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E1765. Hippocampal Diffusion Weighted Imaging (DWI) Review: Patterns That Can Assist Differentiating Disorders
Authors
  1. Manav Bhalla; Medical College of Wisconsin
  2. David Smullen; Medical College of Wisconsin
  3. Sonia Gill; Medical College of Wisconsin
  4. Mohit Agarwal; Medical College of Wisconsin
  5. Andrew Klein; Medical College of Wisconsin
Background
The hippocampus can be involved in a variety of pathologies, ranging from relatively benign entities like transient global amnesia (TGA) to more concerning and morbid conditions like seizure or stroke. Considerable overlap in the clinical presentation of these neurologic disorders poses a challenge in the diagnoses. The pattern of Diffusion weighted imaging (DWI) signal abnormalities associated with these etiologies can be helpful in differentiating diseases and guiding diagnoses.

Educational Goals / Teaching Points
1. Common pathologies affecting hippocampi. 2. Pathophysiology underlying DWI signal abnormalities. 3. DWI imaging patterns that can differentiate entities.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
The primary concern for restricted diffusion in MRI brain is acute/subacute stroke. While infarcts can be territorial or embolic in distribution, a punctate focus of hippocampal restricted diffusion in acute/hyperacute setting may be related to non-infarct etiologies such as transient global amnesia (TGA), a diagnosis that does not require extensive investigation or follow up and has an altogether different management protocol, particularly when compared to more serious confounding conditions such as Transient ischemic attack (TIA), stroke, epilepsy, herpes encephalitis etc. Extra-hippocampal involvement, however, excludes TGA. Potentially reversible DWI findings in seizure result from cytotoxic and vasogenic edema related to disproportionate glucose metabolism compared to blood flow. T2 weighted signal alteration is seen simultaneously in seizures, as opposed to infarct where DWI findings precede T2 findings. Paraneoplastic limbic encephalitis which usually has insidious onset typically spares basal ganglia. DWI-MR findings often precede clinical diagnosis in these cases. Concomitant involvement of parieto-occipital cortex and basal ganglia with sparing of thalami, subcortical or deep white matter and cerebellum may suggest hypoglycemia. On the other hand, similar pattern with thalamic involvement may suggest hypoperfusion or hypoxia.

Conclusion
The pattern of hippocampal involvement on MR-DWI can help guiding towards diagnosis by suggesting underlying pathophysiology.