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|LETTERS TO EDITOR
|Year : 2015 | Volume
| Issue : 6 | Page : 969-970
Vanishing infarct on CT—Fogging phenomenon: Perplexing scenario for the beginners
Department of Radiodiagnosis, Sree Balaji Medical College and Hospital, Chennai, Tamil Nadu, India
|Date of Web Publication||20-Nov-2015|
Department of Radiodiagnosis, Sree Balaji Medical College and Hospital, Chennai, Tamil Nadu
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Indiran V. Vanishing infarct on CT—Fogging phenomenon: Perplexing scenario for the beginners. Neurol India 2015;63:969-70
Although fogging of infarct on CT (computed tomography) has been known since its description in 1979, it could still provide a confusing situation for the beginners in radiology as well as neurology. Hence, awareness of the fogging phenomenon is essential to image the patient at an appropriate time and interpret the images accordingly.
A 24-year-old female patient presented with a history of complete paralysis of the left upper and lower limbs along with deviation of mouth to the right side for 2 days. She had a medical termination of pregnancy 10 days earlier for a missed abortion of 2 months clinical gestational age. She also had a spontaneous abortion 5 months before at 7 weeks gestational age. She neither had any previous medical illness nor any relevant family history. She had elevated antiphospholipid antibody IgM (20.76 U/ml) and IgG (290.50 U/ml). Antinuclear antibody test was positive. Her international normalized ratio was 1.58, and prothrombin time was 17.3 seconds. Her hemoglobin was reduced (8.7 g/dL), and blood sugar and lipid profile were normal. Plain computed tomography (CT) scan of the brain done on the second day after the onset of hemiplegia showed a wedge-shaped hypodense lesion measuring approximately 5 × 3 cm in the right temporoparietal lobe and gangliocapsular region [Figure 1]a,[Figure 1]b,[Figure 1]c. "Hyperdense MCA" (middle cerebral artery) sign was also seen [Figure 1]d. She received low-molecular-weight heparin and antiplatelet drugs. Plain CT scan of the brain done 11 days after the onset of hemiplegia, before the patient was discharged (a check CT scan done despite no neurological deterioration), showed no obvious hypodense lesions in the region of the infarcts demonstratable on the previous CT [Figure 2]. The second CT scan was reported as normal on the preliminary evaluation by a postgraduate radiodiagnosis resident without a proper consideration of the patient's history and a review of the previous imaging finding, which led to the confusion. The same film was brought the following day for discussion by the neurology resident. The history and previous films were reviewed by the reporting staff radiologist, and the reversal of hypodensity on the latest CT was diagnosed as "fogging phenomenon" of the cerebral infarct. The patient was discharged with advice to continue antiplatelet drugs, warfarin, and physiotherapy.
|Figure 1: (a-c) Plain CT done on 16 June 2015 shows a wedge-shaped hypodense lesion in the right temporoparietal lobe and gangliocapsular region (white arrows); (d) hyperdense M1 segment of the right middle cerebral artery—“Hyperdense MCA” sign (white arrow|
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|Figure 2: (a-c) Plain CT done on 25 June 2015 shows no obvious hypodensity in the right temporoparietal lobe and gangliocapsular region|
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Fogging phenomenon on CT denotes the absence or reversal of hypodensity in clinically well-documented stroke patients. Fogging may occur in as many as 50% of patients presenting with infarcts., It occurs typically during the subacute phase of the infarct. Fogging has also been described on T1- and T2-weighted images on magnetic resonance imaging (MRI). It has been reported between 6 and 36 days after the onset of stroke. Infarcts showing fogging on MRI are observed as isointense to normal brain in the subacute phase from the mildly to markedly increased signal on T2-weighted images seen earlier; the exact cause is not yet clearly understood. Fogging on a CT scan may be seen due to hemorrhagic transformation of infarcts and may be identified on MRI in the subacute stage. MRI may or may not show fogging at the time when fogging occurs on the CT images. Hence, cerebral infarcts tend to be underestimated or missed when images are taken during the second or the third week following stroke. The possible causes for fogging are resolution of edema, invasion by macrophages, capillary proliferation, and extravasation of blood cells. Fogging is unlikely in cases with large-vessel occlusion owing to poor collateral formation. A contrast-enhanced CT may be helpful, as the infarcted areas show enhancement in the subacute period owing to deranged blood–brain barrier., However contrast enhancement is not routinely used to image an infarct  as it would add to the cost burden apart from the risk of nephrotoxicity.
Imaging early in the course of stroke, diligent review of previous imaging studies, and awareness of the time course of imaging findings and their limitations would avoid misdiagnosis/underestimation of stroke. The learning points for radiologists and neurologists would be to avoid using CT scan as the first modality for stroke imaging in the second or third week after hemiplegia onset, especially if there is no deterioration of neurological status; and, always to review the clinical history and previous imaging findings while interpreting the CT studies, especially those done during the subacute phase of infarcts.
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[Figure 1], [Figure 2]