Radiological Case: Lucent MCA sign on head CT after heart valve surgery

CASE SUMMARY

A patient presented to our institution for mitral valve replacement surgery involving cardiopulmonary bypass. Within 24 hours after the procedure was completed, the patient was noted to have a left hemiparesis on physical examination during the recovery period. A noncontrast CT scan of the head was then performed (Figures 1A and 1B).

IMAGING FINDINGS

Blurring of the gray-white matter and sulcal effacement were present within the right middle cerebral artery (MCA) distribution, raising suspicion for acute ischemic infarct (Figure 1B). In the region of the sylvian fissure within the right MCA, a lucent focus consistent with fat (not gas) was noted. This represented a fat embolus (Figure 1A). This was not present on a CT scan performed 2 months earlier for headache symptoms (Figure 2). The lucency demonstrated Hounsfield density equivalent to fat. The patient eventually stabilized and was discharged. Multiple follow-up studies up to 10 months after the embolic event demonstrated a chronic right MCA infarct with the persistent lucent embolus in the right middle cerebral artery (Figure 3).

DIAGNOSIS

Acute stroke caused by fat macroembolism following mitral valve replacement surgery with lucent MCA sign

DISCUSSION

Thromboembolism is a commonly known complication of cardiac surgery. Atheroembolism is not as well-known and is usually described in the literature as microembolism.¹ We present a case of fat macroembolism causing acute stroke as a perioperative complication diagnosed by noncontrast CT scan following mitral valve replacement surgery.

While thromboembolism has been frequently described in the medical literature as a source of stroke, cholesterol microembolism also occurs. This sometimes is referred to as atheroembolism, which can commonly occur from cardiovascular intervention including surgery or catheterization procedures. This syndrome is recognized clinically from end-organ damage related to microemboli in small arterioles.¹ In an autopsy study of 221 patients who had undergone myocardial revascularization or valve operation between 1982 and1989, cholesterol embolization or abnormalities consistent with cholesterol embolization were noted in 48 patients (21.7%) as compared to thromboemboli (6.3%). Twenty-six percent of coronary revascularization as opposed to 9% of valve replacement surgery demonstrated cholesterol emboli.² The brain was the most common site of atheroembolism and severe atherosclerosis of the ascending aorta increased the risk for cholesterol emboli.²

In contradistinction to cholesterol microemboli, it is possible for larger fat macroemboli to cause larger arterial occlusions. An autopsy series described cerebral fat emboli related to cardiac surgery. Two instances of fat macroemboli were noted following valve replacement and cardiac transplant, respectively. A third case involving valve replacement demonstrated microemboli.³

The imaging diagnosis of stroke caused by perioperative emboli is a challenging one. Computed tomography performed in the acute stages of ischemic stroke often is negative. MRI may be contraindicated because of a cardiac pacemaker or the patient’s clinical condition.CT signs of acute infarct usually include gray-white matter blurring and sulcal edema. Occasionally, hyperdensity is identified within the course of the middle cerebral artery. The “dense MCA sign,” a finding indicative of a thromboembolic clot, is well documented in literature.⁴

Our case is notable in that intracerebral macroscopic fat emboli causing acute cerebral infarct documented by CT have been reported rarely involving only mitral and aortic valve replacement surgery.^5-8 Although cardiopulmonary bypass is needed for both procedures,valve replacement requires different surgical techniques than coronary revascularization. This might explain the contrast in the literature that demonstrated a higher incidence of fat microembolization in revascularization cases.² The etiology of the fat macroemboli is most likely related to surgical incision around the cardiac valve near epicardial and/or pericardial fat. Both an immediate or delayed cerebral event is possible if the fat is initially incorporated into a suture line and then released into the circulation. This is consistent with our clinical presentation. A similar mechanism was also postulated in the autopsy series by Ghatak et al.³

As illustrated with this case, fat macroemboli and thromboemboli as causes of acute stroke can be differentiated by CT.This may have potential prognostic and therapeutic implications if a novel thrombolytic agent or specific interventional angiographic device is to be considered in the future. Current thrombolytic agents normally cannot be used in an immediate postoperative setting; however, a better understanding of the possibility of fat embolism could alter therapy in the near future.

Although a larger case series is needed, macroscopic fat embolism should be considered in the differential diagnostic subset of non-hemorrhagic infarct when a patient presents with neurological symptoms following cardiac surgery, especially after a valve-replacement procedure. To our knowledge, this association of cardiac valve surgery and fat embolism is unique to our case and four other case reports.^5-8 This phenomenon is manifested as a hypodense artery, or the “lucent MCA sign” on CT imaging.

REFERENCES

1. Kronzon I, Saric M. Cholesterol embolization syndrome. Circulation. 2010. 122:631-641.
2. Blauth CI, Cosgrove DM, Webb BW, et. al. Atheroembolism from the ascending aorta: An emerging problem in cardiac surgery. J Thorac Cardiovasc Surg. 1992;103: 11041111.
3. Ghatak NR, Sinnenberg RJ, Deblois GG. Cerebral fat embolism following cardiac surgery. Stroke. 1983.14:619-621.
4. Tomsick TA, Brott TG, Olinger et al. Hyperdense middle cerebral artery: Incidence and quantitative significance. Neuroradiology.1989;31:312–315.
5. Lee T, Bartlett E, Fox A, Symoms S. The hypodense artery sign. Am J Neuroradiol. 2005; 26:2027-2029.
6. Wang YC, Fu JH and Lai PH. Hypodense artery sign in acute cerebral infarction by contrast-enhanced CT. Neurology. 2009;73;e16.
7. Abend NS, Levine JM. Hypodense middle cerebral artery with fat embolus. Neurocritical care. 2007;6: 147-148.
8. Malik, Amer M. Portable head computed tomography in the diagnosis of cerebral fat embolism secondary to cardiac surgery. The Neurohospitalist. 2012;2:154-155.