Approach Considerations
Allergy-specific tear and conjunctival scraping laboratory tests are not available except in academic or commercial research settings. Similarly, impression cytology techniques potentially are enlightening, yet are available to only a few dedicated research centers and ophthalmology-specific diagnostic laboratories. Conjunctival scrapings can be sent to hospital cytology laboratories and may be useful if a pathologist with a particular interest in ocular diseases is readily available.
In seasonal and perennial allergic conjunctivitis, superficial conjunctival scrapings may help to establish the diagnosis by revealing eosinophils, but only in the most severe cases, since eosinophils typically are present in the deeper layers of the substantia propria of the conjunctiva. Therefore, the absence of eosinophils on conjunctival scraping does not rule out the diagnosis of allergic conjunctivitis.
Many investigators have described measurement of tear levels of various inflammatory mediators, such as IgE, histamine, and tryptase, as indicators of allergic activity. Additionally, skin testing by an allergist may provide definitive diagnosis and pinpoint the offending allergen(s). Skin testing is highly practical and readily available to all practicing ophthalmologists, as well as to optometrists in some states.
In vernal keratoconjunctivitis (VKC), conjunctival scrapings of the superior tarsal conjunctiva and of Horner-Trantas dots show an abundance of eosinophils. Conjunctival scrapings of patients with atopic keratoconjunctivitis (AKC) may demonstrate the presence of eosinophils, although the number is not as significant as that seen in VKC. Additionally, free eosinophilic granules, which are seen in VKC, are not seen in AKC.
Advanced point-of-service testing soon may become available through several diagnostic technology companies. Biomarkers such IgE, matrix metalloprotease-9 (MMP-9), or eosinophilic basic protein (EBP) may prove to be clinically useful surrogates for disease activity level and therapeutic response monitoring. Specimens can be obtained by tear sampling or conjunctival scraping techniques.
Histologic Findings
Vernal keratoconjunctivitis
Conjunctival scrapings of the superior tarsal conjunctiva show an abundance of eosinophils. Conjunctival biopsy reveals that there are a large number of mast cells within the substantia propria. Histochemical analysis of mast cells, present in VKC, reveals neutral proteases tryptase and chymase. There is enhanced fibroblast proliferation, which leads to the deposition of collagen within the substantia propria and, as result, induces conjunctival thickening.
B-cell and T-cell lymphocytes are present locally, which combine to produce IgE. Specific IgE and IgG as well as the inflammatory mediators histamine and tryptase have been isolated from tears of patients with VKC. Although VKC is typically recognized as a type I hypersensitivity reaction, evidence supports some involvement of type IV hypersensitivity reaction.
Atopic keratoconjunctivitis
Conjunctival scrapings of patients with AKC may demonstrate the presence of eosinophils, although the number is not as significant as that seen in VKC. Additionally, free eosinophilic granules, which are seen in VKC, are not seen in AKC. Mast cells also may be found within the substantia propria of the conjunctiva in greater numbers.
There is an increased amount of IgE in the tears of patients with AKC. Although AKC typically is recognized as a type I hypersensitivity reaction, evidence supports some involvement of type IV hypersensitivity reaction, as is the case in VKC.
Giant papillary conjunctivitis
Histologic findings in GPC consist of cellular infiltration of the conjunctiva by a number of cell types. Plasma cells, lymphocytes, mast cells, eosinophils, and basophils have been identified within the substantia propria. Mast cells also may be found in the epithelium. Tear levels of immunoglobulin are elevated, especially IgE, and tryptase also is elevated, as in AKC and VKC.
Tables
Characteristics |
VKC |
AKC |
Age at onset |
Generally presents at a younger age than AKC' first decade |
Second to third decade |
Sex |
Males are affected preferentially. |
No sex predilection |
Seasonal variation |
Typically occurs during spring months |
Generally perennial |
Discharge |
Thick mucoid discharge |
Watery and clear discharge |
Conjunctival scarring |
Moderate incidence of conjunctival scarring |
Higher incidence of conjunctival scarring |
Horner-Trantas dots |
Horner-Trantas dots and shield ulcers are commonly seen. |
Presence of Horner-Trantas dots is rare. |
Corneal neovascularization |
Not present, unless secondary to infectious keratitis |
Deep corneal neovascularization tends to develop |
Presence of eosinophils in conjunctival scraping |
Conjunctival scraping reveals eosinophils to a greater degree in VKC than in AKC |
Presence of eosinophils is less likely |
