Advanced MRI reveals spinal biomarker for HIV-positive patients

HIV-positive patients could benefit from the ability of MR diffusion tensor imaging to reveal microscopic changes in their spinal cords before abnormalities show up on conventional MR imaging. The early warning could identify those who would benefit from antiretroviral therapy.

The diagnosis of spinal cord involvement in patients with AIDS is difficult using conventional MRI, said lead author Dr. Christina Mueller-Mang during a scientific session on Sunday.

HIV is associated with vacuolar myelopathy, which is common in the late stages of infection. Changes occur in the lateral and posterior spinal cord. But diagnosis of vacuolar myelopathy is one of exclusion, ruling out infection, degenerative disease, and neoplastic myelopathy, said Mueller-Mang, a radiology resident at the Medical University of Vienna.

Conventional MRI is nonspecific, showing hyperintense signal in the lateral and posterior parts of the cervical spinal core, and atrophy is the most common finding.

"Diagnosis remains challenging," she said.

Mueller-Mang and colleagues decided to see if diffusion tensor imaging (DTI) could find differences in the fractional anisotropy and apparent diffusion coefficient in the spinal cords of asymptomatic HIV-positive patients compared with healthy controls. They also wanted to determine if DTI metrics could be predictive of myelopathy in HIV-positive patients.

As white matter fibers lose their integrity due to pathology, water molecules within them move in all directions (isotropically) rather than unidirectionally along a predetermined route. The reference metric for this is fractional anisotropy.

Conversely, the diffusion of extracellular water molecules is also adversely affected by lesions or damage to white matter fibers. This is measured with the ADC.

Mueller-Mang and colleagues calculated the average fractional anisotropy and ADC at the C2-3, C3-4, and C4-5 levels of 14 asymptomatic HIV-positive patients and 14 volunteers. The researchers used regions of interest at bilateral anterior, lateral, and posterior white matter regions of the cord.

The average age of HIV-positive patients was 35 years versus 33.4 years in the control group.

A conventional T2-weighted MR sequence also was performed in all patients. No patients had abnormalities on conventional T2-weighted MR images. On DTI, however, HIV-positive patients had an overall lower mean fractional anisotropy compared with controls, particularly in the C3-4 and C4-5 levels, but the difference didn't reach significance.

Patients also had decreased ADC compared with controls, again more so in the C3-4 and C4-5 levels, but not significantly.

The researchers are already into their next leg of study, examining symptomatic HIV-positive patients who have unremarkable conventional MR images.

"We think we will find a significant difference in symptomatic patients," Mueller-Mang told Diagnostic Imaging.

Researchers also want to divide asymptomatic patients into groups according to their immunological status. They may be better able to predict fractional anisotropy and ADC thresholds in patients who are "immunologically on the edge" and becoming symptomatic, senior investigator Dr. Majda Thurnher told Diagnostic Imaging.

"Those patients would then be the target for antiretroviral therapy," Thurnher said.