Etiology

• The obturator nerve originates from the spinal cord at L5 and L6 .
• The nerves pass along the medial aspect of the ilium before exiting the pelvic cavity through the obturator foramen.
• The anatomical situation of the nerves makes them very susceptible to direct trauma.
• The nerve provides motor function to most of the adductor muscles of the hind limb.
• The nerve has no sensory function.
• Partial or complete paralysis affects the adductor muscles, with consequent loss of function .
• Pressure on the nerves from an oversized calf can cause localized ischemia with subsequent loss of nerve function.
• Sustained pressure of 20 minutes of more will lead to temporary or long term loss of nervous function.
• Affected nerves may take up to a year to recover full function.
• The nerves may be directly traumatized and torn, or even transected, leading to total and permanent dysfunction.

Predisposing factors

General

• Breeds and individuals with small pelvic canals and those predisposed to dystocia (eg Belgian Blue Belgian Blue).
• Primiparous animals.
• Relative fetal oversize, with large calves due to breed or nutrition in late pregnancy.

Specific

• Internal pelvic measurements which result in small pelvic areas (less than 140 cm² at 13 months of age) will predispose to dystocia and subsequent obturator paralysis in first calf heifers.
• Internal pelvic dimensions can be measured with a Rice Pelvimeter and size compared to a chart that will predict the likelihood of dystocia.

Pathophysiology

• Pressure on the obturator nerve can cause damage by hypoxia, which disturbs neural function.
• Hypoxic damage to nerves can occur after less than 20 minutes of direct pressure, and recovery may take months or years.
• Tearing or transection of nerves leads to permanent loss of function.
• Loss of motor function leads to paralysis which causes an instant inability to use the adductor muscles of the hind limb, which function to pull the hind limbs medially.
• The condition is most often associated with other traumatic damage to the soft tissues of the pelvic canal, or possible dislocation of the sacroiliac joint Dislocations, which causes pain.

Timecourse

• Paralysis is instant when associated with direct trauma through parturition.
• Paralysis associated with inflammation after parturition, or other space occupying lesions of the pelvis, may be delayed.
• If the obturator nerves are permanently damaged through tearing or complete transection, the adductor muscles will be permanently paralyzed and the subsequent locomotion will be life-long.
• If the damage is not permanent, muscle function will partially, or completely recover, but the function may take several weeks or months, during which time the affected animal will be predisposed to uncontrolled abduction of the limbs (splits).