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Intermediate uveitis is diagnosed when the signs of uveitis are most pronounced in the vitreous, although lesser degrees of inflammation may co-exist in the anterior and posterior segments.
Pars planitis is the most common form of intermediate uveitis, and is characterised by inflammatory exudates over the inferior peripheral retina, ora serrata and pars plana. It typically affects young adults and is usually bilateral. The usual clinical course comprises chronic, low-grade inflammation with occasional exacerbations resulting in blurred vision and floaters. Although the etiology is currently unknown, occasional associations with systemic diseases including sarcoidosis, multiple sclerosis, Lyme disease, Whipple's disease and intraocular lymphoma are well-documented.
Symptoms
Floaters and blurred vision are the principal symptoms. Blurred vision may be due to vitreous infiltrates or cystoid macular oedema (CMO). Mild symptoms may exist for many months before the patient seeks medical attention. Occasionally, asymptomatic cases of pars planitis are diagnosed on routine examination. Red eye, ocular pain and photophobia are more suggestive of anterior uveitis.
Signs
The anterior segment is clear or shows low-grade inflammation only similarly, there are no focal inflammatory fundus lesions. A variable degree of vitreous haze is usual, with floaters and inflammatory cells in the anterior vitreous.
The critical signs of pars planitis can often be seen only with indirect ophthalmoscopy and scleral depression. White vitreous cellular aggregates ('snowballs') form near the ora serrata, usually inferiorly. With disease progression, these lesions organise into a smooth, white, fibrovascular membrane over the inferior pars plana ('snowbanking' - the pathognomonic sign in pars planitis).
Progressive fibrosis and contraction of this membrane occasionally results in vision-threatening complications including vitreous or retinal detachment, vitreous haemorrhage and neovascularisation of the optic disc and peripheral retina.
CMO is the most common cause of reduced visual acuity in intermediate uveitis. Chronic, cystoid changes rarely cause permanent visual impairment from lamellar macular hole formation.
Other signs may include mild peripheral periphlebitis, posterior subcapsular cataract (possibly corticosteroid-induced) and elevated intraocular pressure with secondary glaucoma.
Prevalence
Very rare (less than 1 per 100,000).
Significance
The majority of patients experience low-grade, chronic inflammation and have a very good visual prognosis without treatment. Nevertheless, potential vision-threatening complications and systemic associations necessitate thorough assessment and review.
Differential diagnosis
Sarcoidosis, Toxoplasmosis, Toxocariasis, Syphilis, Lyme disease, Multiple sclerosis (see Optic neuritis).
See also
Posterior uveitis, Cystoid macular oedema, Retinal detachment, Macular hole.
Management
Blood tests, imaging investigations
Baseline investigations include chest X ray (hilar lymphadenopathy in sarcoidosis), serum angiotensin-converting enzyme (often elevated in sarcoidosis) and syphilis serology. Serological tests for toxoplasmosis and toxocariasis may also be helpful, depending on the clinical presentation. When multiple sclerosis is suspected, referral to a neurologist is appropriate. Fluorescein angiography is often performed to detect retinal vasculitis. Macular oedema can also be assessed by observation, angiography or ocular coherence tomography (OCT).
Treatment
The principal indication for treatment of pars planitis is impaired visual acuity (often worse than 20/40). The initial treatment is corticosteroids. Posterior sub-Tenon's injection (for example, methylprednisolone, at intervals of 1-2 months) is favoured in cases of significant vitreous reaction, CMO or periphlebitis. Topical corticosteroids are administered for significant or symptomatic anterior chamber inflammation. Oral or intravenous therapy - steroid or other immunosuppressive agents - are generally reserved for resistant cases. Transcleral cryotherapy to the area of snowbanking, pars plana vitrectomy and laser photocoagulation to areas of peripheral neovascularisation are also performed occasionally.
Review
Initial review is at one to four week intervals, then tapered according to severity and response to treatment. Ten per cent of cases are self-limited 90 per cent are chronic. Patients receiving prolonged corticosteroid therapy are monitored for elevated intraocular pressure and cataract.