Recovery after Unilateral Phrenic Injury Associated with Coronary Artery Revascularization: Discussion (4)

When demyelination is more extensive, recovery occurs by synthesis of myelin by proliferating Schwann cells. Although this type of injury usually resolves by 12 weeks, internodal distances may be permanently shorter than normal, leading to a reduction in maximal conduction velocity. The time course of recovery in this study is most compatible with an axonal degeneration. Recovery after axonal degeneration is a complex process. If the original basement membrane remains intact, the Schwann cells proliferate and align themselves longitudinally. Sprouts from the proximal axons are evident by seven to ten days, becoming remyelinated as they grow. Regeneration may occur over several months depending on the distance over which regrowth must occur.
Potential insults leading to phrenic nerve axonal degeneration and demyelination during bypass surgery include nerve sectioning, stretch, cold exposure and compromise in blood flow. The intimate location of the phrenic nerve to the pericardium makes these nerves vulnerable to surgical manipulation, particularly during internal mammary artery grafting. Mobilization of the internal mammary artery also could compromise blood flow to the vasavasorum of the phrenic nerve. The observation that four of five patients in our study with left hemidiaphragmatic paralysis had ipsilateral internal mammary grafting suggests that these mechanisms may be important in phrenic nerve injury. This is further supported by the study of Abd et al, who found concordance between the side of internal mammary grafting and phrenic nerve injury in 18 of 20 instances after CAB surgery.