Cranioencephalic Malformation with Atlanto‐Occipital Luxation in an Andalusian Neonate Foal

A 45-kg, 12-hour-old Andalusian colt was referred to the Equine Teaching Hospital of Barcelona for evaluation of abnormal mental status and weakness with inability to stand from birth. Foaling was attended and reported to be uneventful. On physical examination, the foal was recumbent and showed signs of immaturity (eg, domed forehead and flexor tendon laxity of all 4 limbs), as well as an abnormal skull characterized by a prominent occipital crest on palpation. On neurological examination, the colt was stuporous, suckle reflex was absent, and pupilary reflex of the left eye was slow and incomplete (pupilary reflex of the right eye could not be assessed because of hyphema). Oculocephalic movements were normal. The foal had tachypnea (80 breaths/ min) with apneic periods induced by cervical manipulation, which made assisted ventilation necessary during examination. The foal’s heart rate was within normal limits (80 beats/min), but it had a weak arterial pulse, congested mucous membranes, and prolonged capillary refill time (3 seconds). All of these later signs were consistent with mild dehydration. Gastrointestinal sounds were absent and meconium impaction was detected on digital rectal palpation. Hematology and plasma biochemistry performed upon admission disclosed mildly increased lactate concentration (4.5mmol/L; reference range, o2.5mmol/L), PCV (55%; reference range, 40– 52%), total protein concentration (6 g/dL; reference range, 4.5–4.7 g/dL), PvCO2 (60mmHg; reference range, 37–43mmHg) and bicarbonate (26.6mEq/L; reference results, 23mEq/L), and decreased glucose (2mmol/L; reference range, 6–12.5mmol/L) and potassium (2.9mmol/L; reference range, 3.5–5.5mmol/L) concentrations, which was interpreted as a mixed acidemia because of respiratory and metabolic lactic acidosis. Septic score was 12 (normal index,o11). A cranio-cervical radiological study was performed. Images in stress position could not be taken because the foal showed apneic periods during vertical mobilization of the neck. The skull and cranial cervical vertebrae evidenced a domed forehead, occipital crest enlargement, no articulation between occipital and atlas bones, and hypoplasic occipital condyles (Fig 1). A definitive diagnosis of malformation of the caudal cranium and atlanto-occipital luxation was reached. Because of the poor prognosis, the colt was euthanized. Cranio-cervical radiographs in full flexion and extension, as well as collection of cerebrospinal fluid (CSF) from the atlanto-occipital space, were performed immediately after euthanasia. Cytological examination and lactate determination, as well as total protein concentration and creatine kinase activity, were performed. The liquid was clear but xanthochromic, had normal nucleated cell count and total protein concentration (117mg/ dL, reference range 99–120mg/dL), but creatine kinase activity and lactate concentration were increased (14 IU/ L, reference range 0–8 IU/L, and 4.4mmol/L, reference value o4mmol/L, respectively). Postmortem myelography also was performed by injecting 25mL of contrast (sodium amidotrizoate and meglumine amidotrizoate in a proportion of 10 : 66 in aqueous solution) in the atlanto-occipital space. Myelography in flexed-stressed position showed a 50% reduction of the subarachnoid space at the atlantooccipital junction as well as mild spinal cord compression between the 3rd and 4th cervical vertebrae because of dynamic subluxation (Fig 2A and B). At necropsy, a skull malformation was confirmed. The cranial dome was figure 8-shaped and the occipital crest was enlarged. There was caudalization of the hindbrain toward the foramen magnum. The medulla oblongata was situated between the occipital condyles, and the 4th ventricle could be seen through the foramen magnum (Fig 3). The cerebellar vermis had an S-shaped form due both to the pressure of adjacent bony structures and to the presence of hydrocephalus. In addition, there was an apparent vascular proliferation on the brain surface. Blood vessels were congested and associated with hemorrhages in the falciform and tentorium ligaments, and diffusely on the subarachnoid space. All of the ventricular system was enlarged without the presence of cellular debris in its lumen. Histological examination of the brain was performed. Hematoxylin-eosin, immunoperoxidase, and glial fibrillary acidic protein stains were used. Microscopically, the From the Servei de Medicina Interna Equina (Viu, Armengou, Jose-Cunilleras, Cesarini, Monreal) and the Unitat de Neuropatologia Veterinària (Pumarola), Departament de Medicina i Cirurgia Animals, Facultat de Veterinària, Universitat Autònoma de Barcelona, Barcelona, Spain. Corresponding author: Lara Armengou, DVM, Dipl ECEIM, Servei de Medicina Interna Equina, Departament de Medicina i Cirurgia Animals, Facultat de Veterinària, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain; e-mail: lara.arme ngou@uab.cat. Submitted September 25, 2009; Revised October 27, 2009; Accepted November 19, 2009. Copyright r 2010 by the American College of Veterinary Internal Medicine 10.1111/j.1939-1676.2010.0478.x Abbreviations: