From histology to neurocognition: the influence of tumor grade in glioma of the left temporal lobe on neurocognitive function

The morphology of a tumor and the histologic grade assigned to it represent the behavioral nature of a neoplasm and, generally, the prognostic value that can be expected. Such a histologic criterion is applied to tumors throughout the body, and the brain is not exceptional in this regard. Where the brain is exceptional is in the fact that it functions in a neurocognitive context that can be encumbered by way of pathologic insult, including neoplastic transformations. Concertedly, it has been proposed in practice that the degree of tumor grade may influence the neurocognitive function (NCF) in the brain as established by the behavioral context of the tumor.

This supposition is explored by Noll et al¹ in a study to determine the relationship between histologic grade and NCF in patients with glioma of the left temporal lobe prior to surgical resection. By assessing the preoperative neuropsychological profile of each patient with a glioma histologically low-grade (II) or high-grade (III or IV), Noll et al found that patients with confirmed high-grade lesions experienced greater rates of neurocognitive impairment. The frequency of neurocognitive impairment was demonstrated with greater relative frequency in patients with confirmed high-grade tumors than those with low-grade tumors. Measures of verbal learning, processing speed, executive functioning, and language were evaluated and, as the report would indicate, patients with high-grade tumors fared more poorly than their lower-grade counterparts, although it is important to note that the majority of patients, irrespective of neoplastic grade, exhibited impaired NCF (74% of all patients on at least one neurocognitive measure).¹

Interestingly, deficit of NCF in high- and low-grade gliomas was observed independently of lesion size, supporting the notion that it is the rate of neoplastic growth, and hence neuronal damage, that most greatly impacts NCF as defined by what can be collectively referred to as “tumor momentum.”² In this regard, lesions of higher histologic grade involve greater deficit of NCF on account of their more aggressively infiltrative nature, which neoplastically enhances at a greater rate. Greater neuroplasticity may be observed in such lower-grade tumors, thereby permitting more pronounced migration of NCFs to nearby anatomic structures with temporal regard.^3–6 The similarity in decreased NCF was more analogous in grades II and III lesions than IV. This may be attributed to the lack of combined necrosis and vascular proliferation in diffuse astrocytoma (grade II) and anaplastic astrocytoma (grade III) than in glioblastoma (grade IV), which also has more noted brisk mitotic activity.

As glioblastoma is markedly polymorphic with variances in astrocytic cytology and differentiation—some being well differentiated with admixed anaplastic cells and others with densely anaplastic cellularity—there is greater potential for increased intracranial pressure that may occasionally result in hemorrhage. Such high-grade lesions are oftentimes more expansive, resulting in edema that produces marked mass effect that can, as one would expect, produce more immediate deficits of NCF. These deficits are more prevalent in glioblastoma than in more well differentiated astrocytomas which infiltrate into the intact parenchyma without very much disturbance to neighboring structures.

In effect, it is perfectly sensible to conclude that histologic grade would correlate to the degree and extent of impairment in NCF as established by the morphology and behavioral cytology evident by histopathologic evaluation. Glioblastomas would very well present with more frequent deficits of NCF, and the dissemination patterns would support this supposition.

Conflict of interest statement. None declared.