Filamentary keratitis, first described by Leber (1) in 1882, is a chronic disorder of the cornea characterized by one or more filaments that hang from the surface of the cornea (2). Filamentary keratitis occurs secondary to various diseases or conditions (Table 36-1), but there are some cases in which no associated disease can be identified; these cases are then reported as cases of idiopathic essential filamentary keratitis (3,4).

Observation by a slit-lamp biomicroscope has disclosed that filaments appear as short tails, variable in length but usually less than 2 mm long, being connected to the surface of the cornea (Fig. 36-1) or, very rarely, to the conjunctiva. The location of filaments on the cornea varies in cases and may be related to the mechanism of the background disease. Filaments are stained with rose Bengal, or Lissamine green and less brightly, with fluorescein (3) (Fig. 36-1). At the base of each filament the superficial cells are observed to be elongated and stretched toward the filament (5). The superficial corneal epithelial cells between lesions are normal, but superficial punctate keratopathy is sometimes seen (Fig. 36-1), because filaments are the most common complication of tear-deficient dry eye. Beneath the attachment to the epithelium, there may be a gray subepithelial granular opacity (3), implying a possible association with inflammation due to the filament itself or its association with the background disease. Epithelial detachment is sometimes seen at the base of the filament, which can be detected by the diffusion of fluorescein into the detached subepithelial space (Fig. 36-2).

Histologically, filaments are known to be composed of a central mucin core, which is stained by a periodic acid-Schiff (PAS) or Alcian blue, and degenerated epithelial cells surrounding the core (Fig. 36-3). Also, a previous study reported that at the base of the filaments, just below the basal epithelium, scattered groups of inflammatory cells and fibroblasts were demonstrated, suggesting that filamentary keratitis occurs as a result of damage to either the basal epithelium, epithelial basement membrane, or both (6).

Filaments start out as quite small but grow larger, and in cases where filaments appear in the interpalpebral region, they are likely to be pulled off at the time of blinking. Because of their connection to the corneal surface, patients experience ocular pain, foreign-body sensation, photophobia, and lacrimation, not unlike those symptoms experienced in cases where a foreign body is present. Patients may also experience blepharospasm. (If they already suffer from this, there is the possibility of it becoming worse).

As summarized in Table 36-1, filamentary keratitis is related to dry eye, especially the tear-deficient type, and this association is most prevalent. Filamentary keratitis has also been associated with superior limbic keratoconjunctivitis, acute viral keratoconjunctivitis such as epidemic keratoconjunctivitis (Fig. 36-4), eye surgery such as penetrating keratoplasty (Fig. 36-5) or cataract surgery (Fig. 36-6), recurrent erosions, ocular trauma, vernal or atopic keratoconjunctivitis, neurotrophic keratitis, and prolonged closed eye conditions (ptosis, blepharospasm, or large angle strabismus) (3,7, 8, 9). Systemic diseases such as sarcoidosis, diabetes mellitus, and atopic dermatitis, or medications such as preservative-containing artificial tears, antiglaucoma eyedrops, and diphenhydramine hydrochloride (10) have also been reported as being connected to filamentary keratitis.

In filamentary keratitis, long-term treatment is often ineffective and recurrences are common. It is therefore important to take the pathophysiology (Fig. 36-7) into consideration and treat any underlying disease, although the mechanism of filamentary keratitis remains obscure.

In the normal, healthy ocular surface, interaction between the tear film and the ocular surface epithelium is well maintained with a delicate balance. It is the case, however,
that in eyes with filamentary keratitis, both major factors, the tear film and corneal epithelium, are disrupted and both of these abnormalities seem to be essential for the establishment of corneal filaments. Moreover, mucus glycoproteins in the tear film play an important role in maintaining the stability of tear film and thus keep the corneal surface intact. Alteration in the chemical composition of mucus, which occurs in dry eye, results in the change of physical properties, with increased viscosity causing some of the distinctive clinical features seen in keratoconjunctivitis sicca, superior limbic keratoconjunctivitis, vernal keratoconjunctivitis, and neuroparalytic keratitis (11).