When considering field defects in the context of inherited retinal diseases (IRDs), it’s important to approach the case with a structured and multi-faceted diagnostic process. Field defects may point to a variety of conditions, including optic neuropathies, maculopathies, or more widespread retinal diseases, and distinguishing between them can be challenging.  

1. Correlating Structural and Functional Changes 

  • Symmetry vs Asymmetry: In IRDs, retinal changes are usually symmetric, although there are exceptions. For example, diseases like retinitis pigmentosa can exhibit more asymmetric presentations, while conditions like Stargardt disease typically show symmetric macular changes. If the visual field defects are asymmetric, this might suggest a different diagnosis, such as optic neuropathy or another neurological disorder. 
  • Use of Structural Imaging: OCT (Optical Coherence Tomography) can show retinal layers, optic nerve head characteristics, and macular involvement. The thickness of the retinal nerve fiber layer (RNFL) and ganglion cell layer (GCL) can be measured. Thickening or thinning of these layers can guide the diagnosis.
  • Retinal Changes Beyond the Macula and Optic Disc: IRDs often cause peripheral retinal changes that can be missed if attention is only focused on the macula and optic disc. Wide-field imaging, such as fundus autofluorescence (FAF) or ultra-widefield retinal photography, is essential to visualize broader areas of retinal dysfunction.

 

2. Understanding Retinal Diseases that Overlap with Neurological Conditions 

Some retinal diseases overlap with neurological conditions, and these should be considered when evaluating field defects. Retinal conditions like central serous retinopathy, plastic retinopathy, acute macular neuroretinopathy, and toxic retinopathies may present with visual field defects that mimic or overlap with IRDs. For example: 

  • Toxic Retinopathies: These can be caused by medications, such as metronidazole, which can lead to vision loss and retinal changes similar to macular degeneration. Such conditions need to be differentiated from IRDs like Stargardt disease, which can present with similar macular dysfunction.
  • Retinal Vascular Insufficiency: Conditions affecting retinal blood flow, such as retinal vein occlusion or ischemic optic neuropathy, may also present with visual field defects that overlap with those seen in IRDs.
  • Identifying whether the patient’s history involves potential exposure to toxins, medications, or systemic diseases is key to guiding the diagnostic approach. 

 

3. Evaluating Visual Field Defects

Visual field testing is essential to characterize the extent and pattern of defects: 

  • Central and Paracentral Defects: Central or paracentral scotomas (blind spots) are common in conditions like Stargardt disease and can be identified using visual field tests (e.g., Humphrey or Goldmann perimetry). These defects are usually associated with macular involvement, which can help distinguish between macular and optic nerve pathology.
  • Field Patterns: In optic neuropathies, defects are usually more diffuse or exhibit characteristic patterns (e.g., a central scotoma, arcuate defects, or altitudinal defects). If the visual field defect is localised and confined to a specific area, it is less likely to be an optic neuropathy and more likely to involve retinal pathology. 

 

4. Advanced Ancillary Testing 

Electroretinography (ERG): ERG is crucial in evaluating the functional status of the retina. It tests the electrical responses of the retina to light stimulation and can provide insight into the health of the rod and cone systems. For example:

  • Full-field ERG: It measures global retinal function, and abnormalities can indicate widespread retinal dysfunction, as seen in IRDs.
  • Pattern ERG and Multifocal ERG: These tests assess macular and central retinal function and can identify early dysfunction in conditions like Stargardt disease, which primarily affects the macula.
  • Electrooculography (EOG): Can be used to assess retinal pigment epithelium (RPE) function and is valuable in diagnosing conditions like Best disease or cone-rod dystrophy.
  • The pattern of dysfunction in ERG tests can help identify whether the disease is affecting the rod system (night vision, peripheral retina) or cone system (day vision, central retina).

 

Imaging: Imaging modalities like OCT, fundus autofluorescence, and wide-field retinal photography should be utilised to evaluate the extent of retinal involvement.

  • OCT can identify structural changes in retinal layers (e.g., macular edema, retinal thinning).
  • Fundus autofluorescence highlights areas of retinal dysfunction, often revealing early changes before they are clinically visible, particularly in macular and peripheral regions.
  • Wide-field imaging (e.g., Optos or others) is essential for detecting peripheral retinal changes that are characteristic of certain IRDs but may be missed by conventional imaging.
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5. Case Examples of Overlapping Diagnoses 

  • Case 1: A 33-year-old woman referred for progressive vision loss. She had cloudy vision and headaches. Although the retinal nerve layer thickened, the MRI and MRV were normal. But the abnormal electrophysiology and visual fields suggested retinopathy rather than optic neuropathy. The full field ERG revealed both rod and cone dysfunction, indicating a retinal issue rather than an optic nerve problem. Wide-field imaging showed broader retinal involvement, which helped clarify the diagnosis.
  • Case 2: Another patient with symmetric central visual field defects and significant macular dysfunction was initially suspected of having an optic neuropathy. However, pattern ERG and multifocal ERG revealed significant macular dysfunction, leading to a diagnosis of Stargardt disease.
  • Case 3: A patient with superior altitudinal defects was misdiagnosed with glaucoma. However, wide-field imaging revealed subtle pigmentary changes in the retina, and autofluorescence highlighted sectoral changes consistent with an IRD, not glaucoma. 

 

6. Look Beyond the Optic Disc and Macula 

Often in busy clinics (e.g., glaucoma or medical retina clinics), the optic disc and macula receive most of the focus, but this can lead to missed diagnoses. Peripheral retinal changes in IRDs like retinitis pigmentosa or cone-rod dystrophy may not be immediately obvious in traditional imaging. Wide-field imaging, especially in undilated pupils, can uncover these changes and provide more comprehensive information. 

 

7. Summary: Key Considerations in Suspecting an IRD 

  • Visual Acuity: Impaired acuity in both maculopathy and optic neuropathy requires careful analysis to differentiate between the two.
  • Field Defects: Pay attention to the pattern of defects and symmetry. A localized or central defect suggests macular involvement, whereas a diffuse or arcuate defect is more consistent with optic nerve pathology.
  • Ancillary Testing: Use OCT, fundus autofluorescence, and electrophysiology to assess both structural and functional aspects of the retina. These tools are crucial in differentiating IRDs from other conditions that may mimic them.
  • Retinal Involvement: Always consider the possibility of broader retinal disease by looking beyond the optic disc and macula using wide-field imaging and advanced testing.
  • Multidisciplinary Approach: Engage a multidisciplinary team (e.g., geneticists, ophthalmologists, electrophysiologists) to work through complex cases where the diagnosis remains unclear. 

Diagnosing IRDs in the presence of field defects requires a comprehensive approach that integrates clinical history, visual field analysis, structural imaging, and electrophysiological tests. A thorough examination and the use of advanced imaging and testing can help distinguish IRDs from other retinal and optic nerve conditions, ensuring accurate diagnosis and optimal patient management.