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Soothing solutions: Acidity
Richard M. Hill, OD, PhD, contributing editor
The Eye und the Contact Lens
Soothing Solutions: Acidity
Solutions intended to soothe constitute a substantial part of the ophthalmic drug market. Yet, in the absence of a knowledgeable practitioner, the patient appears to depend primarily on descriptors such as redness relieving, lubricating, wetting, rewetting, irritae tion reducing, etc., for guidance in their needs. ’ Success then seems more often based on a serendipitous pairing of patient and product than upon the careful consideration of constituents involved. Still, soothing solutions are a widely popular and growing class of product and do reveal in their own way how varied the needs and thresholds of patients can be.
Two were found below pH 6.8, the commonly cited minimum threshold for comfort, but are serving many patients well today.4 This reconfirms that traditional “thresholds” can be taken as general references at best, but cannot be applied to any particular patient with certainty.
the direction and magnitude of pH change for these solutions are illustrated in Figure 2. In every case, the shift was toward greater acidity, the units indicated being those associated with the 24 months following the first opening.
Old Solution Outlooks Slipping Toward Acidity The acidity of a solution
is rarely stable, however, and changes may well be accelerated after opening.5 Both
An “old” solution might be best defined as one whose chemical and physical properties have shifted sufficiently to alter either safety or efficacy of ac-
[s00~~lNG soLmor4s
I
Acidic Drift Over 24 Months
I I I
Indulging Intuition If the label were more revealing, what property would you pick to “assure” a soothing result? For many, relative acidity might be high on the list of intuitive concerns. Based on the favorable experiences of early fluid lens fitters using bicarbonate of soda as a means of extending wearing time (minimizing blur and discomfort), the prescriber might reason ably assume that soothing solutions would all be alkaline. As Figure J from the recent studies of Carney et al.’ and Barr and colleagues3 shows, that assumption would prove only partially true. Among the eight highly successful, soothing RGP compatible solutions illustrated here, six were above pH 7.0.
0 1991 Butterworth-Heinemann
I
I
(4
(W ((7 PI
(E)
F) I
(G)
I
(HI
6.0
5.0 “Risk Zone”
j 7.0
PH
8.0 “Comfofl Zone”
Figure 1. The pH values found on first opening the containers of eight ophthalmic solutions designed to improve comfort.
ICE,
Vol.
18, March/April 1991
75
The Eye and the Contact Lens
[SOOTHINGSOLUTIONS 1j Acidic Drift Over 24 Months
-++-
/
-j-
j
+-(F)
(4
/
+-
tB)
(C)
j
References
I ,
6.0
5.0 ‘Risk Zone”
/
PH
7.0
8.0 “Comforf Zone”
Figure 2. The ranges of pH shift found for the eight solutions in Figure 1, based on follow up measurements made 24 months later. tion. Since with slow or intermittent use this status may develop well before the expiration date, discomfort from “acid bite” could be an important
76
ICLC, Vol. 18, March/April
1991
It must be realized that many other shifts (salinity, buffering capacity, wetting angle, and sterile capacity) could and probably are underway as well. Certainly the continued use of any “old” solution represents a peril far beyond the economic value of those few remaining .milliliters. Yet this temptation remains one of the most fundamental but difficult problems to manage in patient education.
warning to the average or sensitive patient. Conversely, it may be the more acid tolerant patient who is at increased risk of missing this signal.
1. Physicians’ Desk Reference fur Nonprestiption Drugs, 10th Ed. Oradell, NJ, Medical Economics Company, Inc., 1989. 2. Camey LG, Barr JT, Jochum KL, Hill RM: Contact lens lubricants, Part 1. Contact Lens Spectrum 1988;3:24-26. 3. Barr JT, Camey LG, Jochum KL, Hill RM: Contact lens lubricants, Part 2. Contact Lens Spectrum 1988;3:44-46. 4. Moses RA: Adler’s Physiology of the Eye; 7th Ed. St. Louis, C.V. Mosby, 1981. 5. Camey LG, Hill RM, Barr JT, Habenicht BL: The “aging” of contact lens solutions: 1. Lubricants. Contact Lens Spectrum (in press).
The Eye und the Contact Lens
Soothing Solutions: Acidity
Solutions intended to soothe constitute a substantial part of the ophthalmic drug market. Yet, in the absence of a knowledgeable practitioner, the patient appears to depend primarily on descriptors such as redness relieving, lubricating, wetting, rewetting, irritae tion reducing, etc., for guidance in their needs. ’ Success then seems more often based on a serendipitous pairing of patient and product than upon the careful consideration of constituents involved. Still, soothing solutions are a widely popular and growing class of product and do reveal in their own way how varied the needs and thresholds of patients can be.
Two were found below pH 6.8, the commonly cited minimum threshold for comfort, but are serving many patients well today.4 This reconfirms that traditional “thresholds” can be taken as general references at best, but cannot be applied to any particular patient with certainty.
the direction and magnitude of pH change for these solutions are illustrated in Figure 2. In every case, the shift was toward greater acidity, the units indicated being those associated with the 24 months following the first opening.
Old Solution Outlooks Slipping Toward Acidity The acidity of a solution
is rarely stable, however, and changes may well be accelerated after opening.5 Both
An “old” solution might be best defined as one whose chemical and physical properties have shifted sufficiently to alter either safety or efficacy of ac-
[s00~~lNG soLmor4s
I
Acidic Drift Over 24 Months
I I I
Indulging Intuition If the label were more revealing, what property would you pick to “assure” a soothing result? For many, relative acidity might be high on the list of intuitive concerns. Based on the favorable experiences of early fluid lens fitters using bicarbonate of soda as a means of extending wearing time (minimizing blur and discomfort), the prescriber might reason ably assume that soothing solutions would all be alkaline. As Figure J from the recent studies of Carney et al.’ and Barr and colleagues3 shows, that assumption would prove only partially true. Among the eight highly successful, soothing RGP compatible solutions illustrated here, six were above pH 7.0.
0 1991 Butterworth-Heinemann
I
I
(4
(W ((7 PI
(E)
F) I
(G)
I
(HI
6.0
5.0 “Risk Zone”
j 7.0
PH
8.0 “Comfofl Zone”
Figure 1. The pH values found on first opening the containers of eight ophthalmic solutions designed to improve comfort.
ICE,
Vol.
18, March/April 1991
75
The Eye and the Contact Lens
[SOOTHINGSOLUTIONS 1j Acidic Drift Over 24 Months
-++-
/
-j-
j
+-(F)
(4
/
+-
tB)
(C)
j
References
I ,
6.0
5.0 ‘Risk Zone”
/
PH
7.0
8.0 “Comforf Zone”
Figure 2. The ranges of pH shift found for the eight solutions in Figure 1, based on follow up measurements made 24 months later. tion. Since with slow or intermittent use this status may develop well before the expiration date, discomfort from “acid bite” could be an important
76
ICLC, Vol. 18, March/April
1991
It must be realized that many other shifts (salinity, buffering capacity, wetting angle, and sterile capacity) could and probably are underway as well. Certainly the continued use of any “old” solution represents a peril far beyond the economic value of those few remaining .milliliters. Yet this temptation remains one of the most fundamental but difficult problems to manage in patient education.
warning to the average or sensitive patient. Conversely, it may be the more acid tolerant patient who is at increased risk of missing this signal.
1. Physicians’ Desk Reference fur Nonprestiption Drugs, 10th Ed. Oradell, NJ, Medical Economics Company, Inc., 1989. 2. Camey LG, Barr JT, Jochum KL, Hill RM: Contact lens lubricants, Part 1. Contact Lens Spectrum 1988;3:24-26. 3. Barr JT, Camey LG, Jochum KL, Hill RM: Contact lens lubricants, Part 2. Contact Lens Spectrum 1988;3:44-46. 4. Moses RA: Adler’s Physiology of the Eye; 7th Ed. St. Louis, C.V. Mosby, 1981. 5. Camey LG, Hill RM, Barr JT, Habenicht BL: The “aging” of contact lens solutions: 1. Lubricants. Contact Lens Spectrum (in press).