Methodological aspects of tear flow determination by means of a radioactive tracer

Mathematical Models of Ocular Drug Delivery

Drug delivery is an important factor for the success of ocular drug treatment. However, several physical, biochemical, and flow-related barriers limit drug exposure of anterior and posterior ocular target tissues during drug treatment via topical, subconjunctival, intravitreal, or systemic routes. Mathematical models encompass various barriers so that their joint influence on pharmacokinetics (PKs) can be simulated in an integrated fashion. The models are useful in predicting PKs and even pharmacodynamics (PDs) of administered drugs thereby fostering development of new drug molecules and drug delivery systems. Furthermore, the models are potentially useful in interspecies translation and probing of disease effects on PKs. In this review article, we introduce current modeling methods (noncompartmental analyses, compartmental and physiologically based PK models, and finite element models) in ocular PKs and related drug delivery. The roles of top-down models and bottom-up simulations are discussed. Furthermore, we present some future challenges, such as modeling of intra-tissue distribution, prediction of drug responses, quantitative systems pharmacology, and possibilities of artificial intelligence.

Spontaneous eyeblink activity

Spontaneous blinking is essential for maintaining a healthy ocular surface and clarity of vision. The spontaneous blink rate (SBR) is believed to reflect a complex interaction between peripheral influences mediated by the eye surface and the central dopaminergic activity. The SBR is thus extremely variable and dependent on a variety of psychological and medical conditions. Many different methods have been employed to measure the SBR and the upper eyelid kinematics during a blink movement. Each has its own merits and drawbacks, and the choice of a specific method should be tailored to the specific needs of the investigation. Although the sequence of muscle events that leads to a blink has been fully described, knowledge about the neural control of spontaneous blinking activity is not complete. The tear film is dynamically modified between blinks, and abnormalities of the blink rate have an obvious influence on the ocular surface.

Effect of blinking on tear volume after instillation of midviscosity artificial tears

PURPOSE

To determine by optical coherence tomography (OCT) the effect of blinking on ocular surface tear volume after instillation of artificial tears.

DESIGN

Experimental study.

METHODS

Normal human eyes (n = 21) were imaged to obtain dimensions of the tear film and menisci during blinking. The imaging was carried out immediately and five minutes after the instillation of 35 microl midviscosity artificial tears (1.0% carboxymethylcellulose; Refresh Liquigel; Allergan, Irvine, California, USA). The exposed ocular surface area and the lid lengths were used to calculate the volumes.

RESULTS

Immediately after the instillation, total tear volume was increased (P < .001) compared with five minutes after the instillation, with the major increases in the lower tear meniscus volume and tear film volume. After the instillation, blinking caused tear loss in total tear volume because of the decrease of the lower tear meniscus volume (P < .05). In contrast, blinking increased the tear film volume (P < .05). At the end of the eye-opening period, tear film volume decreased and lower tear meniscus volume increased significantly (P < .05), with no significant changes in total tear volume (P > .05). During the blink cycle immediately after the instillation, net loss was evident in tear film volume, lower tear meniscus volume, and total tear volume (P < .05).

CONCLUSIONS

Blinking plays a crucial role in distribution and removal of instilled tears. When the tear system is overloaded, the increase in blink output helps restore balance.

Quantitative assessment of dacryoscintigraphic images in the evaluation of epiphora

BACKGROUND

At the present time, only visual analysis is implemented on dacryoscintigraphic images and quantitative assessments are not routinely obtained. The authors have designed a novel, simple, quantitative method for the diagnosis of sac and post-sac obstructions based on the dacryoscintigraphic images.

METHOD

Fifty-nine patients with severe unilateral epiphora were included and the contralateral asymptomatic eyes were used as controls. After ocular instillation of 0.1 cc isotonic saline containing 4 MBq 99mTcO4, dynamic images were obtained for 20 minutes. Visual interpretation was done by two blinded specialists in nuclear medicine. A Time-Activity Curve (TAC) was generated for each eye and its pattern was divided to plateau type, shallow-down sloping and deep-down sloping. The excretion ratio (expressed as the percentage of drained activity) in the 2nd, 3rd, 5th, 7th, 10th, 15th and 20th minute of the study was calculated based on the following formula: (the total count in the region of interest (ROI) in the 1st minute - the total ROI count in the 2nd, 3rd, 5th, 7th, 10th, 15th or 20th minute/the total ROI count in the 1st minute) x 100. The results of the quantitative analysis were compared with the clinical symptoms, visual analysis and TAC pattern.

RESULTS

At the optimal cut-off points for the 2nd, 3rd, 5th and 7th minute, the sensitivity of the prediction of obstruction was 76%, 72%, 71% and 69%, respectively. The correlation between visual interpretation and the TAC pattern was statistically significant.

CONCLUSION

The quantitative evaluation of dacryoscintigraphic images would contribute greatly to achieving an easier and more objective interpretation of the scintigraphic results and also provides a reliable tool for inter-individual comparison and the follow-up of patients. As it is the first time that this innovative method has been examined clinically, it will be necessary to investigate its clinical utility in a larger series of patients.

Clinical value of dacryoscintigraphy using a simplified analysis

PURPOSE

To study the discriminatory ability of dacryoscintigraphy in differentiating between patients with epiphora and volunteers using a simple method.

METHODS

Twenty eyes in ten volunteers and 66 eyes in 55 patients with severe epiphora were studied. Dacryoscintigraphy (15 frames of 1 min) was performed after administration of 4 MBq (99m)Tc-pertechnetate (10 mul) in both eyes. By mapping a single region of interest (ROI) over the conjunctival sac we determined T1 (%dose in first minute) and linear clearance rate (LCR, defined as [see text] from the tracer disappearance curve. Reproducibility was determined in volunteers. Conjunctival resorption was determined from completely obstructed systems. Sensitivity and specificity were calculated and receiver operating characteristics (ROC) analysis was conducted.

RESULTS

In volunteers mean T1 was 52.8+/-11.9% (95% CI 47.1-58.4%), and LCR was 74.2+/-11.1% (95% CI 69.0-79.4%). Reproducibility was good (mean difference 4.1+/-13.3% for T1 and 0.7+/-17% for LCR). Epiphora patients had clearly higher T1 (82.1+/-15.2%, P<0.0001) and lower LCR (38.9+/-22.5%, P<0.0001) values. Tracer resorption was 24%. Based on ROC analysis 70% for T1 and 50% for LCR were considered optimal cut-off levels to separate patients from volunteers. Sensitivity/specificity were 77/95% for T1 and 71/100% for LCR. T1 and LCR values did not correlate with symptom scores or Anel test results.

CONCLUSION

Dacryoscintigraphy, using LCR and T1 as parameters, is a reliable and objective method to detect tear-flow abnormalities.

99mTc-diflunisal and the human iris: topical application reveals localization

Following the instillation of a drug into the eye, drainage mechanisms will commence at once. In this report, an attempt was made to assess the dynamics of an instilled nonsteroidal anti-inflammatory drug (NSAID), diflunisal, labeled with 1 MBq 99mTc followed by twenty minutes of scintigraphy. The results obtained with this labeled drug were compared with instillation of the same volume and activity of 99mTcO4-. Although the pertechnetate anion is an excellent and innocuous indicator for detecting the external lacrimal drainage system of the eye, it cannot visualize the internal structures. A clear scintigraphic difference was noted between labeled diflunisal and the pertechnetate anion. Scintigraphic activity surrounding the pupil of the eye provides evidence of visualization of the iris/ciliary body. This seems reasonable as the cyclooxygenase enzyme is located in this structure, and NSAIDs exert their mechanism of action via this complex. With this technology, visualization of some internal structures of the eye may be facilitated.

Quantitative study of the effect of dacryocystorhinostomy on lacrimal drainage

Using scintigraphy, we have studied the lacrimal drainage from the conjunctival sac of normal subjects and patients who have undergone dacryocystorhinostomy. A mathematical model of drainage was constructed that accurately described the complete activity curves for both groups. The initial rate of drainage was shown to be a relevant parameter to characterize drainage, and it was found to be different from normals to patients, indicating that the lacrimal pump mechanism is affected by dacryocystorhinostomy.

In vivo evaluation of dosage forms: application of gamma scintigraphy to non-enteral routes of administration

The trend to deliver drugs to defined areas of the body involves sophisticated carriers systems. In addition to the in vitro drug release profile one must be aware of the in vivo behaviour of the dosage form and the drug. Gamma scintigraphy is an elegant way to gain insights of the actual in vivo distribution pattern of dosage forms. This technique relies on the use of radioactive tracers included into the medicament and selected so as to enable an optimum detection by a gamma ray camera. The choice of a convenient label enables the in vivo determination of the targeting of the formulation administered through a large number of routes. The present paper reviews applications of gamma scintigraphy for the evaluation of dosage forms administered by the parenteral, rectal, buccal, nasal, pulmonary, and ophthalmic routes.

Scintigraphic assessment of an ophthalmic gelling vehicle in man and rabbit

Studies of the rate of clearance of a gellan gum formulation (Gelrite) radiolabelled by the inclusion of technetium-99m labelled diethylenetriaminepentaacetic acid were carried out in volunteer subjects and in rabbits. Disposition was followed by gamma scintigraphy and compared with 0.5% w/v hydroxyethylcellulose (HEC) solution and isotonic saline administered to the same subjects. Clearance of all solutions was found to follow bi-exponential kinetics with differences in clearance rates between the two species studied. A significant retention of the gellan gum formulation compared to HEC (p = 0.006) or saline (p = 0.009) was noted in man, but not in the rabbit. In this latter species the HEC showed greater retention compared to Gelrite. The species-specific differences in the precorneal residence of the formulations are attributed to the different physiological responses following instillation of solutions into the eye.

Concentration-dependent precorneal loss of pilocarpine in rabbit eyes

Precorneal loss of pilocarpine was studied in pigmented and albino rabbits. We instilled 25 microliter of isotonic pilocarpine solution, pH 6.4, into rabbit eyes and monitored drug concentration in the precorneal tear film. Increased concentration (0.2% - 2.0%) of pilocarpine accelerated precorneal drug loss from the tear film from 0.686 min-1 to 1.064 min-1. This increase was mainly due to induced lacrimation. Polyvinyl alcohol (1.4%) retarded precorneal loss of pilocarpine. The rate of loss was the same in pigmented and albino rabbits. The effects of the changed precorneal loss of pilocarpine on corneal drug absorption are discussed.

Lacrimal dacryoscintigraphy

Lacrimal dacryoscintigraphy facilitates definite diagnosis of obstructions and stenosis of the lacrimal drainage system with little stress to the patient. Together with x-ray dacryocystography it is an important diagnostic tool especially for pre- and postoperative evaluation of the drainage apparatus. Dacryoscintigraphy is the best method for measuring the dynamics of tear drainage especially in the canaliculi. Although it is not a substitute for other methods in general use, it complements them and expands their diagnostic accuracy.

Lacrimal scintigraphy. III. Physiological aspects of lacrimal drainage

Lacrimal scintigraphy (LS) was performed on asymptomatic lacrimal drainage systems and the results were evaluated to understand the physiology of lacrimal drainage. It was found that a physiological obstruction can exist at the level of the nasolacrimal duct in normal asymptomatic individuals, and it is suggested that this obstruction is due to the resistance offered by the valve of Hasner, which in turn is dependent on (a) the volume of fluid in the lacrimal sac and the nasolacrimal duct, and (b) the anatomical integrity of the valve. The LS observations are taken into account to postulate a mechanism of drainage from the lacrimal sac into the nose, which hitherto has not been very clear. It is also suggested that reflux can occur between the various compartments of the lacrimal drainage system and that the various valves in the membranous passageway can become incompetent in an obstructed system.

Lacrimal scintigraphy. I. Compartmental analysis of data

Linear compartmental analysis was applied to lacrimal scintigraphy data from asymptomatic and obstructed lacrimal systems. Failure to achieve satisfactory results in most of the asymptomatic systems studied suggests that variable tear flow is a feature of normal lacrimal drainage which precludes linear analysis. However, in cases where the analysis was successful the results were consistent with the occurrence of reflux between the conjunctival and lacrimal sacs, and between the lacrimal sac and nasolacrimal duct. Further support for the existence of reflux between compartments was provided by the analysis of obstructed systems, particularly those with obstruction at the upper end of the nasolacrimal duct. In such cases there was rapid equilibrium of the tracer between the conjunctival and lacrimal sacs, which in the absence of tear flow could be explained by reflux between these 2 compartments.

Effect of stimulus on the chemical composition of human tears

We examined the effect of the nature of the stimulus on the chemical composition of human tears. The protein concentration of emotional tears from women exceeded that of irritant-induced tears by 24% (P less than .01). Polyacrylamide disk gel electrophoresis disclosed no significant difference between the distribution of positively or negatively charged proteins of irritant-induced and emotional tears. Manganese concentrations in tears (30 ng/ml) exceeded serum concentrations from the same subjects by 30-fold. These manganese concentrations in tears were considerably less than previously reported values. We found no differences for the concentrations of protein or manganese in human tears between the sexes.

A human and in vitro study on the exchange of water and solutes from soft contact lenses

Elimination of technetium (pertechnetate in normal saline solution) was studied from various types of contact lenses placed on normal human eyes by means of computer assisted gamma camera using "region of interest technique" with the designated area corresponding to the conjunctival sac. An elimination 4 times slower was found from a highly hydrophilic lens (Scanlens) than from a HEMA-lens (Softlens), 0,5% min and 2.0% min, respectively. From an ultrathin lens (U3-Softlens) was eliminated 2.4% min. Hard lenses did not absorb the isotope. In a laboratory study the lenses were pre-soaked in pertechnetate, blotted and washed at 2 min intervals in 0.5 ml saline. By this procedure 3% min of the technetium was eliminated from Scanlens, 16% min from Softlens and 28%/min from a thin, therapeutic lens (Plano-T). The ratio Softlens/Scanlens was in the human study 4.0 and in the laboratory study 4.9. Radioactivity was very rapidly eliminated from CAB-lenses. A similar study was carried out with radioactive water. More than 100% min was eliminated in the first 10 min followed by a slower elimination from 10-20 min. Then an increased elimination was seen for a few min. This increased elimination was in repeated studies constantly found after 20 min. It was not found in the studies with technetium and labelled leucine.

Lacrimal pathology evaluated by dynamic lacrimal scintigraphy

This study of pathological lacrimal systems demonstrated the usefulness and sensitivity of dynamic lacrimal scintigraphy (dynamic use of computer assisted gamma camera). The method was very sensitive; even small lacrimal obstructions caused a distinct change of the outflow curves. This technique complements other tests in lacrimal assessment especially in patients with epiphora and normal conventional tests.

Tear flow and soft contact lenses

Tear flow was determined in 14 persons before and after one month of adaptation to a soft contact lens, (Soflens, Bausch & Lomb), by means of a radioactive tracer (technetium, Tc99m, as pertechnetate), a gamma camera and a computer system. The elimination of the radioisotope from the conjunctival sac was diphasic with a significant increase of the fractional turnover rate after the adaptation period in the initial phase with a rapid elimination, but no change in the basal phase with a slower elimination. The fractional turnover rate was also determined with the soft contact lens placed on the cornea after having been pre-soaked in the technetium solution resulting in a value of 0.021 +/- 0.003 min-1 (mean +/- SEM, n = 12). The fractional turnover rate resulting from instillation of the radioisotope on the non-pre-soaked soft contact lens placed in the eye was found to 0.020 +/- 0.004 min-1 (mean +/- SEM, n = 6). In the studies with the soft contact lens in the eye the elimination curves was monophasic and not diphasic as in normal tear flow studies.

Tear flow in normal human eyes. Determination by means of radioisotope and gamma camera

Tear flow was measured in normal human eyes by means of a radioisotope (technetium as pertechnetate in a normal saline solution), a gamma camera and a computer. By "region of interest" technique the elimination was shown to have two phases: an initial rapid elimination followed by a slower elimination after 5--7 min. The mean fractional turnover rate in the initial phase was 0.197 min.-1 (n = 35, SEM = 0.013) and in the basal phase 0.083 min.-1 (n = 35, SEM = 0.003). Assuming a constant tear volume these values corresponded to a tear flow of 1.4 microliter min.-1 and 0.6 microliter min.-1, respectively. There was no significant difference between the fractional turnover rate in the upright and supine position. No difference was found in tear flow between males and females. With the eyes closed the fractional turnover rate was low with intermittently rapid outflow of tears. Irritation to the contralateral eye with a filterpaper caused a stimulated tear flow of 4.4 microliter min.-1. A nomogram facilitating background corrections was constructed.

Improved radiological technique for evaluating the lacrimal pathways with special emphasis on functional disorders

A radiological study of the lacrimal drainage apparatus was made in 50 patients with temporary or permanent epiphora. The authors state that the present radiologic technique (macrography, seriography, subtraction) allows the detection on the various lacrimal levels of not only organic lesions but also of functional disorders. They assert the definite superiority of dacryocystography over the other clinical and radiological investigations (scintigraphy, roentgencinematography) in identifying the site and the nature of the various organic lesions, in evaluating and interpreting the functional alterations as well as following and judging the effectiveness of treatments.

Conjunctival transport of technetium-99m pertechnetate

Using a gamma camera coupled to a computer and a tape unit the transport of pertechnetate (Tc99m) across the conjunctiva was determined by the 'region of interest' technique in human subjects. In 11 patients with their lacrimal sacs removed a fractional turnover rate of 0.021 min-1 was found. In 21 patients with inflammed conjunctiva due to chronic dacryocystitis a fractional turnover rate of 0.027 min-1 was found, whereas the value in 11 normal individuals was 0.015 min-1. Neglecting the relatively small transcorneal route of disappearance, the values found for technetium disappearance could be regarded as representative for the transconjunctival transport of the radioisotope. This assumption was confirmed by correlating the radioactivity in the blood with the fractional turnover rate.