4-month-old Miniature Schnauzer

MRI system: 1 T magnet (Magnetom HarmonyTM, Siemens Medical Technologies, Malvern, PA)

Sequences (Brain):

• Sagittal T2-W turbo spin echo (TSE) images (TR 3300 ms, TE 112 ms, 3 mm slice thickness)
• Transverse T2-W TSE images (TR 4460 ms, TE 98 ms, 3 mm slice thickness)
• Transverse T1-W SE images (TR 635 ms, TE 15 ms, 3 mm slice thickness)
• Transverse fluid attenuated inversion recovery (FLAIR) images (TR 8000 ms, TE 92 ms, TI 2300 ms, 3 mm slice thickness)
• Transverse T2*-W gradient recalled echo (GRE) images (TR 831 ms, TE 26 ms, flip angle 20º, 3 mm slice thickness)
• Post contrast images (0.2 ml/kg (0.1 μmol/kg) gadopentetate dimeglumine (Magnevist®) i.v.):
• T1-W SE images in transverse, (dorsal and sagittal plane not shown) (TR 470-635 ms, TE 15 ms, 3 mm slice thickness)

MRI Findings

The corpus callosum is smaller than normal on all sequences and image planes, best seen on sagittal T2-weighted images. Best observed on image #13 on the transverse T2-weighted sequence there is a triangular area measuring 5mm in greatest length immediately rostral and dorsal to the interthalamic adhesion on midline which is isointense to CSF and is continuous with the ventricular system. Best appreciated on transverse T2-weighted images the lateral ventricles have unusually upturned corners. No additional abnormalities are identified.

Diagnosis

Dysgenesis of the corpus callosum

Discussion

Agenesis or dysgenesis of the corpus callosum is a congenital anomaly. It may be associated with a more complex malformation (holoprosencephaly; HPE) but may also occur as an isolated abnormality. Staffordshire Bull Terriers and Miniature Schnauzers are predisposed to both conditions but other breeds may be affected. The age at presentation is very variable and has been reported to range from 3-81 months (1).  Adipsia/hypodipsia (associated with hypernatremia) is the most common presenting complaint (2-5).  Animals may be neurologically normal or show variable signs including tremors, proprioceptive deficits, ataxia, reduced menace, obtundation and vestibular signs. Dermatologic abnormalities including hyperkeratosis, ichthyosis, seborrhea and alopecia have also been reported (1, 2). MRI findings in dogs with lobar HPE and agenesis or dysgenesis of the corpus callosum include (1-5):

• Agenesis or dysgenesis of the corpus callosum which is best visualized on mid-sagittal or transverse T2W images
• Unusually upturned, pointed corners of the lateral ventricles seen on transverse images
• Less commonly, a focal circular T2-hyperintensity dorsal to the midbrain thought to represent either dilatation of the pineal recess or an arachnoid diverticulum
• In case of lobar HPE, fusion of the ventral frontal lobes, cingulate gyri, lateral ventricles and part of the diencephalon with associated loss of some normal midline structures

While dysgenesis of the corpus callosum was able to explain the profound hypernatremia seen in this patient, its association with the clinical signs is uncertain. A CSF tap yielded a diagnosis of mild mononuclear pleocytosis, and an inflammatory condition may have contributed to the neurologic symptoms. The owners elected euthanasia due to lack of improvement following attempted medical management, and necropsy did not reveal any significant structural abnormalities beyond changes seen on MRI.

References

1. Goncalves R, Volk H, Smith PM, Penderis J, Garosi L, MacKillop E, et al. Corpus callosal abnormalities in dogs. J Vet Intern Med. 2014;28(4):1275-9.
2. Chapman PS, Petrus D, Neiger R. Hypodipsic hypernatremia in eight dogs. Tierarztliche Praxis Ausgabe K, Kleintiere/Heimtiere. 2009(37):15-20
3. Jeffery ND, Watson PJ, Abramson C, Notenboom A. Brain malformations associated with primary adipsia identified using magnetic resonance imaging. Vet Rec. 2003;152(14):436- 8.
4. Shimokawa Miyama T, Iwamoto E, Umeki S, Nakaichi M, Okuda M, Mizuno T. Magnetic resonance imaging and clinical findings in a miniature Schnauzer with hypodipsic hypernatremia. J Vet Med Sci. 2009;71(10):1387-91.
5. Sullivan SA, Harmon BG, Purinton PT, Greene CE, Glerum LE. Lobar holoprosencephaly in a Miniature Schnauzer with hypodipsic hypernatremia. J Am Vet Med Assoc. 2003;223(12):1783-7, 78.