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Acute Amnestic Syndrome – “Watch Out” for Fornix Infarct
Vikram V Holla1, Shailesh Shivraj Pene2, MN Rakesh Sharma2
1 Department of Neurology, Narayana Multispecialty Hospital, Mysuru, Karnataka, India
2 Department of Radiology, Narayana Multispecialty Hospital, Mysuru, Karnataka, India
Vikram V Holla,
Department of Neurology, Narayana Multispecialty Hospital, Mysuru - 570 019, Karnataka
Source of Support: None, Conflict of Interest: None
The amnestic syndrome is a disorder of recent and short-term episodic memory with normal immediate working memory, semantic, motor and procedural memory and other cognitive domains unaffected. It is commonly caused by impairment in structures forming Papez circuit due to transient global amnesia, thiamine deficiency, trauma, encephalitis, stroke etc. We describe a case of acute amnestic syndrome due to bilateral fornix infarct with a peculiar pattern of infarct on MRI (Magnetic Resonance Imaging) scan.
A 55-year-old hypertensive woman presented with acute onset confusion and difficulty recalling recent events for five days. The patient was able to identify relatives and take care of herself, along with assisting in household activities. There was no history of significant progression since onset. There were no other symptoms. Family members attributed the onset of symptoms to a trivial head injury she had a week earlier. A CT scan done on the 4th day in view of acute symptoms post head injury did not show any acute infarct or haemorrhage. On examination, patient was conscious, alert, cooperative and partially disorientated to time. MMSE was abnormal (26 out of 30) with points deducted for errors in recall and partial disorientation to time and place. The rest of the neurological assessment was normal. The patient was diagnosed as a case of acute amnestic syndrome. Complete blood count, serum creatinine, electrolytes, liver enzymes, fasting plasma glucose, thyroid stimulating hormone and vitamin B12 level were within normal limit. An MRI brain was planned despite a normal CT brain in view of acute non-progressive nature of illness in a hypertensive middle aged woman, suspecting either a strategic lacunar infarct or a demyelinating lesion. CSF analysis was planned if MRI brain is normal to rule out encephalitis. MRI of the brain showed acute infarct in bilateral anterior fornix and genu of corpus callosum which in axial section looked like “watch out” or “warning” sign [Figure 1]. MR angiography did not reveal any significant stenosis. Echocardiography was normal. The diagnsosis of acute amnestic syndrome secondary to fornix infarct was made. The patient was managed with antiplatelet, statin and donepezil for memory impairment. The patient's memory was significantly better at 3rd month follow with MMSE of 29/30 (1 point deducted for impaired recall).
|Figure 1: Axial section of DWI (a), ADC (b) and FLAIR (c) of MRI brain of patient showing acute infarct of bilateral anterior pillar of fornix (black arrow) and genu of corpus callosum (white arrow). Infarct pattern appears like “watch out” sign (inset)|
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The fornix is a discrete white matter tract bundle that connects the hippocampus to the mamillary body, thalamus and cingulum. As a part of Papez circuit, it plays an important role in new memory formation with damage to it resulting in amnestic syndrome. Our patient presented with acute amnesia with normal initial CT scan but on MRI brain there was acute bilateral fornix and genu of corpus callosum infarct. The pattern of infarct in MRI axial image was interesting as it was resembling “watch out” or “warning” sign, as if prompting us to watch out for bilateral fornix infarct in a case of acute amnestic syndrome [Figure 1]. The subcallosal artery, a branch of anterior communicating artery, supplies both anterior pillars of fornix along with rostrum and genu of the corpus callosum. This artery can be occluded either by microangiopathic process, cardioembolic or iatrogenic occlusion secondary to aneurysm repair. Although rare, bilateral fornix infarction resulting in acute amnestic syndrome have been reported in literature [Supplementary Table]. Many other pathological conditions can involve the fornix such as midline tumours, thiamine deficiency, limbic encephalitis (viral or autoimmune), multiple sclerosis or as post-surgical complication of anterior communicating artery aneurysm repair or transcallosal approach for mid line tumour resection. Presentation of acute amnesia as predominant symptom in a stroke is rare representing <1% of all acute ischaemic stroke/transient ischaemic attacks. Infarcts in the hippocampus, anterior thalamus, mamillary body either bilaterally or mostly left if unilateral can also result in ischaemic amnesia other than fornix stroke., Acute amnesia can also result from various other causes [Table 1]. Transient global amnesia, limbic encephalitis (Viral or autoimmune), epileptic seizure (ictal or post ictal), thiamine deficiency, drug/alcohol intoxication, post traumatic concussion, anoxia, hypoglycaemia or psychogenic. Transient global amnesia is one of the most common causes of amnestic syndrome which is transient and should resolve within 24 hours after onset, unlike in our case, where amnesia lasted more than 24 hours. Trauma can also lead to amnesia by contusion of temporal lobe, subdural hematoma or by diffuse axonal injury. Head injury in our patient was trivial and is unlikely to have caused the MRI changes seen. Wernicke's-Korsokoff amnestic syndrome due to thiamine deficiency should be suspected in chronic alcoholics or malnourished patients. In our case, there was no history suggestive of chronic malnutrition.
To conclude, amnestic syndrome presentation of stroke although rare, should be a differential diagnosis especially in patients with stroke risk factors and acute presentation. Occlusion of subcallosal artery lead to infarct of bilateral anterior pillar of fornix and genu of corpus callosum which in MRI axial section may appear as “watch out” sign reminding us to watch out for fornix infarct in a case of acute amnesia.
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