Case reveal

• A large unilateral fluid accumulation is noted in the right subdural space (yellow arrowheads), which conforms to the right lateral aspect of the cerebrum. This fluid is homogeneously strongly T2 hyperintense, T1 hypointense, has no signal on FLAIR, and lacks contrast enhancement. This fluid results in a mass effect and mild leftward midline shift (pink arrowheads), with flattening of the associated gyri and sulci as well as mild to moderate compression of the right lateral ventricle, and midbrain. The right cerebellar hemisphere is minimally compressed.

• Surrounding the lateral ventricles, bilaterally, there is a thin, ill-defined rim of T2w hyperintensity with no associated contrast enhancement. 
• At the rostral aspect of the occipital lobe, and centered to the left of the dorsal sagittal venous sinus, a small, well-defined, T2 hyperintense, T1 hypointense, FLAIR signal nulling structure is noted. 
• No abnormal contrast enhancement or susceptibility artifact is noted. 
• No associated bony or soft tissue abnormality is identified.

Summary:

• Large, right sided, extra-axial subdural fluid accumulation, with associated cerebral compression. 
• Possible small periventricular, intracranial lesion. 
• Small cyst, dorsal sagittal venous sinus, left of midline (occipital lobe) 

Conclusions:

Primary consideration for subdural fluid accumulation is given to external hydrocephalus of which, differentials can include traumatic, congenital or infectious/inflammatory etiologies. Hydrocephalus ex-vacuo is considered unlikely given the associated cerebral compression and mass effect. The possibility of this lesion representing a chronic subdural hematoma is considered less likely, given the lack of associated soft tissue trauma or susceptibility artifact. The periventricular lesions might represent edema; however, given the lack of associated ventriculomegaly and contrast enhancement, normal variation should also be considered.

How we got here:

• Computed tomography angiography with an esophagram was performed the day prior to further assess the reported dysphagia. No esophageal/pharyngeal abnormalities are identified; however, right-sided intra-calvarial and extra-calvarial fluid accumulation was seen and prompted the MRI.

Follow-up:

• Due to concerns for long-term quality of life, given the dysphagia and possibility of an underlying neurogenic cause, euthanasia was elected. Note: Compression of the corticobulbar tracts in hydrocephalus is reported to cause oropharyngeal dysphagia in humans.

Necropsy:

1. Muscular fibrosis and thickening, segmental, chronic, moderate, with luminal narrowing and focal adventitial fibrosis and hematoidin/hemosiderin deposition, cranial esophagus.
2. Bronchopneumonia, neutrophilic and histiocytic, acute to subacute, mild to moderate, with rare intralesional foreign material (consistent with aspiration), lung
3. Subdural fluid accumulation, moderate, right cerebral hemisphere.

The segmental muscular thickening and fibrosis in the cranial esophagus with narrowing of the
luminal diameter was consistent with an esophageal stricture. Esophageal strictures in horses are classified
into three types; mural, mucosal-submucosa, and annular (involves all layers). Mural esophageal stricture
involves only the adventitia and muscularis, consistent with gross and histologic changes seen in this case.
Stricture of the esophagus in horses can be acquired as result of trauma to the esophageal wall, and fibrosis
and hematoidin/hemosiderin in this case would be consistent with prior trauma. Based on a conversation with
the submitting clinician and surgical intern, this foal had previously had an indwelling esophagostomy tube in
place in the same region, and this speculatively could have been the cause for the stricture at this site.
There was bronchopneumonia associated with particulate material consistent with prior barium
aspiration in a portion of the lung, but there were also lesions in other sections consistent with more recent aspiration of foreign material, which likely occurred secondary to esophageal stricture.
Consistent with antemortem imaging, there was significant cerebral spinal fluid accumulation
surrounding the right cerebral hemisphere on gross examination. Grossly, the right hemisphere of the brain
was equivocally smaller than the left after removal from the skull, so it is possible there was some mild
hypoplasia of the right cerebrum that could have resulted in the fluid accumulation. Histologically, there were
no significant differences between the right and left hemispheres, and a cause for fluid accumulation was not
identified.

A little bit more…

• External hydrocephalus in horses is a condition marked by excessive cerebrospinal fluid (CSF) accumulation in the subarachnoid space, particularly around the cerebral hemispheres. It is often congenital and may be associated with developmental anomalies or trauma. Unlike internal hydrocephalus, the ventricles may appear normal or only mildly dilated.

MRI Features in Equine Patients:

• MRI is the preferred modality for diagnosing external hydrocephalus due to its ability to differentiate fluid-filled spaces and assess surrounding brain structures. Key features include:
• Enlarged subarachnoid spaces, especially in the dorsal cerebral convexities.
• Normal or slightly dilated ventricles, helping distinguish from internal hydrocephalus.
• Hyperintense CSF signal on T2-weighted images and suppression on FLAIR sequences.
• No contrast enhancement, confirming the non-inflammatory nature of the fluid.
• Associated findings may include cortical thinning or midline structural anomalies.
• MRI also aids in evaluating concurrent conditions such as cerebellar hypoplasia or porencephaly, which may influence prognosis and treatment.

Clinical Significance in Horses

• Clinical signs vary with severity and include:
• Mild cases: subtle neurologic deficits, poor performance, or behavioral changes.
• Severe cases: ataxia, seizures, vision impairment, or macrocephaly in foals.
• Early MRI-based diagnosis is crucial for differentiating benign variants from progressive or symptomatic hydrocephalus. Treatment may range from conservative monitoring to surgical CSF diversion in rare cases.

Insights from Human Medicine:

• In pediatric neurology, external hydrocephalus is often seen in infants and is typically benign and self-limiting. MRI findings mirror those in equine cases, with enlarged subarachnoid spaces and normal ventricles. Human studies emphasize the importance of distinguishing external hydrocephalus from subdural collections or atrophy, which carry different prognoses. These principles are increasingly applied in veterinary neurology to refine differential diagnoses and guide management.

Conclusion:

• External hydrocephalus in horses, though uncommon, is a clinically relevant condition that benefits from advanced imaging. MRI provides critical anatomical and pathological insights, enabling accurate diagnosis and informed treatment decisions. Comparative understanding from human medicine enhances diagnostic confidence and supports evidence-based care.

References:

• Source: AAEP – Application and indications of magnetic resonance imaging and computed tomography of the equine head.  [aaep.org]
• Human pediatric neurology literature on benign external hydrocephalus (e.g., Di Rocco et al., Child’s Nervous System, 2006).