Steroid Control of Acute Middle Ear Inflammation in the Mouse

Abstract

Objective

The middle ear innate immune response to bacteria leads to acute inflammation consisting of fluid accumulation, infiltration of inflammatory cells, and mucosal thickening. Although inflammation from otitis media generally subsides after 5–7 days, suppression of this response would help alleviate suffering and minimize risk to the inner ear. Glucocorticoids and mineralocorticoids have differential effects on inflammation and fluid absorption, but little is known of their control of middle and inner ear manifestations of acute otitis media. Therefore, steroids were investigated for their potential for therapeutic approaches to control of otitis media.

Design

Both glucocorticoid (prednisolone, dexamethasone) and mineralocorticoid (aldosterone, fludrocortisone) steroids were investigated for their ability to reduce inflammatory symptoms in a mouse otitis media model.

Subjects

Acute inflammation was induced by transtympanic injection of heat killed Streptococcus pneumoniae to 100 Balb/c mice.

Interventions

Twenty mice in each experimental group (prednisolone, dexamethasone, aldosterone, fludrocortisone) were given a steroid in their drinking water the day before inoculation and continued on these treatments until histologic observance. Twenty control mice were treated with water only.

Results

Histologic middle ear morphometrics showed significant steroid effects at both 3 and 5 days in reduction of fluid area, cell number and tympanic membrane thickness.

Conclusions

Glucocorticoids were most effective in controlling inflammation. Interestingly, the mineralocorticoids were also effective in reducing the inflammatory response at 5 days, suggesting their fluid transport function helped clear disease. Thus, steroid control of middle ear disease may be useful in alleviating symptoms faster and reducing risk to the inner ear.

Introduction

Otitis media (OM) is one of the most prevalent inflammatory diseases in the pediatric population¹. The only effective primary treatment for acute otitis media (AOM) is antibiotics. However, a consequence of antibiotic therapy is bacterial death and release of bacterial inflammatory products (lipopolysaccharide, peptidoglycan, and DNA), which can exacerbate and prolong inflammation in the middle ear^2,3,4. Alleviation of symptoms can be accomplished by oral anti-inflammatory agents, oral antibiotics, or the infection can resolve on its own up to 70% of the time⁵. Recent efforts to withhold the use of antibiotics for this disease entity have been put forward to decrease the number of antibiotic prescriptions written every year for this common disorder⁶. Decreasing the use of antibiotics should decrease bacterial resistance to antibiotics, an emerging problem in many parts of the world. Therefore, new strategies are needed for alleviating the pain and hearing loss that accompany AOM.

Inflammation results from the innate immune response to bacteria present in the middle ear. Thus, strategies to decrease fluid and inflammation would be potentially beneficial in the treatment of AOM. The lack of therapeutic options beyond antibiotics or tympanostomy tubes has led to considerable animal research efforts to better understand the mechanisms of AOM and develop new strategies for its prevention or treatment.

Glucocorticoids lessen inflammation by suppressing the immune response, but they also bind to the mineralocorticoid receptor to enhance Na⁺ transport and fluid absorption. Mineralocorticoids, such as aldosterone, only act on fluid homeostasis by increasing production of Na⁺-K⁺-ATPase and the epithelial sodium channel. It has been reported in both in vivo and in vitro studies that the middle ear epithelium is involved in active fluid transport via the epithelial sodium channel^7,8. This fluid homeostasis function of the middle ear epithelium is likely to have a major role in removing fluid that accumulates during inflammation such as seen with acute and chronic OM. If the fluid were controlled, hearing would be restored and pain from inflammation would be alleviated.

The possibility that both glucocorticoids and mineralocorticoids could both impact middle ear inflammation in OM was investigated using a previously developed animal model of AOM⁹. The goal of the study was to differentiate which steroid roles, fluid homeostasis (mineralocorticoids) or immune suppression (glucocorticoids), would experimentally provide the control of middle ear fluid and cellular infiltration in this model of acute OM.

Methods

Balb/c mice (N = 100) were examined with the otomicroscope to establish absence of infection in the external or middle ear. They were then bilaterally inoculated transtympanically with 3.5 μl of heat-killed Streptococcus pneumoniae (10⁹ organisms per ml) and their middle ears examined after 3 days or 5 days of treatment. Control mice (N=20) received no steroid treatment and 10 were killed at each time point. The remaining mice (N=80) were given steroid treatments that began the day before inoculation and continued until sacrifice. Oral steroid treatments were given in the drinking water per our previous protocol¹⁰ and consisted of either prednisolone (5 mg/kg/day), fludrocortisone (10 μg/kg/day), or aldosterone (15 μg/kg/day). Animals drank the medication-containing water ad libitum. Dexamethasone was given as a subcutaneous injection daily (0.75 mg/kg/day). Ten mice were examined at each time point for each treatment.

The animals were euthanized at either the 3 or 5 day treatment endpoint by an overdose of anesthetic (ketamine and xylazine), fixative was intracardially perfused (1.5% glutaraldehyde -3% paraformaldehyde in 0.1M phosphate buffer), and the dissected skulls immersed in fixative overnight. The middle and inner ears were left intact and connected to each other by the skull base so both ears were processed together for histology and sectioning. Tissues were microwave decalcified in EDTA, embedded in glycol methacrylate plastic, sectioned in the horizontal plane at 5 μm, serially mounted on glass slides, stained with basic fuchsin and methylene blue, and coverslipped. One ear from a 5 day aldosterone treated mouse was not available for evaluation, leaving 19 ears in this group for analysis.

Middle ears were assessed at 10x power at a standard middle ear section (level of the stapedial artery) for fluid area, number of inflammatory cells, and TM thickness. These parameters were previously determined to best assess acute inflammation, with significant inflammation first occurring at day three, increasing at day five, and resolving by day seven⁹. Therefore, measurements were made at days 3 and 5 because these post-inoculation times will provide maximum inflammation for demonstration of any potential steroid effects.

The fluid area was measured with a calibrated micrometer grid in the eyepiece and the cells within the fluid area were counted. Tympanic membrane thickness was measured with a micrometer scale within the opposite eyepiece. Three sections were measured and the three individual measures for each middle ear parameter were averaged to derive one mean value for each parameter for each ear. Statistical analyses (ANOVA, SPSSR®) were then performed on data from the treatment groups and controls to determine if steroid treatments suppressed middle ear inflammation.

All animal procedures were approved by OHSU IACUC. Institutional guidelines regarding animal experimentation were followed.

Results

Histopathology

Control (bacteria only) ears often had significant inflammation of the middle ear at both days 3 and 5 on histologic analysis. Features of inflammation commonly seen were accumulation of inflammatory exudate, hypertrophy of the middle ear mucosal epithelium, and thickening of the round window membrane and TM (Fig. 1A, B, C). Extensive middle ear fluid and inflammatory cells were present as well. The inflammatory cells were primarily neutrophils (acute phase inflammatory cells). Often fluid filled the entire middle ear space. By contrast, steroid-treated ears had less inflammation (Fig. 1D). Often these middle ears were clear of fluid and inflammatory cells and the mucosal epithelium was seldom hypertrophied. Many ears looked normal.

Figure 1.

Impact of bacterial inoculation on middle ear inflammation A: Middle ear (ME) inflammatory changes seen at3 days (10x) in a control mouse. There is considerable inflammation in the middle ear (ME) space, essentially filling the cavity with fluid and inflammatory cells. The fluid opposes the round window membrane (RW), although the cochlea (C) is clear. B: ME mucosal epithelium thickening and hypertrophy seen at higher magnification (20x). C: Appearance of similar control ear section taken at 5 days (20x). Again note the hypertrophy of the middle ear mucosal epithelium, and inflammatory infiltrate nearly filling the middle ear space. D: Prednisolone-treated ear 5 days (20x) is completely clear of the typical inflammatory changes seen in untreated mice.

Stains: basic fuchsin/methylene blue.

ME = middle ear; C = cochlea; A = stapedial artery, RW = round window membrane.

Assessment was made of the number of ears with any degree of inflammation, such as fluid or inflammatory cells. Occasionally an ear will not manifest an inflammatory response in spite of bacterial inoculation. This could be due to the inoculate draining before an innate immune response is initiated, a particular animal being more resistant to bacterial stimulation, or slower inflammation development. Therefore, the total ears showing inflammation at the two ages was evaluated to determine if the steroid treatments altered the normal progression of pathology. Control mice showed 4 middle ears completely free of inflammation out of the 40 inoculated (Table 1). In comparison, glucocorticoid treated ears showed a significantly higher incidence of inflammation-free ears. Prednisolone-treated mice had 10 middle ears without symptoms, while 11 dexamethasone-treated ears were clear. Chi-square analyses showed both of these were statistically better than controls (Table 1). On the other hand, mineralocorticoid treated ears were no different than bacteria only controls (p > 0.05), suggesting these steroids did not reduce the overall incidence of inflammation.

Table 1.

Chi-Square of inflammation free ears at 3 and 5 days

Treatment	# Ears		X2 Value	Probability
	Clear	Inflamed
Control	4	36
Prednisolone	10	30	10.00	0.0016
Dexamethasone	11	29	13.61	0.0002
Aldosterone	4	35	0.028	0.868
Fludrocortisone	5	35	0.278	0.598

X² compared to controls

Inflammation Measures

To determine if there were quantitative differences in the extent of inflammation, ANOVA was conducted on the measures of fluid area, inflammatory cells, and TM thickness. Control mice without steroid treatment showed the normal progression of inflammation measures that paralleled the qualitative changes above. Fluid averaged 1,902 μm² at 3 days, increasing to 3,158 μm² by day 5, while the number of inflammatory cells within the fluid increased proportionately from 106.5 to 167.9 (Fig. 2). The steroid treatments served to reduce this inflammation. The ANOVA showed significant overall group differences with regard to steroid effects at both 3 and 5 days (Fig. 2). This significant steroid effect was an overall reduction in all histologic measures at both 3 and 5 days with the exception of fluid area at day 3 (Fig. 2).

Figure 2.

Morphometric results. ANOVA (F and p values) shown for overall treatment effects compared to controls. Post-hoc analysis indicated by asterisk over bars. * = p < 0.05. pred = prednisolone; dex = dexamethasone; aldo = aldosterone; fludro = fludrocortisone.

Post-hoc comparisons showed that the steroids had an impact as early as day 3 on number of cells and TM thickness (Fig. 2). Prednisolone and dexamethasone both appeared to reduce cell number, but the post-hoc test showed statistical significance only between the prednisolone and fludrocortisone results. The thickness of the TM is measured away from the injection site to reflect changes due to the bacterial stimulation and not due to needle trauma. Both glucocorticoids were significantly different from control TM thickness at day 3 (Fig. 2). The reduction in TM thickness measured in the mineralocorticoid treated mice was not different from controls, in spite of their averages appearing less.

By day 5, all of the steroid treatments had some impact when compared to controls (Fig. 2). Overall treatment effects were seen with regard to fluid area, with all steroid averages being reduced. Post-hoc tests showed all steroid results were not different from each other, however the dexamethasone average was statistically less than control mice. The number of inflammatory cells was significantly reduced in all steroid groups compared to controls, generally showing only a third of the cells seen in control mice. The TM healing that occurs between days 3 and 5 is reflected by the reduction in TM thickness during this period. There was an overall group effect for the steroids and they did not differ from each other at day 5. However, the dexamethasone mice were statistically different from controls.

Thus, the steroids were generally similar to each other in their reduction of middle ear inflammation during the observation period. Post-hoc comparisons generally showed one or both of the glucocorticoids were better at reducing inflammation than the mineralocorticoids. However, all steroids were effective in reducing inflammatory cells by day 5, suggesting that some aspects of inflammation were sensitive to the two steroid groups.

Discussion

While systemic steroids have been used extensively for ear diseases such as sudden sensorineural hearing loss, and in earlier trials for treatment of otitis media with effusion (OME)^11,12,13, animal research on the use of various steroids for OM has not been conducted. Specifically, investigation of the use of steroids to clear the middle ear fluid that accompanies OM has not been explored. Middle ear fluid present during OM causes conductive hearing loss by virtue of decreasing transmission of sound through the middle ear. Therefore, earlier resolution of this fluid would alleviate troubling sequelae of OM such as pain and decreased hearing. Currently, no good immediate therapy exists to clear fluid with the exception of myringotomy or tympanostomy tube placement. While clearance of middle ear fluid has long been ascribed to the function of the Eustachian tube, there is now evidence that the middle ear epithelium plays a major role in fluid absorption, while the role of the Eustachian tube is more to allow gas pressure equilibration⁷. Other laboratories have reported that fluid regulation in the middle ear is via the Na⁺ channel dependant process in the middle ear mucosal epithelium. The use of mineralocorticoids offers some appeal as use of such an agent would avoid the systemic morbid sequelae of glucocorticoids.

The glucocorticoids, prednisolone and dexamethasone, were the most effective in our study in reducing middle ear inflammation in response to bacterial challenge to the middle ear at both 3 and 5 days. Presumably, this is due to the strong immunosuppressive and anti-inflammatory action of these steroids. Interestingly, prednisolone also has a modest mineralocorticoid effect, one-fifth that of the glucocorticoid effect. One could speculate that the mineralocorticoid action of prednisolone could also be acting in a positive way to decrease middle ear fluid. Aldosterone, is basically a pure mineralocorticoid (10,000:1 mineralocorticoid: glucocorticoid action), and fludrocortisone has a much stronger mineralocorticoid than glucocorticoid effect (12.5:1 mineralocorticoid: glucocorticoid action)¹⁴. Interestingly, the mineralocorticoids were also found to reduce middle ear inflammatory response to bacterial injections, but only at the 5 day endpoint. Thus, while the glucocorticoid agents were effective in decreasing fluid and cell count in the middle ear, the mineralocorticoids were also moderately effective at the low therapeutic does employed here. This lends support to the concept that sodium transport of middle ear fluid may be an important function in clearing fluid and thus in controlling inflammation in the middle ear. Also, the mineralocorticoids may have been more effective at the 5 day endpoint since their actions (clearance of fluid) would be expected to take longer than the stronger pure immune suppressive glucocorticoids. While we did not see significant effect on fluid at the three day endpoint for the mineralocorticoids compared to the glucocorticoids, nor an overall effect, both steroid groups were equally effective at day 5. At the five day endpoint, dexamethasone was the only one significantly better at decreasing fluid compared to the other steroids, but overall effect was essentially equal in the remaining steroid groups. This would indicate that fluid was impacted by both types of steroids and equally well by the mineralocorticoids.

The immune-suppressive actions of glucocorticoids were most effective in reducing the severity of the middle ear innate immune response. Nevertheless, mineralocorticoids were also moderately effective in reducing the inflammatory response at 5 days. These studies offer insight into the potential steroid control of middle and inner ear disease during acute otitis media. Thus, steroid control of middle ear disease may be useful in alleviating symptoms faster and reducing risk to the inner ear. Further investigations are ongoing into the relative efficacy of steroids and antibiotics for suppression of inflammatory response to OM in the mouse model.