Response to: An objection to "Congenital external hydrocephalus in a dog".

Abstract

We appreciate the interest in our article detailing the diagnosis of external hydrocephalus (Gomes et al. 2019). Dr. Hasegawa raises an important point: that definitive differentiation between subdural haemorrhage and external hydrocephalus currently requires histopathological confirmation. Veterinary nomenclature is currently ill-defined, since external hydrocephalus and subdural haemorrhage are both rare in dogs (De Lahunta et al. 2015). The subdural space is a “potential” space which does not occur in the normal living animal, only developing between the dura mater and the arachnoid under certain circumstances (Haines et al. 1993, De Lahunta et al. 2015). Dr Hasegawa quotes three publications describing subdural fluid accumulation, two case reports and a small case series. In the first case, (Kitagawa et al. 2008) MRI of the brain revealed a compressive mass and marked enlargement of the ventricles. The mass had heterogeneous signal, T1W hypointense and T2W hyperintense centre and a T1W isointense and T2W hypointense periphery, with partial contrast enhancement. This mass was interpreted to be a chronic subdural haematoma with suspected re-haemorrhage, confirmed during craniotomy. These imaging characteristics are clearly distinct from those in our case, describing a clear-fluid filled cavity and no imaging characteristics of haemorrhage. The second case (Asakawa et al. 2010) described a brain MRI with ventricular asymmetry, a large bilateral subdural accumulation of fluid, slightly T1W hyperintense compared with ventricular cerebrospinal fluid (CSF), not supressing on fluid attenuated inversion recovery (FLAIR). A gravity-dependent fluid line was present as well as a T2*W susceptibility artefact. Post-mortem histopathology confirmed the presence of serosanguinous fluid containing clotted blood in the subdural space, with investigations confirming a neuronal ceroid lipofuscinosis. In our case, there was a more extensive generalised dilation of the subarachnoid space, with the ventricular system size being largely preserved. The fluid signal characteristics in our case indicated a slightly more proteinaceous fluid than CSF, clearly supressed on FLAIR, with no susceptibility artefact on T2*W sequences. Considering the fluid characteristics on MRI, we feel confident that no haemorrhage was evident, although we agree that fluid analysis would have been necessary to definitively confirm this. The case series (Lehner et al. 2020) described dogs that underwent ventriculo- and cystoperitoneal shunting, alongside follow-up MRI. Relevant cases for comparison were: case 2 where follow-up MRI revealed the presence of “subdural fluid” compressing the cerebral parenchyma and case 6 where follow-up MRI revealed a unilateral lesion and bilateral extra-axial fluid accumulation interpreted to be a subdural haematoma and bilateral hygroma (causing brain compression), despite being later described as being a massive subarachnoid fluid accumulation. These cases underwent invasive procedures that could indeed have created a subdural space precluding comparison with our case. MRI signal characteristics of the fluid were not described in detail. Published images in these cases reveal a severe degree of brain compression, resembling reports previously described and published under the term external hydrocephalus (Dewey 2002, Dewey et al. 2003). External hydrocephalus had been described previously in veterinary medicine in two dogs, two cats and a foal (Dewey 2002, Dewey et al. 2003, Oey et al. 2011). Based on the questions raised by Dr. Hasegawa, the diagnosis of an external hydrocephalus would also be debatable in those cases where a post-mortem analysis was not performed. In the absence of histopathological confirmation, this becomes a terminology/nomenclature discussion. We are reminded of a similar exchange between researchers describing the controversy around subdural myelography years ago (Lamb 1997, Scrivani et al. 1997). On imaging findings alone, it is currently challenging to differentiate between extra-axial fluid in a subdural or subarachnoid location. We consider that a subdural space fluid accumulation would most likely be secondary to a pathological process (inflammation, neoplasia, trauma) particularly in the presence of haemorrhage or haematoma, or following surgical intervention. In the absence of predisposing factors leading to the formation of a subdural space, as in our case, we believe it is more reasonable to interpret a bilateral extra-axial generalised fluid accumulation as an external hydrocephalus. Appendix S1. Supporting information Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.