Fourth Nerve Palsy After Hair Replacement Surgery and Prolonged Vertical Position in a Patient With Mega-Cisterna-Magna: Report of a Case.

Hair replacement surgery is a common cosmetic procedure. We present the case of a healthy 47-year-old man who suffered a right fourth nerve palsy five days after surgery. Magnetic resonance imaging revealed the presence of a mega-cisterna magna. We hypothesize that in the presence of this anatomical variant, the reduction of pressure within this space, caused by prolonged vertical posture during the postoperative period, may have led to cerebrospinal fluid shifts causing brainstem anteroposterior instability. The special configuration of the fourth cranial nerve, which curves around the brainstem, and the fact that it is very thin and long, make it particularly vulnerable to anteroposterior movements of the brainstem. Therefore, we believe that small anteroposterior displacements of the brainstem could have stretched the nerve, inducing the mild paresis we report.