Brain Injury May Exist Despite Normal Post-surgery Scans

Although both neuroimaging and clinical manifestations can indicate an organic brain injury after major surgery, new research suggests the two indicators may not always be consistent.

“Neurological injury can manifest whether or not you find signs of it on a CT [computed tomography] scan or MRI [magnetic resonance imaging],” said Chiranjeev Saha, MD, a fellow in cardiac anesthesia at Toronto General Hospital, in Ontario, Canada, and lead author of a study presented at the 2010 annual meeting of the Society of Cardiovascular Anesthesiologists, in New Orleans (abstract 32).
Dr. Saha and his team had noted a high rate of neurologic complications following heart surgeries at their hospital. These likely resulted from embolic insults and episodes of hypoperfusion, he said, and presumably were compounded by multiple perioperative factors including a patient’s physiologic condition.
In search of a correlation between resulting neurologic problems and visible evidence of injury using current technologies, the investigators reviewed clinical and neuroimaging data from 106 patients who underwent cardiac surgery with cardiopulmonary bypass (CPB) at the hospital, between June 2008 and May 2009.
All patients (aged 65 to 77 years) included in the analysis had either diffusion-weighted MRI or CT scans of the brain after indications of neurologic deficit, delirium, clinical seizures or a decreased level of consciousness. Because cardiac patients commonly have pacing wires in the chest—a contraindication for an MRI—and due to the longer time that an MRI requires, the CT scanner was used for 89 assessments, and the MRI for 17.
Delirium was diagnosed in 77 patients (72%), seizures in 36 (34%) and clinically manifested perioperative stroke in 18 (17%). Imaging identified 63 patients (59%) with brain infarcts: 25 new and 38 old lesions. These patients were not always the same individuals who presented with symptoms.
The team was surprised to discover that the 43 patients with normal scans were no less likely to have delirium or seizures than were those with detectable brain injuries, either new or old. In fact, seizures were significantly more likely to occur in patients without brain infarcts (P=0.002).
Old brain infarcts were also more closely associated with seizures (24% vs. 16%) and delirium (76.3% vs. 76%) than were new infarcts. That link did not surprise Charles Hogue, MD, associate professor of cardiac anesthesia at The Johns Hopkins University School of Medicine, in Baltimore, who was not involved in the study. “The patient’s inherent cerebral disease state plays a dominant role in the manifestations of brain injury after cardiac surgery,” he told Anesthesiology News.
Given the high resolution of both MRI and CT images, how is an insult missed? One explanation could be that the imaging technologies are not sensitive enough, said David Stump, MD, professor of anesthesiology and cardiothoracic surgery at Wake Forest University School of Medicine in Winston-Salem, N.C. A lesion’s size, for example, would have to be at least three millimeters to be obvious on an MRI scan. “Most microemboli cause smaller lesions and are therefore invisible to MRI,” Dr. Stump said.
Timing, too, is imperative. “From a clinical standpoint, it may take a significant amount of time to identify injured and nonviable brain tissue,” Dr. Saha explained. For this reason, CT scans are repeated every three or four days after a patient presents with a clinical manifestation of a stroke.
To complicate matters, despite a patient having neurologic symptoms, an organic injury may not have occurred. An electrolyte imbalance, inflammatory response and hemodilution can also cause seizures, delirium and loss of consciousness.
At the same time, an organic brain injury can exist without any accompanying clinical manifestations. “Many lesions occur in brain areas not examined with a clinical neurologic exam or with psychometric testing,” said Dr. Hogue, adding that these could still have long-term functional and cognitive significance.
Alternatively, it may be the case that any clinical manifestations of the injury were too subtle. Slight changes in balance, hormone management or neurotransmitter production, as well as blood pressure, temperature or sugar regulation, are all potential symptoms of small brain infarctions, Dr. Stump said.
Unfortunately, the generalized data, combined with the study’s retrospective design, limited the researchers’ ability to tease apart these factors. Dr. Saha noted that his group is working on a paper that will discuss in more detail the relationships between specific manifestations of neurologic insult and their clinical and imaging findings.
Recommendations regarding avoiding or ameliorating neurologic complications, he added, must be individualized based on specific risk factors—prompting further preoperative investigations, modification of intraoperative management and possible changes in the surgical approach—for example, off-pump percutaneous aortic valve replacement in severe aortic atherosclerosis.
If going through with bypass, Dr. Stump said, anesthesiologists should ready the patient’s “internal environment” for the effects of rapid changes in blood sugar, insulin, temperature, blood pressure and hematocrit. “We cannot prevent all emboli, but we can better prepare the patient to deal with the onslaught,” he said. Careful management of all of these variables is vital during surgery and while a patient is waking up, he added.