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Interstitial Keratitis
Cornea
1. Clinical Analogy & Conceptual Framework
Exam pearl
- Clinical Analogy: Think of the corneal stroma as a "battlefield". The epithelium and endothelium are innocent bystanders - the real war is happening in the stroma. Interstitial keratitis (IK) is NOT an infection of the stroma - it is an IMMUNE REACTION within it, triggered by antigens from distant organisms (spirochetes, mycobacteria, viruses). The blood vessels invading the stroma are like "emergency supply lines" - the body's misguided attempt to fight an enemy that is no longer there. This is why IK responds to steroids, not antibiotics.
- The fundamental concept: IK = Delayed hypersensitivity stromal inflammation driven by antigen-antibody (immune complex) deposition and T-cell mediated responses - NOT direct microbial invasion of stroma (with very few exceptions like HSV).
Conceptual Framework: Interstitial Keratitis vs Bacterial/Fungal Keratitis
| Feature | Interstitial Keratitis | Suppurative Keratitis |
|---|---|---|
| Mechanism | • Immune-mediated stromal inflammation • Mechanisms vary by cause and may include antigen–antibody–complement responses and delayed hypersensitivity | • Direct microbial infection • Stromal infiltration and tissue destruction |
| Epithelium | • Usually intact | • Commonly ulcerated or associated with an epithelial defect |
| Suppuration | • Absent or minimal | • May be present • Purulent infiltrate and hypopyon may occur |
| Vascularity | • Characteristic or common stromal vascularisation | • Not typical early • May occur in advanced or chronic disease |
| Treatment | • Corticosteroids • Treat the underlying cause | • Antimicrobials • Avoid corticosteroids initially • Consider corticosteroids selectively only after effective antimicrobial therapy and specialist assessment |
| Pain | • May occur • Severity varies | • Commonly painful • May be severe, especially with an epithelial defect |
| Discharge | • Usually absent or minimal • Tearing is more typical | • Purulent or mucopurulent discharge favors suppurative microbial keratitis |
| Feature | Interstitial Keratitis | Suppurative Keratitis |
|---|---|---|
| Mechanism | • Immune-mediated stromal inflammation • Mechanisms vary by cause and may include antigen–antibody–complement responses and delayed hypersensitivity | • Direct microbial infection • Stromal infiltration and tissue destruction |
| Epithelium | • Usually intact | • Commonly ulcerated or associated with an epithelial defect |
| Suppuration | • Absent or minimal | • May be present • Purulent infiltrate and hypopyon may occur |
| Vascularity | • Characteristic or common stromal vascularisation | • Not typical early • May occur in advanced or chronic disease |
| Treatment | • Corticosteroids • Treat the underlying cause | • Antimicrobials • Avoid corticosteroids initially • Consider corticosteroids selectively only after effective antimicrobial therapy and specialist assessment |
| Pain | • May occur • Severity varies | • Commonly painful • May be severe, especially with an epithelial defect |
| Discharge | • Usually absent or minimal • Tearing is more typical | • Purulent or mucopurulent discharge favors suppurative microbial keratitis |
2. Definition, Classification & Aetiology
- DEFINITION (Kanski): Interstitial keratitis is a non-ulcerative, non-suppurative inflammation of the corneal stroma, characterized by cellular infiltration and vascularization of the stroma,
- with relative sparing of the epithelium and Descemet's membrane in early stages.
Classification by Aetiology
| Category | Specific Cause | Mechanism | Key Feature |
|---|---|---|---|
| INFECTIOUS | Congenital syphilis (Treponema pallidum) | Immune-mediated; possibly antigen–antibody-complex-related | • Typically bilateral interstitial keratitis • Associated with Hutchinson triad |
| Acquired syphilis | Infectious cause of interstitial keratitis | • Ocular syphilis may involve interstitial keratitis | |
| Tuberculosis | Type IV (DTH) | Sector/sectoral vascularisation | |
| Herpes simplex (HSV-1) | Direct invasion + immune | UNILATERAL; post-epithelial keratitis | |
| Herpes zoster (HZO) | Immune-mediated stromal inflammation associated with varicella-zoster virus involving the ophthalmic division of the trigeminal nerve | • Nummular opacities may occur | |
| Epstein–Barr virus | Immune complex | • Rare association with bilateral peripheral interstitial keratitis • May have delayed onset and a chronic relapsing course | |
| Leprosy (M. leprae) | Cell-mediated + immune complex | Beaded nerves, sclerosing IK | |
| Onchocerciasis | Mazzotti reaction (immune) | Sclerosing IK + river blindness | |
| Lyme disease (B. burgdorferi) | Rare ocular association, including interstitial keratitis | • Erythema migrans rash and arthritis may accompany systemic disease | |
| Trypanosomiasis | Immune complex | Rare, tropical | |
| Microsporidia | Infectious-associated stromal/interstitial keratitis | Rare cause of presumed immune stromal keratitis | |
| NON-INFECTIOUS | Cogan's syndrome | Autoimmune (ANCA-assoc?) | IK + vestibuloauditory dysfunction |
| Sarcoidosis | Granulomatous | Mutton fat KPs, iris nodules | |
| Gold (chrysiasis) | Toxic deposition | Gold deposits in stroma | |
| Idiopathic | - | Diagnosis of exclusion |
| Category | Specific Cause | Mechanism | Key Feature |
|---|---|---|---|
| INFECTIOUS | Congenital syphilis (Treponema pallidum) | Immune-mediated; possibly antigen–antibody-complex-related | • Typically bilateral interstitial keratitis • Associated with Hutchinson triad |
| Acquired syphilis | Infectious cause of interstitial keratitis | • Ocular syphilis may involve interstitial keratitis | |
| Tuberculosis | Type IV (DTH) | Sector/sectoral vascularisation | |
| Herpes simplex (HSV-1) | Direct invasion + immune | UNILATERAL; post-epithelial keratitis | |
| Herpes zoster (HZO) | Immune-mediated stromal inflammation associated with varicella-zoster virus involving the ophthalmic division of the trigeminal nerve | • Nummular opacities may occur | |
| Epstein–Barr virus | Immune complex | • Rare association with bilateral peripheral interstitial keratitis • May have delayed onset and a chronic relapsing course | |
| Leprosy (M. leprae) | Cell-mediated + immune complex | Beaded nerves, sclerosing IK | |
| Onchocerciasis | Mazzotti reaction (immune) | Sclerosing IK + river blindness | |
| Lyme disease (B. burgdorferi) | Rare ocular association, including interstitial keratitis | • Erythema migrans rash and arthritis may accompany systemic disease | |
| Trypanosomiasis | Immune complex | Rare, tropical | |
| Microsporidia | Infectious-associated stromal/interstitial keratitis | Rare cause of presumed immune stromal keratitis | |
| NON-INFECTIOUS | Cogan's syndrome | Autoimmune (ANCA-assoc?) | IK + vestibuloauditory dysfunction |
| Sarcoidosis | Granulomatous | Mutton fat KPs, iris nodules | |
| Gold (chrysiasis) | Toxic deposition | Gold deposits in stroma | |
| Idiopathic | - | Diagnosis of exclusion |
Exam pearl
- Syphilis remains the MOST COMMON overall cause of interstitial keratitis (IK) worldwide, though herpes simplex virus (HSV) is the most common in developed countries (Salmon JF. Kanski's Clinical Ophthalmology: A Systematic Approach, 9th Edition. Elsevier; 2019).
- Approximately 90% of syphilis-associated interstitial keratitis (IK) is congenital; congenital disease is typically BILATERAL, whereas acquired disease is generally UNILATERAL.
- Interstitial keratitis (IK) can occur in secondary and tertiary syphilis, but the hallmark association is CONGENITAL syphilis.
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Keep reading Interstitial Keratitis
13 more sections are included with the Clinical plan.
- Pathophysiology & Immunology
- Syphilitic IK - Congenital & Acquired
- Tuberculosis-Related IK
- Viral IK - HSV, HZO, EBV, CMV
- Other Causes - Cogan's, Leprosy, Onchocerciasis, Lyme
- CLINICAL FEATURES - ACUTE vs HEALED IK