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Temporally-Specific Retrograde Amnesia in Two Cases of Discrete

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Temporally-Specific Retrograde Amnesia in Two Cases of Discrete HIPPOCAMPUS 9:247–254 (1999) Temporally-Specific Retrograde Amnesia in Two Cases of Discrete Bilateral Hippocampal Pathology Narinder Kapur1,2* and David J. Brooks3 1Wessex Neurological Centre, Southampton General Hospital, Southampton, England 2Department of Psychology, University of Southampton, Southampton, England 3MRC Cyclotron Unit, Hammersmith Hospital, London, England ABSTRACT: The role of the hippocampus in retrograde amnesia re- tion.’’ As Squire and Alvarez (1995, p 173) have pointed mains controversial and poorly understood. Two cases are reported of out, data relating to the severity and pattern of retro- discrete bilateral hippocampal damage, one of which was a rare case of grade amnesia have implications for theories of memory limbic encephalitis secondary to the human herpes virus 6. Detailed memory testing showed marked anterograde memory impairment, but consolidation. They concluded that ‘‘the facts of tempo- only mild, temporally-limited retrograde amnesia that covered a period of rally-graded retrograde amnesia, and virtually all ac- several years in both autobiographical and factual knowledge domains. counts of this phenomenon that involve the concept of The absence of extensive retrograde amnesia in these two cases points to a memory consolidation, suggest that memory storage and time-limited role for the hippocampus in the retrieval of retrograde memories, and suggests that entorhinal, perirhinal, parahippocampal, or retrieval come eventually to be supported by neocortex neocortical areas of the temporal lobe may be more critical than the as the result of gradual changes in connectivity within hippocampus proper for long-term retrograde memory functioning. Our neocortex.’’ findings offer general support to theories of memory consolidation that Human lesion studies have offered some clues as to propose a gradual transfer of memory from hippocampal to neocortical the role of the hippocampus in retrograde memory, dependency. Hippocampus 1999; 9:247–254. ௠ 1999 Wiley-Liss, Inc. although many of the published studies suffer from some KEY WORDS: human herpes 6 virus; limbic encephalitis; remote limitations. Investigations relating to the patient HM memory have variously reported a mild retrograde amnesia of 1–2 years to a retrograde amnesia of up to 13 years. As has The role of the hippocampus and related structures in retrograde been pointed out elsewhere (e.g., Ogden and Corkin, memory functioning has only recently become the focus of research in 1991; Kapur et al., 1994a), there are some difficulties in cognitive neuroscience. The status of retrograde amnesia in the human interpreting HM’s retrograde memory performance. In amnesic syndrome and its neural basis is more uncertain and controversial addition, his lesion is not restricted to the hippocampus than the status of anterograde amnesia. As noted by Hodges (1995, p 101) (Corkin et al., 1997), and so his case study has a limited in a recent review, ‘‘Much has been learnt, but uncertainty remains in many amount to offer with regard to our understanding of the areas.’’ While the hippocampus has always been strongly implicated in role of the hippocampus in retrograde memory function- memory formation, there has been a greater emphasis on neocortical or ing. hippocampal-neocortical interaction in the case of retrieval of stored Schnider et al. (1994) reported dense anterograde and memories (Eichenbaum, 1997). In his classic paper, Squire (1992, p retrograde amnesia in their patient who suffered bilateral 222–223) concluded on the basis of evidence up to that point, ‘‘The facts of infarction of the hippocampi, with extension of the retrograde amnesia can be summarized as follows: 1. When damage is infarct on the left side to more lateral structures, limited to the CA1 region of human hippocampus, retrograde amnesia is including the parahippocampal gyri and neocortex of limited to a period of a year or two at the most. 2. In patients with more the posterior temporal/anterior occipital lobe. Although complete damage to the hippocampal formation, retrograde amnesia can be formal retrograde memory test data were not reported, extensive and temporally graded across a decade or more, with sparing of the memory loss for pre-illness information appeared to very old memories. 3. Hippocampal damage causes retrograde amnesia for equally affect autobiographical and semantic memories, both factual information and autobiographical, event-specific informa- and to extend back to the patient’s childhood with little in the way of a temporal gradient. The patient also had a significant naming deficit, and there may have been Grant sponsor: Wellcome Trust; Grant number: 045045. some contribution of this deficit to part of his retrograde *Correspondence to: Professor N. Kapur, Wessex Neurological Centre, Southampton General Hospital, Southampton, SO16 6YD England. amnesia. Lee et al. (1992) reported a 4-month period of E-mail: [email protected] autobiographical retrograde amnesia in their patient Accepted for publication 3 March 1999 who suffered an amnesic attack associated with a tempo- ௠ 1999 WILEY-LISS, INC. 248 KAPUR AND BROOKS ral lobe seizure. The anterograde memory impairment in this subtle histological changes in other areas or remote hypoactivation patient was more variable than in a classical amnesic syndrome. It effects of the hippocampal infarcts, particularly with regard to is of note that an MR scan carried out several days after the adjacent cortical areas and the mammillo-thalamic system. A apparent resolution of the retrograde amnesia showed bilateral more recent PET study of this patient (Kapur et al., 1999) showed hippocampal lesions, with no visible extension into more lateral additional pathology in the right thalamus, and so the role of the structures. hippocampal damage in this patient’s retrograde amnesia remains In a later study, Schnider et al. (1995) found that their patient, uncertain. a 55-year-old farmer with bilateral hippocampal lesions who was In light of the above studies, it is clear that further evidence different from the case reported by Schnider et al. (1994), was needs to be gathered from well-documented cases of discrete unaware of personal events over the previous 10–15 years. He hippocampal lesions in order to clarify the role of the hippocam- recognized the photographs of cattle that he owned more than 15 pus in retrograde memory function. Although such patients are years ago, but not more recent ones. He was also reported to have rare, we were able to study two such cases whose hippocampal a temporally-graded retrograde amnesia for politicians who had lesions could be verified using advanced brain imaging proce- become prominent over the previous 10–15 years. The lesions dures. One of these cases was an instance of paraneoplastic appeared to be restricted to swelling of both hippocampi, though encephalitis induced by the human herpes virus 6 in a patient who it is difficult to be certain that there was no swelling of adjacent was immune-compromised due to a bone marrow transplant hippocampal formation structures. The aetiology of the lesion is procedure (Drobyski et al., 1994). uncertain, though the primary medical condition of the patient was systemic lupus erythematosus (SLE). The relatively sudden onset of the amnesia suggests a vascular aetiology, though the authors themselves suggested a pathology related to an autoim- CASE 1 mune process. Reported cerebral complications of SLE tend to be more diffuse than selective hippocampal pathology of the type BE (dob March 24, 1946) is a right-handed man who worked reported here, and there remains the possibility of additional as a college lecturer. His past medical history was unremarkable lesions in this case that were not detected on MRI. A further note except for mild symptoms in the early 1980s related to a possible of caution is that the patient was a farmer, and it is not clear the diagnosis of rheumatoid arthritis. These symptoms resolved with a extent to which he was interested in public events of the type that change in dietary regime, and he only took medication for a short were used as part of the retrograde amnesia assessment; no formal period of time. He developed a viral encephalitis in May 1991, test scores or control data were reported in this respect. which was accompanied by acute memory loss for recent events, Reed and Squire (1998) found extensive episodic and factual an odd sense of smell, and episodes of de´ja` vu. In the days retrograde amnesia following hippocampal pathology only when following admission, he had two grand mal seizures. There were that pathology was accompanied by lesions in additional temporal no positive findings to suggest a herpes simplex virus, though lobe neocortex and not when it was confined to the hippocampus antiviral treatment was given. A CT scan on the day of admission alone. These patients with extensive retrograde amnesia had was reported as normal, as was a routine MRI scan carried out 11 lesions associated with herpes simplex encephalitis. Mattioli et al. days after admission. EEG investigations around this time showed (1996) reported FDG (fluorodeoxyglucose) PET scan data in a bilateral temporal lobe abnormalities. case of focal retrograde amnesia, and found evidence of bilateral In May 1992, a further MRI scan was carried out using a hippocampal and anterior cingulate glucose metabolic values. The specially designed protocol (described in Kapur et al., 1994b) that resolution of their scan did not permit the more detailed gave detailed views of memory-related brain structures. The scans anatomical fractionation of the lesion. were independently reviewed by two neuroradiologists, who were In one of the few post-mortem studies of amnesic patients blind to the neuropsychological test data. The scans were (Rempel-Clower et al., 1996, p 5252), the authors concluded that interpreted as showing discrete bilateral lesions (Fig. 1) that were ‘‘temporally-graded retrograde amnesia covering 15 years or more restricted to the hippocampus (due to some degree of artefact on can occur after relatively circumscribed damage to the hippocam- axial slices, only coronal images are shown here).
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