Revisiting retrosplenial amnesia: Injury of the crus of the fornix following splenial hemorrhage - A 3D magnetic resonance imaging case report.

Lesions involving the region immediately posterior to the splenium of the corpus callosum have traditionally been associated with "retrosplenial amnesia." However, the crus of the fornix (FxCr), which runs immediately beneath the splenium, is anatomically vulnerable to secondary involvement that may be overlooked on conventional magnetic resonance imaging (MRI). Because the fornix constitutes a critical efferent pathway of the hippocampal Papez circuit, disruption of the FxCr can contribute to amnesia that may resemble the effects of retrosplenial cortical dysfunction. This possibility was raised in the original single-case description of "retrosplenial amnesia," although direct assessment of the fornix was not feasible with the imaging techniques available at that time." We report an adult who developed episodic memory impairment following a splenial region hemorrhage with intraventricular extension. Formal neuropsychological assessment demonstrated profound anterograde amnesia with preservation of other cognitive functions, indicating selective impairment of episodic memory. Initial conventional MRI demonstrated that the hemorrhage was confined to the splenium of the corpus callosum, without definite involvement of the retrosplenial cortex. On follow-up imaging, high-resolution 3D T1-weighted MRI with multiplanar reconstruction revealed focal discontinuity of the left FxCr immediately beneath the splenial lesion with chronic encephalomalacia, while the hippocampi and medial temporal structures remained structurally intact. I-123 IMP single photon emission computed tomography showed hypoperfusion in the left hippocampus and the anterior and posterior cingulate gyri, without significant hypoperfusion in the retrosplenial region, consistent with downstream functional disruption of the Papez circuit secondary to FxCr disconnection. These findings provide contemporary structural and functional evidence that injury to the FxCr may represent a critical substrate of amnesia previously attributed primarily to retrosplenial lesions. Routine evaluation of the fornix, particularly the FxCr, should be incorporated into the diagnostic assessment of patients with splenial or retrosplenial pathology accompanied by amnesia.