degenerative changes are also common in senility. Thus, it seems that two observations have close association.
In the study of Norn,  cornea was found to be most sensitive after which followed the lid margin. The caruncle was less sensitive and the conjunctiva the least so. The mean corneal sesitivity was 12 mgm. which is just equivalent to our findings of 11.8 mgm pressure per 0.0113 mm 2 in the centre. Further he noted that sensitivity appreciably declines with increasing age.
Like us, Norn,  also supports the views expressed by Boberg-ANS  , that corneal sensations decrease with advancing age. Norn measured corneal sensations by nylon thread of 0.11 mm. diameter. In his observation average central corneal sensitivity in normal eyes was 15 mgm. or less and limbal sensitivity was 20 mgs.
| Summary and Conclusions|| |
1. Corneal sensitivity of 18.8 eyes has been measured.
2. Instrument (Asthesiometer) used for this study is most sensitive, handy and is recommended for routine clinical use.
3. Cornea is most sensitive in the centre and sensation decreases as we approach the periphery.
4. Corneal sensitivity is reduced in advanced ages.
| References|| |
|1.||Boberg, ANS, J., 1955, Brit. J. Opthal. 39, 705. |
|2.||Chochet, P. and Bonnet, 1961, Bull. Soc. Ophthal., 241. |
|3.||Dixon, J.M., 1964, Amer. J. Ophthal., 58, 424. |
|4.||Forsious, H., 1958, Acta. Ophthal., 48, 789. |
|5.||Norn, M.S., 1973, Acta. Ophthal., 51, 58. |
[Figure - 1], [Figure - 2]
[Table - 1], [Table - 2], [Table - 3]