Imaging diagnosis–occipitoatlantoaxial malformation in a miniature horse foal.

This research article presents a case study of a one-month-old Miniature Horse colt that was diagnosed with a rare congenital condition called occipitoatlantoaxial malformation (OAAM).

Case Presentation

• The foal was reported to show progressive weakness in its pelvic limbs. The symptoms were first noted when the foal was exercising with its mother.
• A local veterinarian identified ataxia (lack of muscle control) accompanying the weakness and referred the colt for in-depth diagnostic tests.

Physical Examination

• The examination demonstrated that the colt was fully alert with normal vital parameters, but displayed ataxia, weakness, spasticity (increased muscle tone causing stiffness), and hypermetria (overestimation of distance) in all four limbs.
• Further, the colt showed an increased intensity of these symptoms when its head was elevated, resulting in the foal collapsing on its pelvic limbs.
• X-ray images of its cervical spine were taken under normal and flexed positions for deeper analysis.

Radiographic Findings

• The x-ray images revealed malformations in the cervical spine. The first cervical vertebra was underdeveloped and rotated, and the occipital condyles (round prominences at the base of the skull) were not visible.
• When the spine was flexed, the distance between the coattail margin of C1 (the first cervical vertebrae) and the dorsal arch of C2 (the second cervical vertebrae) expanded excessively. Meanwhile, the occipitoatlantal junction (the connection between the skull and the spine) remained static.

Post-mortem Examination

• Due to the poor prognosis, the colt was put down humanely. A post-mortem CT scan was done to further study the lesions.
• The CT scan revealed more severe malformations, notably in the occipital bone and vertebrae.
• The research team diagnosed the condition as Occipitoatlantoaxial malformation (OAAM), a rare congenital malformation that has previous cases reported in cows, dogs, cats, sheep, and horses.

Discussion

• This condition has varied symptoms such as tetraparesis (partial paralysis), ataxia, proprioceptive deficits (lack of awareness of body positioning), cervical scoliosis, and a head tilt.
• Previous cases of OAAM were most commonly described in Arabian and half-Arabian foals, although a few cases have also been reported in breeds like Standardbred, Morgan, a Quarter Horse, and an Appaloosa.
• This case study reports the first case of OAAM in a Miniature Horse.
• The colt’s lesions were consistent with a type 2 deformity of occipitoatlantoaxial malformation.