Assessing the Chronic Neuropsychologic Sequelae of Human Immunodeficiency Virus–Negative Cryptococcal Meningitis by Using Diffusion Tensor Imaging

MR Imaging Acquisition

All our patients received MR imaging within 3 days after hospitalization, and these 15 patients underwent both MR imaging and DTI study between March and May 2010. The mean interval between first MR imaging (MR imaging during acute phase but without DTI study) and first including both MR imaging and DTI studies was 60.5 ± 39.6 months. For initial evaluation on admission and follow-up, MR imaging was performed by using a 3T unit (Signa Excite HD; GE Medical Systems, Milwaukee, Wisconsin). All sections were acquired along the anterior/posterior commissure plane, which was ensured by a multiplanar T1-weighted localizer image before each study. Axial T1-weighted images, axial T2-weighted images, and T1-weighted enhanced images were acquired to evaluate intracranial lesions.

DTI Data Acquisition

During follow-up, DTI datasets were acquired by using single-shot diffusion spin-echo, echo-planar imaging with a TR/TE of 15 800 millisecond/minimum, a 2.5-mm section, a matrix of 128 × 128, number of excitations of 3, and a FOV of 25.6 × 25.6 cm, yielding an in-plane resolution of 2 mm, with a total acquisition time of 12 minutes. DTI encoding entailed 13 noncollinear directions with b = 1000 s/mm² and a nondiffusion T2-weighted image. Contiguous sections (n = 55) were obtained without an intersection gap to achieve total cerebral coverage.

The DTI sets were transferred to an off-line workstation for further analysis by the FuncTool diffusion tensor protocol (Advanced Workstation 4.2; GE Healthcare), which contained a preprocessing function to remove echo-planar imaging distortions, including scaling, shearing, and translation due to eddy current effects from diffusion gradient. The distortion-corrected data were then interpolated to attain isotropic voxels and decoded to obtain the tensor field for each voxel. The algorithm computed each pixel location the 6 coefficients of the diffusion tensor. The tensor field data were then used to compute the DTI metrics, including the mean diffusivity (ADC) and FA for each voxel.

Neuropsychologic Testing

A clinical psychologist blinded to the patients' exposure status performed the neuropsychologic tests. Subjects completed a test battery assembled to assess executive function, attention, and memory. Neuropsychologic evaluation was performed by using subtests of WAIS-III.¹⁷ These included information that measured general knowledge, digit span, vocabulary ability to define 35 words, arithmetic, letter-number sequencing, comprehension, similarities, picture completion, matrix reasoning, block design, picture arrangement, digit symbol, and object assembly. The subtests of vocabulary, similarities, and general knowledge yielded the verbal comprehension; picture completion, block design, and matrix reasoning yielded the perceptual reasoning; and arithmetic, digit span, and letter-number sequencing yielded the working memory.

CASI¹⁸ was obtained in all subjects. The CASI, developed as a brief screening cognitive assessment test for comparative studies in Asian and American populations, had 9 domains of cognitive function (ie, attention, concentration, orientation, short- and long-term memory, language ability, visual construction, word list generation, abstraction, and judgment), with scores ranging from 0 (worst) to 100 (best). The mean cognitive scores for measures of neuropsychologic testing were generated by averaging the standardized data from verbal comprehension, perceptual reasoning, working memory of WAIS, and the CASI. Mean cognitive scores were applied for further correlation by DTI parameters.

ROI Analysis of DTI Data

The MR imaging findings in CNS cryptococcosis included abnormal dilated Virchow-Robin spaces, cryptococcomas, abnormal lepto-meningeal enhancement, hydrocephalus, and focal gray matter and WM lesions.^19–22 Meningitis also could lead to periventricular WM injury by glial cell infiltration.¹⁵ The spectrum of abnormalities on MR imaging reflected potential anatomies of fungal invasion. Based on these reports, several major WM and deep gray matter areas were selected for ROI analysis (Fig 1).

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Fig 1.

Location of the ROIs in a healthy control. A, WM of the high frontal and parietal lobes located anterior and posterior to the central sulcus on the most caudal section on which they were visible and on the following cranial section. B, Periventricular WM was measured on the subsequent caudal section. C, ROIs at the genu and splenium of the corpus callosum were placed on 3 consecutive sections. The anterior and posterior internal capsules, caudate nucleus, globus pallidus, putamen, and thalamus were measured on 2 contiguous sections. D, Orbito-frontal WM was placed on the most caudal section of the lateral ventricles. Occipital-lobe ROIs were placed in the optic radiations of 2 contiguous sections, starting from the most caudal section on which the occipital horn of the lateral ventricle was imaged. Temporal lobe WM ROIs were placed lateral to the temporal horn of the lateral ventricle (not shown).

A radiologist measured the ROIs, 50–100 mm² depending on the anatomic region, which were confirmed by another radiologist to avoid malpositioning. All ROI analyses were conducted on DTI-derived maps, including the null image (b value = 0) and their corresponding color-coded FA maps to ensure exact anatomic locations. The regions where ROIs were placed appeared normal on conventional MR imaging and were fixed a priori in all patients as well as in controls. These measurements were performed according to the methods described in previous studies.^14,15,23

In WM analysis, ROIs were placed on the genu and splenium of the corpus callosum; the anterior and posterior limbs of the internal capsule; and the subcortical orbito-frontal, high frontal, parietal, occipital, temporal, and periventricular WM. For deep gray matter analysis, ROIs of the caudate nucleus, globus pallidus, putamen, and thalamus were selected. Raters were blinded to the subjects' details, and the ADC and FA for the 24 brain areas were measured.

Statistical Analysis

First, the age and education level between the 2 patient groups were analyzed by using the Student t test. Sex difference was analyzed by χ² test or the Fisher exact test, where appropriate. The neuropsychologic test scores between patients and control subjects were estimated by 1-way analysis of covariance model with the participant age and sex as covariates. Statistical differences in the DTI parameters of each ROI between the 2 groups were estimated by MANCOVA model, with the participant age and sex as covariates. The MANCOVA model was defined by one between-subject factor and 24 ROIs as dependent variables, with multiple comparison collection (Bonferroni correction). Secondary partial correlation analyses (2-tailed) between nonparametric tests and DTI parameters showing differences between the 2 groups were performed, controlling for age and sex.

High CSF cryptococcal antigen was reportedly associated with poor outcome and high prevalence of cryptococcal-related intracranial lesions.⁵ Cognitive impairment, as a CM complication, has previously been attributed to hydrocephalus and elevated intracranial pressure in HIV subjects.²⁴ Last the partial correlation test was chosen to examine the relationship between CSF cryptococcal antigen and declining DTI parameters, controlling for age, sex, and ventricle size (Evan ratio). Statistical significance was defined at P < .05. All statistical analyses were performed by using the SPSS software, version 10.0 (SPSS, Chicago, Illinois).