Division of Ophthalmology and Visual Sciences, Larry A Donoso Laboratory for Eye Research, University Hospital, Queen’s Medical Centre, Nottingham NG7 2UH, UK

ABSTRACT

Impression cytology refers to the application of a cellulose acetate filter to the ocular surface to remove the superficial layers of the ocular surface epithelium. These cells can then be subjected to histological, immunohistological, or molecular analysis. Proper technique is essential as the number of cells sampled can vary considerably. Generally two to three layers of cells are removed in one application but deeper cells can be accessed by repeat application over the same site. Applications for impression cytology include diagnosing a wide range of ocular surface disorders, documenting sequential changes in the conjunctival and corneal surface over time, staging conjunctival squamous metaplasia, and monitoring effects of treatment. It is also a useful investigational tool for analysing ocular surface disease with immunostaining and DNA analysis. It is non-invasive, relatively easy to perform, and yields reliable information about the area sampled with minimal discomfort to the patient. Major ophthalmic centres should develop and introduce this technique into routine clinical practice. This is best achieved with a team approach including the ophthalmologist, pathologist, microbiologist, and the immunologist.

Abbreviations: OSSN, ocular surface squamous neoplasia; PAS, periodic acid Schiff; RT PCR, reverse transcriptase polymerase chain reaction; TDC, total dye content

Keywords: impression cytology; ocular surface

Impression cytology refers to the application of cellulose acetate filter to the ocular surface to remove the superficial layers of the ocular surface epithelium. The cells thus removed can be subjected to histological, immunohistological, or molecular analysis. Egbert et al first described this minimally invasive method of studying conjunctival goblet cells in 1977.1 Since then the technique has been used to evaluate several ocular surface disorders and modifications to the original technique have been introduced. It is non-invasive, easy to perform, and yields reliable information about the area sampled with minimal discomfort to the patient. This makes it a valuable tool in the understanding of ocular surface disorders. Applications of impression cytology include the aetiological diagnosis of various ocular surface disorders, documenting sequential changes in the conjunctival and corneal surface over time, monitoring effects of treatment and staging conjunctival squamous metaplasia, and as an investigational tool for analysing ocular surface disease with immunostaining and DNA analysis.2

IMPRESSION CYTOLOGY TECHNIQUE

Egbert et al used Millipore filters to collect conjunctival specimens, which were then air dried and stained with periodic acid Schiff (PAS) and haematoxylin.1 Tseng modified the method of collection of specimens and stained them with a combination of PAS and Papanicolaou stains.2 Maskin and Bodé have described a technique with which conjunctival epithelial cells acquired by impression cytology can be studied by electron microscopy.3 Pure nitrocellulose membranes and Biopore membrane devices have been used to enable immunocytochemical staining.4,5

SPECIMEN COLLECTION

The type of filter paper used and the technique of cell collection depend on the purpose for which the specimen is collected. The size of the filter paper pores affects the consistency of epithelial cells collected and the resolution of cell detail. Larger pore sizes collect cells better, but the cell detail is less well preserved. Treatment of the filter paper with surfactant also reduces cell pick up. Most authors use surfactant free filter paper of a pore size between 0.22 μm and 0.44 μm.6

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