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Pigment dispersion syndrome (PDS)

Ocular disorder characterized by melanin pigment granule liberation from the iris pigment epithelium (IPE).

Ocular disorder characterized by melanin pigment granule liberation from the iris pigment epithelium (IPE).

  • Can lead to secondary open-angle pigmentary glaucoma (PG)
    • Due to reduction of the outflow of aqueous humour and consequent increase in intraocular pressure leading to glaucomatous optic neuropathy

Pathophysiology

The released pigment granules are carried by aqueous convection currents and deposit on anterior segment structures including the corneal endothelium, iris surface, trabecular meshwork (TM), lens surfaces, and zonules. Accumulation of pigment in the TM may lead to increased resistance of aqueous humour outflow, and result in the development of pigmentary glaucoma (PG).

The defining characteristic of PDS is the bilateral shedding of pigment from the posterior iris pigment epithelium (IPE) and the subsequent deposition of this pigment in the anterior segment.

Schematic representation of PDS/PG models. In patients with PDS, pigment liberated from the posterior surface of the iris (green) circulates into the anterior chamber following the flow of aqueous humor where it deposits into the cornea and trabecular meshwork (black dots). High IOP can maintain iris bowing (red arrows) due to the reverse pupillary block in which the lens and iris act together in a ball-valve pressure system which normally acts to maintain unidirectional aqueous humor flow. There are two models of PDS/PG which differ in respect to the origin of pigment dispersion from the ciliary body to the trabecular meshwork. The structural model of PDS/PG proposes that posterior iris bowing creates inappropriate iridozonular contacts (black arrowhead, top circle) and that mechanical rubbing between the iris, zonules, and lens is responsible for liberating pigment from the IPE (asterisks, top circle). Although these structural features are well established, it still remains unclear if they predate pigment dispersion as the underlying mechanism. Animal models support IPE dysfunction as the primary driver of this dispersion. In this model, pigmented melanocytes die and/or detach from the IPE (bottom right circle) due to release of cytotoxic melanin synthesis intermediates from dysfunctional melanosomes (bumpy ovals, bottom left circle). | Lahola-Chomiak, A. A., & Walter, M. A. (2018). Molecular Genetics of Pigment Dispersion Syndrome and Pigmentary Glaucoma: New Insights into Mechanisms. Journal of ophthalmology, 2018, 5926906. https://doi.org/10.1155/2018/5926906

Diagnosis

PDS is characterized by the presence of Krukenberg spindles, iris trans-illumination defects, trabecular meshwork pigmentation and backward bowing of the iris.

Diagnostic criteria:

At least two of the following three signs
  • Krukenberg spindle
  • Homogenous moderate-to-heavy (⩾Scheie II) TM pigmentation
  • Any degree of zonular, and/or lenticular pigment granule dusting
(a) Krukenberg spindle (white arrow) seen in case 2. It was more like a triangle rather than a spindle. (b) Homogeneous heavy trabecular meshwork pigmentation was seen in all patients. (c) Dusting of pigment granules on lens zonules. (d) Pigment accumulated at the zonular attachments to the lens, where it formed a Zentmayer ring, also referred as Scheie’s line (green arrow). | Qing, G., Wang, N., Tang, X. et al. Clinical characteristics of pigment dispersion syndrome in Chinese patients. Eye 23, 1641–1646 (2009). https://doi.org/10.1038/eye.2008.328

Management

Treatment of PG should be initiated early to hinder disease progression, glaucomatous damage, and vision loss.

Peripheral laser iridotomy:

Reverse backward bowing of the iris and may prevent progression of pigmentary glaucoma.

Antiglaucoma medications

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