Heavy marijuana use impairs cerebral blood flow

Chronic pot smokers may experience problems with thinking or remembering even after abstaining, and decreased cerebral blood flow may be the cause, according to researchers at the National Institute on Drug Abuse.

"Marijuana users who smoke 11 joints or more per week have deficits in cerebral blood flow. In heavier users, these deficits persist for at least a month," said lead author Ronald I. Herning, Ph.D., a research scientist at the institute.

These changes in the brain observed early during the quitting process may explain previous reports of cognitive deficits observed in marijuana users, he said.

Herning and colleagues used transcranial Doppler sonography to measure flow velocity in the anterior and middle cerebral arteries in a study published in the February issue of Neurology.

Four groups underwent sonography monitoring during a month of abstinence:

• 11 light users who had smoked about 11 joints a week

• 23 moderate users who had smoked nearly 44 joints a week

• 20 heavy users who had smoked about 130 joints a week

• 18 controls

Researchers established a baseline by measuring blood flow for all subjects within three days of admission and then again at 28 to 30 days after monitored abstinence. All noncontrol subjects were observed on an inpatient basis.

During the first round of sonography, both mean and systolic blood velocities for the left and middle cerebral arteries were higher in the marijuana-using subjects compared with controls. Right and left anterior cerebral arteries in the marijuana groups exhibited higher systolic blood velocities compared with controls. The pulsatility index (PI), which measures the amount of resistance to blood flow, was higher for both sets of cerebral arteries in the marijuana users as well.

For both sets of middle and anterior cerebral arteries, blood velocities were not significantly different among the three marijuana-using groups. After a month of abstinence, however, light to moderate marijuana users experienced a significant improvement in PI values and systolic velocity, while the heavy users showed no improvement.

"The PI values of about 1.00 that we observed in the heavy users are similar to those found in patients with chronic hypertension and might put marijuana users at risk for small vessel disease," Herning said.

Because the study observed abstinence for only a month, further research is needed to determine whether the blood flow deficits in heavy users are permanent, he said.

For more information from the online Diagnostic Imaging archives:

Imaging untangles mystery of drug-addicted brain

Mood disorders hinder drug abuse treatment

Neuroimaging reveals damage from nightclub drug Ecstasy

MR brain scan documents substance abuse damage