To the Editor:
The importance of recognizing pseudo-subarachnoid hemorrhage (PSAH) in a case of massive valproic acid overdose was well brought by Min et al.Citation1 The cerebral edema in valproic acid overdose is due to the disruption of osmotic gradient which usually occurs 48–72 hours after the drug over dosage, despite decreasing serum levels of valproic acidCitation2. However, the appearance of PSAH within the basal cisterns, in conjunction with diffuse cerebral edema, has received limited attention in medical literature. We would like to highlight the usefulness of Hounsfield units to differentiate PSAH from subarachnoid hemorrhage (SAH) with reference to the report published.Citation1
Even though the literature proposes that autopsy and CSF study are the ways to rule out PSAH, meticulous measurement of the density of different visible elements is possible through application of Hounsfield unitsCitation3 (HU)which helps to discriminate hyperintensity of SAH from PSAH. Many modern CT software programs have the ability to determine the HU of even small areas with precise differential values for normal and oedematous parenchyma, as well as for blood and the subarachnoid space, rich in protein and congested by displaced structures, by which it may help to prove or disprove suspicion of acute hemorrhage. The hyperintensity of SAH varies between 60 and 70 HU which is higher than both normal gray matter (30–40 HU) and white matter (20–30 HU),whereas that of PSAH varies between 29 and 33 HU.Citation4 With lower attenuation values, in association with signs of diffuse cerebral edema, one should probably consider pseudo-SAH and avoid more invasive testing. While treating emergency cases, erroneous diagnosis of subarachnoid hemorrhage or failure to distinguish PSAH from SAH may have direct implications on patient care. So, application of HU assists emergency physician to distinguish one condition from the other.
Declaration of interest
The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.
References
- Min YG, Tse ML. Image in toxicology: Pseudo-subarachnoid hemorrhage in a case of severe valproic acid poisoning. Clin Toxicol (Phila) 2011; 49:699–700.
- Sztajnkrycer MD. Valproic acid toxicity: Overview and management. J Toxicol Clin Toxicol 2002; 40:789–801.
- Avrahami E, Katz R, Rabin A, Friedman V. CT diagnosis of non-traumatic subarachnoid haemorrhage in patients with brain edema. Eur J Radiol 1998;28:222–225.
- Given CA, Burdette JH, Elster AD, Williams DW. Pseudo-subarachnoid hemorrhage: a potential imaging pitfall associated with diffuse cerebral edema. Am J Neuroradiol 2003;24:254–256.