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Original Contribution

Subretinal Abscess Causing Restricted Diffusion on Magnetic Resonance Imaging

Peeler, Crandall MD; Parmar, Hemant MD; Trobe, Jonathan D. MD

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Journal of Neuro-Ophthalmology: December 2013 - Volume 33 - Issue 4 - p 354-358
doi: 10.1097/WNO.0b013e318294314c
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Abstract

Diffusion-weighted imaging (DWI) is a magnetic resonance imaging (MRI) pulse sequence based on the movement of water molecules within brain parenchyma. DWI assigns an apparent diffusion coefficient (ADC) to biological tissues (1). When the extracellular diffusion of water via Brownian motion is impeded by edema, restricted diffusion is seen on DWI (2–8).

Most commonly found in acute ischemic infarction, restricted diffusion also has been noted in intracranial neoplastic conditions (lymphoma, meningioma, epidermoid) (1,2) and inflammatory disorders (abscess, empyema, acute demyelinating plaques) (3). In head and neck cancer, DWI has been applied to differentiate benign from malignant tumors, discriminate between recurrent tumors and post-therapeutic changes, and detect lymph node metastasis for staging purposes (4). In spine and spinal cord disease, DWI aids in distinction of acute demyelination from infarction and trauma, and helps to identify extradural osseous lesions (5). In orbital disease, DWI has been used to demonstrate ischemic and infiltrating optic neuropathies (6), endophthalmitis (7), and thrombosis of the superior ophthalmic vein (8).

Only 1 case of restricted diffusion has been reported in a case of subretinal abscess (9). We describe 2 additional cases to emphasize the diagnostic value of DWI in this unusual setting.

CASE REPORTS

Case 1

A 16-year-old boy presented with pain and blurred vision in the right eye of 1-day's duration. For the previous 10 days, he had experienced fever, generalized weakness, cough, joint pain, and headaches that were attributed to mononucleosis based on a positive Monospot test. He had received nebulizer treatment, ketorolac, and intravenous fluids.

Examination disclosed that visual acuity was hand movements in the right eye and 20/20 in the left eye. There were no ocular adnexal abnormalities or tenderness to palpation. The right pupil measured 4 mm and the left pupil measured 2 mm in dim illumination, and there was a right relative afferent pupillary defect. Ocular ductions were full and the eyes were aligned. The visual field was unobtainable in the right eye due to poor visual acuity and full to finger counting in the left eye. Slit-lamp biomicroscopy and intraocular pressures were normal in both eyes. In the right eye, ophthalmoscopy revealed a large (3 disc diameters horizontally by 4 disc diameters vertically), yellow-white, round subretinal lesion with indistinct margins. In the left eye, there was a smaller (1/2 disc diameter) subretinal fluid collection nasal to the optic disc. Fundus photography was not performed at that time.

MRI of the brain and orbits was performed on a 1.5 T magnet (Siemens Medical Systems, Enlargen, Germany) using an 8-channel head coil. Diffusion imaging data were obtained using an echo-planar single-shot technique with the shortest TR, 89 ms TE, and a 90 flip angle, and with a b value of 1,000 seconds/mm2. The data were recorded on a 128×128 matrix and were zero filled for a final resolution of 128×256. Axial slices with 5-mm slice thickness and a 0.5-mm interslice gap were obtained.

DWI revealed a small nodular abnormality in the posterior aspect of the right globe that exhibited restricted diffusion (Fig. 1). In addition, multiple foci of restricted diffusion were visualized in both cerebral and cerebellar hemispheres in cortical, subcortical, and periventricular locations (Fig. 2A). Most of these foci exhibited dark signal on the corresponding ADC maps, confirming that the DWI signal reflected true areas of restricted diffusion without any T2 effect (Fig. 2B). Susceptibility sequences demonstrated punctate hemorrhagic components within some of the brain lesions. After contrast administration, many of the lesions demonstrated peripheral enhancement (Fig. 2C). The brain findings were interpreted as septic infarctions and the retinal abnormality as an abscess.

FIG. 1
FIG. 1:
Case 1. Subretinal abscess attributed to methicillin-sensitiveStaphylococcus aureus. A. T2 axial magnetic resonance imaging (MRI) shows an area of low signal (arrow) in the right posterior subretinal space. B. Diffusion-weighted MRI shows a corresponding area of high signal (arrow). C. Apparent diffusion coefficient MRI shows corresponding dark signal (arrow), indicating restricted diffusion.
FIG. 2
FIG. 2:
Case 1. Septic brain infarctions.A. Diffusion-weighted magnetic resonance imaging (MRI) shows areas of restricted diffusion in both cerebral hemispheres. B. Apparent diffusion coefficient MRI shows corresponding areas of dark signal, indicating restricted diffusion. C. Postcontrast T1 axial magnetic resonance imaging demonstrates ring enhancement of the cerebral lesions.
FIG. 3
FIG. 3:
Case 1. Fundus photographs show a large subretinal lesion in the right eye (A) with associated hemorrhage and extension into the vitreous cavity. A smaller subretinal fluid collection is evident nasal to the optic nerve in the left eye (B).

Transesophageal echocardiography revealed a small thrombus in a right pulmonary vein at its junction with the left atrium. Additional imaging showed numerous embolic foci involving the lungs, liver, kidneys, and left hip. Methicillin-sensitive Staphalococcus aureus (MSSA) was cultured from the blood, left hip joint, right hand skin lesion, and pericardial fluid.

The patient was treated with intravenous rifampin, nafcillin, and gentamicin for MSSA bacteremia. On hospital day 4, the subretinal abscess appeared to extend beyond the plane of the retina into the vitreous cavity (Fig. 3), so the patient underwent a vitreous aspiration for culture, followed by intravitreal injection of vancomycin and ceftazidime. Culture of vitreous fluid revealed no growth of microorganisms.

On hospital day 7, the patient underwent open heart surgery for removal of a right pulmonary vein vegetation. Mild noncompaction of the left ventricular apex in the setting of heavily trabeculated endocardium was cited as the likely etiology of thrombus formation. Blood cultures remained negative for MSSA following thrombectomy. The size and elevation of the right fundus lesion began to decrease following a second vitreous injection of vancomycin in the right eye on hospital day 9.

One month following presentation, the patient's visual acuity had improved to 20/200 in the right eye and remained 20/20 in the left eye. There was a persistent right relative afferent pupillary defect but no anisocoria. Ocular ductions and alignment remained normal. Visual field testing showed a superonasal defect to finger counting on the right eye and remained full on the left eye. Ophthalmoscopy of the right eye showed gliosis and retinal pigment epithelial atrophy, and the lesion in the left eye also appeared atrophic with resolution of subretinal fluid.

Case 2

A 62-year-old woman with necrotizing pancreatitis complicated by multiple enterocutaneous fistulas presented with headache, altered mental status, and abdominal pain. She had awakened the previous night with a severe frontal headache and fever. Brain computed tomography (CT) suggested multifocal abscesses in the right cerebral hemisphere.

Ophthalmologic consultation was requested to evaluate chemosis and periocular swelling of the right eye. Complicated by depressed mental status, examination disclosed visual acuity of finger counting in both eyes. There was swelling of the right upper and lower eyelids with minimal erythema. The right pupil measured 3 mm and the left pupil measured 5 mm in dim illumination, and there was a right relative afferent pupillary defect. Ocular ductions and visual field testing were unobtainable given the patient's mental status. Slit-lamp biomicroscopy revealed chemosis temporally in the right eye and was normal in the left eye. Intraocular pressures were normal in both eyes. In the right eye, ophthalmoscopy revealed dense vitritis with no view to the posterior pole. In the left eye, ophthalmoscopy was normal. B-scan ultrasonography of the right eye showed dense vitreous membranes.

Brain MRI revealed a prominent area of restricted diffusion in the region of the far anterior retina of the right eye (Fig. 4). In addition, at least 3 lesions were identified in the right cerebral hemisphere with a peripheral rim of restricted diffusion (Fig. 5A). These foci exhibited dark signal on the corresponding ADC maps (Fig. 5B), confirming that the DWI high signal reflected restricted diffusion. The brain findings were interpreted as septic emboli and the retinal abnormality as an abscess.

FIG. 4
FIG. 4:
Case 2. Subretinal and cerebral abscesses attributed toBacillus species. A. T2 sagittal magnetic resonance imaging (MRI) shows an area of high signal (arrow) in the right superomedial subretinal space and the cerebral hemisphere corresponding to abscesses. B. Diffusion-weighted MRI shows the high signal of restricted diffusion in the anterior medial portion of the right eye (arrow) indicative of a subretinal abscess. C. Apparent diffusion coefficient MRI shows corresponding dark signal (arrow), indicating restricted diffusion.
FIG. 5
FIG. 5:
Case 2. Cerebral abscess (septic brain infarction).A. Diffusion-weighted magnetic resonance imaging (MRI) shows an area of high signal in the right frontal lobe (arrow). B. Apparent diffusion coefficient MRI reveals corresponding low signal, indicating restricted diffusion (arrow).

Vitreous aspiration for culture was negative. A Bacillus species (not anthracis or cereus) was cultured from the patient's blood. She was treated with intravenous vancomycin, cefepime, and metronidazole, intravitreal vancomycin and ceftazidime, and topical moxifloxacin, prednisolone acetate, and atropine.

A transesophageal echocardiogram was negative for valvular vegetations, and imaging of the abdomen showed no evidence of an abscess as the source of the patient's bacteremia. The enterocutaneous fistulas were considered the probable source of the retinal and cerebral abscesses.

Despite treatment, repeat MRI of the brain and orbits on hospital day 4 revealed interval enlargement of the areas of restricted diffusion in the brain and subretinal abscesses. Intravenous voriconazole and levofloxacin were added to the treatment regimen. A repeat vitreal injection was performed with vancomycin and voriconazole.

The patient underwent stereotactic biopsy and drainage of the most anterior brain abscess, which revealed purulent material but demonstrated no growth on culture.

One month following admission, the patient's visual acuity remained hand motions in the right eye. A view of the posterior pole continued to be obstructed by fibrin, although repeat B-scan ultrasonography showed interval clearing of the vitreous space. At this point, she was deemed neurologically stable and discharged to a long-term care facility for ongoing antibiotic treatment.

DISCUSSION

We report the finding of restricted diffusion on DWI in 2 cases of subretinal abscess. The only previously reported case concerned a 41-year-old woman in whom DWI showed restricted diffusion in the subretinal space, and Gram stain of purulent subretinal fluid from the enucleated globe revealed filamentous gram-positive bacteria consistent with Nocardia (9).

Our cases differ from the previously reported case in that the restricted diffusion in the subretinal space was apparent on DWI at an earlier stage in the disease process. Based on a correlation of the ophthalmoscopic and imaging findings in the retina and brain, a presumptive diagnosis of abscess could be made promptly. In the first case, intensive treatment allowed preservation of useful vision, as has been described in subretinal abscess caused by S. aureus (10) and Aspergillus (11). Early diagnosis is critical, as subretinal abscess caused by other organisms such as Nocardia asteroides (12) tends to respond poorly to antibiotics. In our second case, DWI identified an anteriorly located abscess that could not be visualized on ophthalmoscopy or B-scan ultrasonography. Although the visual outcome in the second patient was poor, prompt treatment spared the patient loss of the eye.

Subretinal abscess is a rare presentation of endogenous endophthalmitis, with Nocardia species being the most commonly reported organism (11). Risk factors include endocarditis or structural cardiac abnormality, immunosuppression, and intravenous drug use (10). Differentiating between bacterial or fungal infection and a sterile or viral uveitis may be complicated by the low sensitivity of vitreous culture and an often poor view to the fundus in the setting of vitritis. As our patients lacked typical features of endophthalmitis (hypopyon, vitritis), the imaging findings were particularly helpful.

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