The Temporoparietal Fiber Intersection Area and Wernicke Perpendicular Fasciculus

Abstract

In their recent article, Martino et al1 highlighted the anatomy and surgical importance of the temporoparietal fiber intersection area (TPFIA). We very much enjoyed reading their detailed description based upon postmortem fiber dissections, in vivo diffusion tensor imaging, and tractography as well as clinical case reports, but we noticed one significant mistake. The authors consistently mislabel the outermost, ie, lateral, of the 7 white matter tracts identified in the TPFIA as the “horizontal portion of the superior longitudinal fasciculus (SLF)” in all of their figures, the abstract and part of the text (second page of the article). Yet, the tract the authors refer to is obviously not taking a horizontal but a vertical course (cf. Figures 2 and 6 in their article). At the beginning of the results section, the authors identify it with the "posterior portion of the SLF." According to Makris et al,2 the only vertical component of the SLF belongs to its fourth subdivision (SLF IV), ie, the vertical part of the arcuate fasciculus (AF), which Martino et al1 identify with the second fiber tract in the TPFIA. So if the outermost, vertically running fiber pathways of the TPFIA do not belong to the SLF or AF, respectively, what are they and are these fibers surgically relevant? These particular association fibers had originally been discovered by Wernicke3 in monkey brains and have subsequently been called Wernicke perpendicular (or vertical occipital) fasciculus (WpF).4-7 According to the early seminal investigation by Sachs,8 a disciple of Wernicke in Wrocław (the former Silesian Breslau), WpF is part of the stratum verticale, ie, the stratum profundum convexitatis, lateral to the AF and the stratum sagittale externum which itself contains the inferior longitudinal fasciculus. The WpF connects the inferior parietal with the lower temporal and occipital lobe, and its fibers are crossed by posterior callosal commissure fibers, projection and association pathways. These crossing fibers may cause failures to identify this tract (i) in one-fourth of the postmortem dissections of Martino et al1 and (ii) in diffusion tensor imaging as well as streamline tractography. According to our own data,9 probabilistic tractography with crossing fibers modeling can reduce such false-negative trackings in the presence of perifocal tumor edema, for example. Crossing fibers also emphasize that the tracts of the TPFIA are not arranged in strictly separate but partially interwoven layers. WpF is clinically relevant and should be preserved from surgical damage whenever possible. Lesions to WpF have—in addition to those involving Mill's basotemporal language or the visual word forming area, posterior callosal fibers and the inferior longitudinal fasciculus —been associated with preangular alexia without agraphia.10-13 In this peculiar syndrome, patients are unable to read but can still write (eg, text short-message-service messages). The closer a lesion is located to the left inferior parietal lobule, the more likely it is accompanied by agraphia or other neuropsychological symptoms subsumed by Gerstmann syndrome, whereas the closer a lesion is to the inferior temporal lobe, the more likely it will involve disorders of face or color identification (eg, hyperfamiliarity for faces or color anomia).14,15 Furthermore, the anterior portion of WpF may be part of a brain network processing the age of faces.16 We have ourselves observed 4 cases in which tumors of and/or surgical access through the TPFIA (eg, to approach subsplenial or posterior hippocampal lesions from laterally) have led to alexia without agraphia that we relate to damage to the WpF. Such WpF disconnection syndrome may be transient, but the associated deficit may also persist and turn out to be quite disabling (eg, rendering a shop assistant unable to read labels of goods and to sort items onto the shelves). Therefore, we think that knowledge of the anatomy of the WpF and its correct labeling are essential, which is why we have taken the opportunity to alert the readers of Neurosurgery to the above detailed error in the otherwise excellent article of Martino et al.1 Disclosure The authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article.