Anterior chamber paracentesis is an invasive procedure to collect aqueous humor from the anterior chamber of the eye. It is used as a diagnostic aid for uveitis. 1)
Anterior, intermediate, posterior, and panuveitis account for more than 10% of causes of visual impairment in Western countries. Among them, anterior uveitis is the most common, reaching up to 60% of all uveitis cases. Infectious uveitis accounts for 10–20% of all cases.
The diagnostic approach for uveitis is based on a detailed history and clinical examination. However, many cases are difficult to identify the underlying cause, requiring invasive specimen collection techniques. Anterior chamber paracentesis has a lower risk of complications compared to vitreous sampling and can be performed quickly in an outpatient setting. Early definitive pathogen-specific PCR testing facilitates timely diagnosis and appropriate treatment initiation.
Anterior chamber paracentesis for PCR testing is considered in the following situations: atypical clinical presentation, recurrent uveitis of unknown cause, treatment-resistant cases, unclear clinical picture in immunocompromised patients or the elderly, and cases where fundus examination is difficult. Even in uveitis due to primary VZV infection, VZV-DNA positivity may be confirmed by aqueous humor PCR. 2)
When an infectious cause is suspected, PCR is preferred over culture testing due to its higher sensitivity. It can accurately detect minute amounts of pathogen DNA/RNA in aqueous humor.
Major pathogens detectable by PCR:
Aqueous humor PCR tests specific to these pathogens have high diagnostic sensitivity and specificity comparable to vitreous biopsy and serological tests. Aqueous humor PCR can change the diagnosis and treatment plan in a significant proportion of patients.
Example of primary VZV infection: In a case of anterior uveitis and retinal vasculitis associated with primary VZV infection in a diabetic patient, aqueous humor PCR confirmed VZV-DNA positivity, and treatment with valacyclovir plus steroids showed a favorable course. 2)
Disadvantages of PCR include cost, limited sample volume (difficulty in simultaneous testing of multiple items), false amplification of contaminants, and detection failure when cellular components are scarce.
Measurement of cytokines in aqueous humor aids in the differential diagnosis of uveitis etiology.
Cytokine Profile of Inflammatory Uveitis
Viral uveitis: Immunomodulatory cytokines such as IL-6, IL-10, and IFN-γ are present.
Idiopathic uveitis: Inflammatory cytokines such as IL-1, IL-2, IL-6, TNF-α, IFN-γ, IL-8, and MCP-1 are elevated.
Aid in differentiating infectious vs. non-infectious: Specific cytokine patterns support the diagnosis.
Differentiation of Intraocular Lymphoma (Important)
High IL-10 and IL-10/IL-6 ratio >1: Suggests active B-cell lymphoma (primary intraocular lymphoma). In uveitis, IL-6 is elevated.
MyD88 L265P mutation analysis: A useful tool for diagnosing B-cell lymphoma. Usually performed on vitreous samples, but analysis of aqueous humor samples has also been reported recently.
Caveats: Exceptions include elevated IL-10 in infectious uveitis and low IL-10 in early or low-grade lymphoma.
Consider anterior chamber paracentesis when non-invasive tests such as optical coherence tomography (OCT) and fluorescein angiography (FFA) do not confirm the diagnosis. In cases of uveitis refractory to treatment, suspect malignant lymphoma and test for IL-10, IL-6, cytology, and gene rearrangement (monoclonality).
In infectious uveitis (10–20% of all uveitis cases), PCR of anterior chamber fluid via paracentesis directly leads to a definitive diagnosis.
Major infectious causes:
Ophthalmologically, anterior chamber paracentesis is considered in the following cases:
Anterior chamber paracentesis is an outpatient procedure that can be performed using sterile technique. Topical anesthesia is usually sufficient. Avoiding lens damage is most important, and it is essential to keep the needle tip parallel to the iris plane. Miosis is desirable.
Overview of the procedure:
Technical considerations: The puncture site should be chosen closer to the center of the cornea rather than the conjunctival side (puncture near the conjunctiva increases the risk of bleeding and iris incarceration). When withdrawing the needle, apply slight positive pressure to expel a small amount of sample from the needle tip to prevent contamination. Avoid aspirating too much aqueous humor to prevent collapse of the anterior chamber; maintain enough volume to prevent iris incarceration at the puncture site.
The procedure can be performed with or without a slit lamp microscope. If the patient is uncooperative or the procedure is performed in the supine position, the slit lamp may not be used. Performing the procedure in the supine position minimizes the risk of lens damage.
The recommended needle is 30G (attached to a 1 mL tuberculin syringe). Although it is possible to puncture directly with a sharp 25G or 27G needle, compared to the method of aspirating with a 25G needle after creating a perforation with a knife, sharp needles have poorer sharpness and are more prone to changes in the direction of the eye or needle tip, requiring more careful manipulation. Safety studies have shown that anterior chamber paracentesis in patients with uveitis is a safe procedure. 1)
Anterior chamber paracentesis is generally a safe procedure, but the following complications have been reported. 1)
To minimize these complications, strict adherence to sterile technique and proficiency in the procedure are important.
Aqueous humor is a clear fluid that circulates in the anterior chamber and contains intraocular metabolites, immune cells, pathogens, and cytokines. In normal eyes with an intact blood-ocular barrier, immune cells do not easily pass through, but in uveitis, breakdown of the blood-aqueous barrier allows inflammatory cells, proteins, and immune cells to flow into the anterior chamber.
Diagnostic markers present in the aqueous humor:
In recent years, advances in molecular diagnostics such as multiplex PCR for pathogen DNA and intraocular fluid cytokine testing have increased the number of confirmed diagnoses of uveitis. However, approximately 40% of cases remain unclassifiable using conventional diagnostic techniques.
Vitreous fluid biopsy (intravitreal sampling) via vitrectomy is also useful for definitive diagnosis, but anterior chamber paracentesis is superior because it is less invasive, faster, and can be performed on an outpatient basis.
Applying next-generation sequencing (NGS) technology to aqueous humor samples is expected to enable the detection of unknown pathogens that cannot be detected by PCR, as well as simultaneous detection of multiple pathogens.
Noninvasive evaluation techniques for proteins and cells in the anterior chamber using laser flare cell meters and confocal laser microscopes are being developed. If qualitative information on cells and proteins in the anterior chamber can be obtained noninvasively, it may be possible to narrow down the indications for paracentesis and reduce the need for anterior chamber puncture.
Standardization of diagnostic criteria, such as setting thresholds for the IL-10/IL-6 ratio, is needed. Currently, reference values are not unified across multiple studies, and establishing standardized measurement protocols is necessary for widespread clinical use.
Cheung CMG, Durrani OM, Murray PI. The safety of anterior chamber paracentesis in patients with uveitis. Br J Ophthalmol. 2004;88:582–3. [41433_2023_Article_2631.pdfの引用文献31より]
Marín Payá E, Aguilar González M, Rahhal Ortuño M, et al. Anterior uveitis and vasculitis in primary infection with VZV in a diabetic patient. Romanian Journal of Ophthalmology. 2022;66(4):369–372.
