Retinal edema impairs vision in millions. It is diagnosed as abnormal thickening of the retina that is commonly observed in ischemic and inflammatory diseases.
1–4 Retina edema can be classified as cytotoxic or vasogenic, which respectively reflects cell swelling induced by volumetric increase of
intracellular water and vascular leakage induced volumetric increase of
extracellular water.
5 It is commonly accepted that vascular leakage is the primary pathway of retinal edema. However, in ischemic/excitotoxic diseases cell swelling may develop early or coexist with vascular leakage.
6,7 It has also been suggested that the swelling and death of Müller cells contributes to the development of cystoid macular edema.
4 Thus, cell swelling and vascular leakage may have different contributions to retinal edema in individual patients.
Noninvasive methods for direct assessment of retinal cell swelling are lacking. Unlike vascular leakage induced edema that can be indirectly detected using fluorescein angiography,
2,8 retinal cell swelling can be implicated only in certain patients who develop retinal thickening without vascular leakage.
9 Given the different mechanisms underlying cytotoxic and vasogenic edema, the capability to differentially assess retinal cell swelling and vascular leakage is therefore critical to advance the current disease diagnosis and therapy.
In addition to its detectability, the contribution of cell swelling to edematous retinal thickening remains an open question. Knowledge acquired from cerebral diseases suggested that cell swelling may not lead to increased tissue volume if the blood–tissue barrier, which limits extravasation of blood–pool water, is intact.
10 Instead, cell swelling can develop through redistribution of extracellular water, accompanying the ion (primarily Na
+) flux, into the intracellular compartment.
10 Since the neural retina comprises a blood–retina barrier, if the same mechanism applies, cell swelling may not lead to retinal thickening before the onset of retinal vascular leakage.
We hypothesize that retinal cell swelling directly causes retinal thickening. The retina is a thin layer of tissue (200–300 μm thick) located adjacent to vitreous (99% of water content).
11 It continuously experiences a transretinal fluid flux driven by intraocular pressure gradient from the vitreous to the choroid.
5 This unrestricted extracellular fluid supply suggests retinal cell swelling may develop without changing the extracellular water volume. A mouse model of N-methyl-D-aspartate (NMDA) excitotoxicity was used to test our hypothesis. Excessive binding of intravitreally injected NMDA to NMDA receptors causes overexcitation of neuronal cells followed by energy failure. The resulting cell swelling and edematous retinal thickening occur within several hours.
12 Due to the absence of NMDA receptors on photoreceptor cells in the outer retina, NMDA excitotoxicity primarily damages the inner retina that extends from the nerve fiber layer to the outer plexiform layer.
In this study, diffusion MRI–derived water apparent diffusion coefficient (ADC) was measured to evaluate NMDA excitotoxicity–induced cell swelling in the inner and outer retina. Diffusion MRI is a primary clinical imaging method for assessing neural tissue injury.
13 Despite the retinal edema-induced concomitant T1 and T2 changes during the diffusion-weighted MRI measurement, the derivation of ADC takes the ratio of image intensities with and without diffusion weighting, which essentially performed the automatic normalization of T1 and T2 effects eliminating the commonly seen “T2 shine through” effect in diffusion-weighted images.
13 The measured ADC is by nature independent of T1 relaxation, T2 relaxation, or the magnetic field strength.
14,15 It has long been recognized that neuronal cell swelling leads to an ADC decrease.
16,17 When cell swelling and vasogenic edema coexist, such as that observed in ischemia/reperfusion injury,
18,19 the relative contribution of each component at different stages of the disease confounds the interpretation of the longitudinal evolution of ADC.
Gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA) enhanced MRI with T1 mapping was used in this study to evaluate retinal vascular leakage before, at 3 hours, and 1 day after NMDA treatment. Gadolinium is a paramagnetic ion that reduces
1H T1 of surrounding water molecules. The chelated Gd-DTPA has limited permeability through intact blood–brain barrier or blood–retina barrier. In CNS diseases such as multiple sclerosis (MS), the Gd-DTPA from the blood pool into perivascular tissue through the damaged blood–brain barrier in active lesions results in signal enhancement on T1-weighted images.
20–22 The sensitivity of this technique to MS lesions is improved by administering a higher dose of Gd-DTPA and delayed image acquisition
23,24 ; for example, it reached a 95% detection rate for MS lesions when MR images were acquired at 20 minutes after a triple dose (0.3 mmol/kg) of Gd-DTPA.
24 In retinal diseases such as diabetic retinopathy
25 and macular edema,
26 the leakage of Gd-DTPA into avascular posterior vitreous caused local enhancement of the T1-weighted signal. The Gd-DTPA–enhanced MRI-detected retinal vascular leakage was confirmed by histology
27 or electron microscopy.
28 Our recent work showed that retinal vascular leakage in
rd1 mice resulted in a progressive increase of vitreous signal within the first 30 minutes after injection of 1.0 mmol/kg Gd-DTPA.
29 Unfortunately, the exact sensitivity of this MRI technique to retinal vascular leakage, as compared with standard fluorescein angiography and histological methods, has not been defined yet. Based on the previous work of our group and others, we used a high dose of Gd-DTPA (1.0 mmol/kg, 10-folds the standard dose) and delayed image acquisition (between 30 and 45 minutes after Gd-DTPA injection) to optimize MRI sensitivity against retinal vascular leakage.