Introduction

Confabulation is a memory disturbance occurring in clear consciousness in association with disruption of retrieval and encoded quality of memory [1–3]. It is classified as spontaneous and provoked confabulation; spontaneous confabulation is confusion of ongoing reality with past events, which may result from organic amnesia, and provoked confabulation is typically elicited by questions and represents a normal response to a faulty memory [1–3].

Case report

One patient and five age-matched control subjects (three male; mean age 56.7 years, range 51–59) with no history of neurologic disease participated in this study. All subjects provided signed, informed consent, and our institutional review board approved the study protocol.

A 57-year-old female patient was diagnosed as a spontaneous subarachnoid hemorrhage (SAH) due to a rupture of the anterior communicating artery (ACoA) aneurysm and underwent clipping for a ruptured aneurysmal neck at the neurosurgery department of a university hospital (Fig. 1a). Since the onset of the SAH, she showed severe memory impairment and confabulation was provoked by questions. However, she retained her biographical memory, except birthday, birthplace, and academic ability. Brain MRI taken at 3 months after onset showed leukomalactic lesions in the bilateral prefrontal cortex and cingulate cortex (Fig. 1a). The patient showed severe memory impairment and moderate cognitive impairment: in short-term memory (65: 1 ‰), verbal memory (58: <1 ‰), visual memory (60: <1 ‰), global memory (58: <1 ‰), and cognitive function. For the provoked confabulation test, we used the Korean translation of five fables previously used for eliciting confabulations in a structured setting [4]. In the result of the confabulation test, the patient produced two items not present in the original story [4].

Fig. 1
figure 1

a Brain CT at onset shows a subarachnoid hemorrhage, and Brain MRI at 3 months after onset shows leukomalactic lesions in the bilateral prefrontal cortex and cingulate cortex. b Results of diffusion tensor tractography of the Papez circuit in a patient and a control subject. Discontinuations are observed in the bilateral crus of the fornix and anterior and posterior parts of the cingulum, and left thalamocingulate tract (arrows). c Results of thalamocortical pathway between mediodorsal thalamic nucleus and prefrontal cortex in a patient and a control subject

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Diffusion tensor tractography (DTT) data were acquired at 3 months after onset using a 6-channel head coil on a 1.5 T Philips Gyroscan Intera (Philips, Ltd, Best, The Netherlands) and analysis of DTT was performed using FSL software. For each of the 32 non-collinear diffusion sensitizing gradients, 67 contiguous slices were acquired parallel to the anterior commissure-posterior commissure line. Imaging parameters were as follows: acquisition matrix = 96 × 96, reconstructed to matrix = 192 × 192 matrix, field of view = 240 × 240 mm2, TR = 10,726 ms, TE = 76 ms, parallel imaging reduction factor (SENSE factor) = 2, EPI factor = 49 and b = 1000 s/mm2, NEX = 1, and a slice thickness of 2.5 mm with no gap.

Each neural tract of the Papez circuit was determined by selection of fibers passing through seed and target regions of interest (ROI) as follows: Thalamocortical tract: the cingulate gyrus, anterior limb of the internal capsule, anterior thalamic nuclei; Fornix: mammillary body, crus of the fornix; Mammillothalamic tract (MTT): anterior thalamic nucleus, middle portion of the MTT, mammillary body; Cingulum: anterior pole of the superior cingulum, posterior pole of the superior cingulum. Values of fractional anisotropy (FA), mean diffusivity (MD), and tract volume of each neural tract of the Papez circuit were also measured. For the control tract, thalamocortical pathway between mediodorsal thalamic nucleus and prefrontal cortex (Table 1).

Table 1 Diffusion tensor tractography parameters of the Papez circuit in the patient and normal subjects
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In terms of FA value, both the thalamocingulate tract and fornix showed significant decrement by more than two standard deviations of that of normal control subjects. The values of MD in the right thalamocingulate tract and fornix were increased by more than two standard deviations of that of normal control subjects. For the tract volume, the left cingulum and MTT showed a significant decrement by more than two standard deviations of that of normal control subjects. By contrast, both thalamocortical pathways between mediodorsal thalamic nucleus and prefrontal cortex, which for the control tract, did not show significant difference between patient and control subjects. On the DTT configuration of the Papez circuit in the patient, discontinuations were observed in the bilateral crus of the fornix and anterior and posterior parts of the cingulum. The left thalamocingulate tract between the anterior thalamic nuclei and cingulate gyrus was discontinued at the level of genu of the corpus callosum (Fig. 1b).

Discussion

In the current study, injury of the Papez circuit was investigated in a patient with SAH who showed provoked confabulation with severe memory impairment. We believe that extensive and multiple neural injuries of the Papez circuit in this patient might be concerned with provoked confabulation as well as severe memory impairment.

The Papez circuit consists of hippocampal formation, fornix, mamillary bodies, anterior thalamic nuclei, thalamocortical pathway, cingulate gyrus, cingulum, and parahippocampal gyrus. Injury of the Papez circuit is closely associated with memory impairment, and might also be considered as a major neural circuit concerned with occurrence of provoked confabulation. However, no study on the correlation between occurrence of confabulation and injury of the Papez circuit after SAH has been reported. On the other hand, even without precise estimation of the Papez circuit, several studies have demonstrated common occurrence of confabulation in the patient with SAH. In 2005, Schnider et al. reported on a patient with confabulation after ACoA rupture [5]. The patient showed injury of the right orbitofrontal cortex and basal forebrain due to the ACoA rupture, and showed severe amnesia and spontaneous confabulation. Consequently, we demonstrated occurrence of provoked confabulation in a patient with injury of the Papez circuit by SAH without injury of thalamocortical pathway between mediodorsal thalamic nucleus and prefrontal cortex.

In conclusion, extensive and multiple injury of the Papez circuit was demonstrated in a patient with provoked confabulation following SAH. However, it is case report, this study is limited; therefore we suggest conduct of further studies involving large numbers of patients and correlation analysis between results of confabulation test and specific injury of the Papez circuit should be encouraged.