Etiology of White Dot Syndromes

From Self-sufficiency
Jump to: navigation, search

White dot syndromes are inflammatory diseases characterized by the presence of white dots on the fundus, the interior surface of the eye.[1] Classically recognized syndromes include multiple evanescent white dot syndrome (MEWDS), acute posterior multifocal placoid pigment epitheliopathy (APMPPE), acute zonal occult outer retinopathy (AZOOR), and punctate inner choroidopathy (PIC), among others. In addition, birdshot choroidopathy, diffuse unilateral subacute neuroretinitis (DUSN), multifocal choroiditis (MFC), and serpiginous choroiditis are also diseases that present with white dots.[2] The majority of individuals affected with white dot syndromes are younger than fifty years of age. Some symptoms include blurred vision and visual field loss.[3] There are many theories for the etiology of white dot syndromes including infectious, viral, genetics and autoimmune.

Characteristics of Specific White Dot Syndromes

Specific characteristics regarding the white dots and predicted etiology are presented of selected diseases.

Multiple Evanescent White Dot Syndrome

Multiple evanescent white dot syndrome (MEWDS) occurs mostly in females. Symptoms include a sudden loss of central vision, but patients eventually regain normal vision.[2] The white dots are small (100-200 micrometers) and located in the posterior pole at the level of the retinal pigment epithelium (RPE).[4] The white dots may disappear after the first few weeks of the disease. The cause is generally unknown, but a viral illness has been reported prior to MEWDS in one-third of cases.[3] Since the disease occurs primarily in females, “hormonal status” might be a contributing factor.[1]

Multifocal Choroiditis

Multifocal Choroiditis (MFC), similar to MEWDS, occurs mainly in myopic females. The fundus presents with yellow or gray lesions (white dots) at the level of the choroid and RPE.[1] The size of the white dots are between 50 to 500 micrometres and localized in the macula.[5] MFC is characterized by vitritis and anterior chamber inflammation. Decreased vision due to vitreous inflammation may occur. Unlike MEWDS, MFC is a chronic disorder and macular scarring contributes to severe visual loss. Theories regarding the cause include “an exogenous pathogen sensitizing an individual to antigens within photoreceptors, RPE, or choroid.”[3]

Punctate Inner Choroidopathy

Punctate inner choroidopathy also occurs mainly in young females and may be a milder form of MFC. Symptoms include blurred vision and scotomas.[5] Unlike MFC, PIC does not tend to recur in patients. Yellow lesions are mainly present in the posterior pole and are between 100 to 300 micrometres in size.[6]

Acute Posterior Multifocal Placoid Pigment Epitheliopathy

Acute posterior multifocal placoid pigment epitheliopathy (APMPPE) primarily occurs in adults (with a mean age of 27).[1] Symptoms include blurred vision; vision loss usually occurs in both eyes, but onset may occur at a different time in each eye. There are yellow-white placoid lesions in the posterior pole at the level of the RPE. Some suggest a genetic predisposition to the disease, while others postulate an abnormal immune response to a virus.[3]

Acute Zonal Occult Outer Retinopathy

Some discrepancy exists as to whether acute zonal occult outer retinopathy (AZOOR) is actually considered a white dot syndrome. However, AZOOR may definitely be related to other diseases included in the white dot syndrome group. AZOOR occurs in young to middle age adults and may eventually progress to retinal cell death. Symptoms include acute visual field loss and photopsias.[3] Suspected causes for AZOOR include autoimmune, viral, and fungal.[7]

White Dots

The white dots of the white dot syndromes are lesions that vary in their location in the fundus and in most cases tend to disappear. White dots appear early in the disease stages of PIC and MEWDS. In this case, the white dot is localized in the posterior pole, small (between 25 to 100 μm), and do not clump together.[8] In contrast, white dots appear later in the disease stages of birdshot choroidopathy, serpiginous choroiditis, and APMPPE. The white dots in these diseases may be present throughout the entire fundus, larger (50 to 500 μm), and tend to clump together. Among all these syndromes, there exists some retinal vessel inflammation. The differences in the dots are usually in the size, position, and depth of the lesion within the choroid.[8]

The way in which the dots form in some of the white dot syndromes has been reported. The dot appears as a small granuloma which is composed of lymphocytes and macrophages. The lesion may occur within the choroid, between Bruch’s membrane and RPE, or between the RPE and photoreceptors.[8] Despite the differences in location, the white dots all are of similar composition. The center of the lesion consists of macrophages and epithelioid cells. CD4+ T cells are on the periphery of the granuloma. Benezra has theorized that a large amount of CD8+ T suppressor cells are observed in the later stages of the disease in order to down regulate the inflammatory immune reaction.[9]

The formation of a granuloma occurs when activated antigen presenting cells, specifically dendritic cells, “bind to T cells and induce…the release of pro inflammatory cytokines and chemokines.” This response attracts additional antigen presenting cells and will eventually become a granuloma. Choroidal dendritic cells span several levels within the choroid and also associate with the RPE. Usually, the dendritic cells disappear after removing the antigen. If removal did not occur, the formation of a granuloma would result. The a white color of the dots when illuminated may be due to the granulomas composed only of “white cells”. Each granuloma will disappear leaving no trace of its presence or a ‘punched out’ scar. It is important to note that the formation of white dots may occur more frequently but is undetected. In normal cases, inflammation of the retina or choroid does not occur. Mueller and RPE cells normally release immunosuppressive factors, but certain combinations of cytokines may stimulate RPE cells to release factors encouraging inflammation.[8]

Generally, mild intraocular inflammation results in a small, discrete, evanescent lesions. Larger dots, having less discrete borders, are the result of high intensity intraocular inflammation.[9] In essence, an immune response with the normal amount and appropriate cytokine release will result in small white dots and a misregulated response eventually will produce scarring of the retinal tissue. Treatment is required in the latter case to combat loss of vision.[8] The discrete white dots as they usually disappear naturally. Corticosteroids have been shown to speed up this process. The differences in the immune response of each patient may contribute to the differences seen between the white dot syndromes.[9]

Distinct Diseases

Due the number of common features among the multiple syndromes, many suggest that the white dot syndromes are not distinct and represent a spectrum of one disease.[1] Gass described the ‘AZOOR complex’ which consists of MEWDS, MFC, PIC, acute idiopathic blind spot enlargement, acute macular neuroretinopathy, acute annular outer retinopathy, and AZOOR. He suggested these diseases represent one disease due to common factors such as a high occurrence in females, unexplained visual field loss, and reduced electroretinographic amplitudes.[3][10] Reddy et al. conducted a study on the blind spots in MFC, PIC, MEWDS, and diffuse subretinal fibrosis syndrome. Clinical and electroretinographic evidence suggested the diseases are distinct.[11] However, numerous differences do occur in fundus appearance, the clinical course of the diseases, and electrophysiology.[5]

Suspected Etiology

One cause of the White Dot Syndromes as suggested by Gass involves viral or infectious agents. Specifically pertaining to the ‘AZOOR complex,’ Gass has postulated that a virus may enter the retina at the optic head and the infection may spread from one photoreceptor to another.[10] Some unexplained features include the development of more than one disease in the same patient and the majority of cases occurring in females.

According to Becker’s common genetic hypothesis, “unlike mendelian genetic disorders, common autoimmune and inflammatory diseases arise from combinatorial interactions of common non-disease specific loci, disease specific loci, and specific environmental triggers.”[12] An important aspect of this hypothesis pertains to the existence of common non-disease genes that predispose patients to autoimmune diseases. Jampol and Becker insinuate that ‘common susceptibility genes’ are present in patients affected by white dot syndromes. The presence of environmental triggers, such as viral infections, immunizations, and stress, and interactions with other genes contribute to the development of the white dot syndromes. Additionally, Jampol and Becker hypothesize that the predisposing genetic loci can be identified.[10]

Gass points to a lack of evidence in support of the Becker theory. Instead, Gass highlights that although evidence indicates that patients with AZOOR have a greater chance of developing autoimmune diseases, this does not mean that the AZOOR complex of disorders are themselves autoimmune diseases. This is supported by the difficulty in detecting “retinal autoantibodies” in AZOOR patients.[13]

Two other diseases which also present with white dots on the fundus are retinitis punctata albescens and fundus albipunctatus. These diseases are not white dot syndromes, but have much more defined etiology. Retinitis punctata albescens is caused by mutations in RLBP1, the gene for retinaldehyde binding protein 1. In comparison, fundus albipunctatus is caused by mutations in RDH5 gene for an 11-cis-RDH in RPE cells.[14]

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 [1], Tewari A, Elliot D. White Dot Syndromes. 2007. Emedicine from WebMD.
  2. ↑ 2.0 2.1 Forrester JV, IOIS, Okada AA, BenEzra D. Posterior segment intraocular inflammation: guidelines. 1998:184.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Quillen DA, Davis JB, Gottlieb JL, Blodi BA, Callanan DG, Chang TS, et al. The white dot syndromes. American Journal of Ophthalmology. 2004;137(3):538-50.
  4. ↑ Chern KC ZM. Ophthalmology Review Manual. 2000:560.
  5. ↑ 5.0 5.1 5.2 Polk TD, Goldman EJ. Chorioretinal Inflammatory Syndromes. International Ophthalmology Clinics. 1999;39(4):33-53.
  6. ↑ Goldstein DA, Ulanski L. Multifocal choroiditis vs. PIC: Variations on a Theme? Review of Ophthalmology. 2004;11(6).
  7. ↑ Carrasco L, Ramos M, Galisteo R, Pisa D, Fresno M, Gonzalez ME. Isolation of Candida famata from a Patient with Acute Zonal Occult Outer Retinopathy. J Clin Microbiol. 2005;43(2):635-40.
  8. ↑ 8.0 8.1 8.2 8.3 8.4 Ben Ezra D, Forrester JV. Fundal white dots: the spectrum of a similar pathological process. Br J Ophthalmol. 1995;79(9):856-60.
  9. ↑ 9.0 9.1 9.2 Benezra D. Ocular inflammation: basic and clinical concepts. 1999: 512.
  10. ↑ 10.0 10.1 10.2 Jampol LM, Becker KG. White spot syndromes of the retina: a hypothesis based on the common genetic hypothesis of autoimmune/inflammatory disease. American Journal of Ophthalmology. 2003;135(3):376-9.
  11. ↑ Reddy CV, Brown J, Folk JC, Kimura AE, Gupta S, Walker J. Enlarged blind spots in chorioretinal inflammatory disorders. Ophthalmology. 1996; 103(4): 606-617.
  12. ↑ Becker, K. G. (2001). The common genetic hypothesis of autoimmune/inflammatory disease. Curr Opin Allergy Clin Immunol, 1(5), 399-405.
  13. ↑ Gass, J. D. (2003). Are acute zonal occult outer retinopathy and the white spot syndromes (AZOOR complex) specific autoimmune diseases? American Journal of Ophthalmology, 135(3), 380-381.
  14. ↑ Travis GH, Golczak M, Moise AR, Palczewski K. Diseases Caused by Defects in the Visual Cycle: Retinoids as Potential Therapeutic Agents. Annual Review of Pharmacology and Toxicology. 2007;47(1):469-512.

See also

Retrieved from "http://ssf.f15ijp.com/wiki/index.php?title=Etiology_of_White_Dot_Syndromes&oldid=26597"