Wound healing in spontaneous perforation or myringotomy and middle ear reconstruction.

Concerns have been raised about the potential for ototopical agents to adversely affect middle ear or tympanic wound healing. In general, wound healing takes place in three phases--inflammation, proliferation, and maturation (1):

* The inflammatory phase begins at the moment of injury with an immediate vasoconstriction response. Approximately 10 minutes later, vasodilation begins, and it lasts for up to 3 days. Within minutes of the onset of vasodilation, a cellular response occurs that persists for up to 1 week. The cellular response overlaps with the onset of the proliferative phase.

* The first process in the proliferative phase is re-epithelialization, and it begins on the day of injury. During the next few days, fibroplasia, collagen synthesis, wound contraction, and neovascularization occur. These processes go on for more than 3 weeks.

* Between 2 and 3 weeks following the injury, the maturation phase begins, and it continues for approximately 1 year.

Our knowledge of wound healing is rapidly expanding. Healing of the tympanic membrane is similar to that of other tissues. (2) This article contains a description of the general processes involved in wound healing. The specific mechanisms of healing vary among different species, different organs, and patients of different ages, but the basic principles can be applied to tympanic membrane healing and repair.

Inflammatory phase

Vascular response. Following local vasoconstriction, the coagulation cascade begins, starting with platelet adhesion. (3-6) Platelets contain multiple chemotactic and vasoactive factors that stimulate growth and healing (table). (3) The primary factors that are involved in the healing of tympanic membrane perforations appear to be platelet-derived growth factor, epidermal growth factor, fibroblast growth factor, and transforming growth factor.

Table. Chemotactic and vasoactive factors that
stimulate growth and healing (3)

Human growth hormone
Epidermal growth factor
Platelet-derived growth factor
Fibroblast growth factor
Transforming growth factor
Nerve growth factor
Brain-derived neurotrophic factor
Ciliary neurotrophic factor
Insulin-like growth factor 1
Tumor necrosis factor
Interleukins
Interferons
Keratinocyte growth factor

Transforming growth factor has been identified as a contributor to pathologic scarring. It is upregulated in subglottic stenosis, hypertrophic scars, keloids, pulmonary fibrosis, and tympanic membrane perforations. Once bleeding has stopped, histamine release dramatically increases blood flow to the wound bed by vasodilation and elevation of vascular permeability of the wounded tissue. Transforming growth factor, along with other chemotaxins, recruits key effector cells and the proteins they express and induces them to migrate to the edge of the wound.

Cellular response. Granulation tissue serves as a provisional matrix scaffold across which epithelialization occurs. Granulation is initiated by fibronectin followed by the migration of white blood cells and fibroblasts. (7-9) Fibronectin then overtakes the clot, leading to fibroblast proliferation, which in turn produces collagen. This well-orchestrated response occurs along the front and directly behind the leading edge of the injury. In most organs, this proliferation occurs two or three cells behind the leading edge, bringing the edges together. This process changes when the edges come into approximation.

When wound healing occurs in an infected area, granulocytes migrate to the wound, and the regular and orderly process of the healing cascade is significantly altered. (10,11) The macrophages are the main cells of phagocytosis and tissue debridement. Macrophages release chemotactic and growth factors, including transforming growth factor. (12)

Finally, lymphocytes link the immune response to wound repair and further stimulate the fibroblast.

Proliferation phase

There are five stages of the proliferation phase: epithelial regeneration, fibroplasia, collagen formation, wound contraction, and neovascularization.

Epithelial regeneration establishes an environmental barrier by way of active mitosis and the migration of the wound edge. As the wound closes, cell differentiation occurs so that the correct cells are formed to regain the proper histologic cellular stratification of the wounded tissue. After contact inhibition occurs, there is an increase in wound thickness. This increase occurs largely because the fibroblasts manufacture collagen--first collagen III and later, as the scar matures, collagen I. Myofibrils pull the edges of the wound toward each other, and the wound contracts until it completely closes. Macrophages, platelets, lymphocytes, and numerous growth factors combine to promote neovascularization. (13)

Maturation phase

The maturation phase involves remodeling and reorganization. The fibroblasts stop producing collagen III and begin producing collagen I. The fibers reorganize and become stronger. After approximately 1 year, the healed area has regained more than 80% of its tensile strength (it will never regain 100% of its strength).

Tympanic membrane healing

While the general principles of wound healing can be applied to the tympanic membrane, there are some variations that occur secondary to the eardrum's unique anatomy and function. Like most healing tissues, tympanic membranes heal more effectively if the local environment is free of infection, if there is a rich blood supply and oxygen level, and if the area is hydrated. (3)

The three levels of the tympanic membrane are the epithelial outer layer, the fibrous middle layer, and the mucosal inner layer. The blood supply is radially arranged along the edges (forming the vascular ring) and the manubrial plexus, which courses down the handle of the malleus.

The pars tensa, which represents the bottom 80% of the eardrum's structure, is composed mainly of collagen II running in parallel sheets, which is unique. (14) The pars flaccida, which is the superior aspect of the drum, is made up primarily of collagen I; it also has multiple three-dimensional areas of collagen.

Basically, a tympanic membrane perforation closes by a process of circumferential epithelial proliferation followed by connective tissue growth. (15) If healing occurs during a time of infection, there is usually poor alignment of the collagen and poor wound strength. (16)

Attempts to promote tympanic membrane perforation healing with various topically applied growth factors and insulin have been met with mixed results. (17-19) As stated earlier, transforming growth factor appears to play a role in the formation of chronic perforations, but this effect may be reversed by epidermal proliferation, fibroblast production, and angiogenesis stimulated by basic fibroblast growth factor. (18, 20)

Myringosclerosis is scarring and calcification of the middle layer of the eardrum, predominantly adjacent to the malleus and annulus, that can hamper the mobility of the tympanic membrane. (21) This pathologic process begins within 9 hours of injury, mediated by the macrophages during the inflammatory portion of healing, depositing calcium in a process that mimics bone remodeling. (22,23) Myringosclerosis is more likely to occur as a result of infection with Streptococcus pneumoniae type 3 than Haemophilus influenzae. Myringosclerosis can be prevented by antioxidant or anti-inflammatory agents. (24-29)

Effects of steroid therapy

Steroids affect all three phases of wound healing. (30) The addition of a steroid to a topical antibiotic has increased the efficacy of the latter in treating a draining ear and in combating granulation tissue. However, granulation tissue, which is an unwanted result of infection, helps the healing of a perforation. The addition of a steroid, then, may impair tympanic membrane healing. However, the possible clinical advantage of shortening the inflammatory phase and diminishing granulocyte activity in a healing tympanic membrane by the judicious use of a topical steroid is still theoretical; no proof has been demonstrated in any known studies.

Steroids reduce the expression of almost all chemotactic and vasoactive proteins involved in wound healing. (31) The negative effect of steroids on healing has been countered by the administration of topical vitamin A, as well as several types of growth factor. (32-34) Several published studies have indicated that local steroid application has no negative effect on the success rate of tympanoplasty when the steroid is used in the wound packing material. (35,36) Recent animal studies of currently available otic drops compared the effects of antibiotic and antibiotic/steroid combinations on the healing process of a membrane perforation. (37,38) These studies have shown that healing with antibiotic/steroid drops may be delayed, but all the membranes will heal. (37) A similar study has shown that myringotomy healing is transiently down-modulated by treatment with antibiotic/ steroid drops, but healing occurs normally after the drops are discontinued. (38) This is fortunate, as topical steroids have enhanced the treatment of both otitis externa and otitis media with drainage. As we better understand the healing mechanisms of the eardrum and the ways that steroids affect those mechanisms, we should be able to use topical steroids to our advantage in helping tympanic membranes heal in a nonpathologic manner.

References

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Billy Giles, MD