A Rat Model of Second Impact Syndrome in Diffuse Traumatic Brain Injury: Evidence of Facial Hypersensitivity, Neurological and Other Behavioral Alterations, and Immunological and Histopathological Tissue Changes

Traumatic brain injury (TBI) can lead to chronic complications, including headache and facial hypersensitivity. Second-impact syndrome (SIS), occurring when a second TBI precedes recovery from an initial injury, often causes severe long-term symptoms. Neuroinflammation is implicated, but the comprehensive pathophysiology of SIS remains unclear. This study characterized neurological/behavioral outcomes, facial nociception, plasma inflammatory markers, and central nervous system (CNS) histopathology in rat models of SIS and single mild TBI. Female Wistar rats (n = 6/group) were assigned to naïve control, sham surgery, single impact (TBI), or double impact (SIS; intervals: 1h, 24h, 3d, 7d, 14d). Mild TBI was induced using a modified Marmarou weight-drop model. Assessments (facial mechanical sensitivity via von Frey, Neurological Severity Scale (NSS), Neurobehavioral Scale (NBS)) were performed pre-injury and on days 1,3,5,7 post-injury. Plasma IL-6 and TNF-α were measured by ELISA. Histopathology (H&E) assessed CNS inflammation. All impact groups developed transient facial mechanical allodynia and acute neurobehavioral deficits (impaired NSS/NBS at day 1). SIS groups, particularly with longer intervals (≥ 24h), exhibited more prolonged facial allodynia and significant neurological deficits compared to single TBI at specific timepoints. Plasma cytokines showed minimal changes. Histopathology revealed significantly elevated CNS inflammation in both TBI and SIS groups versus controls, with SIS groups (intervals 24h-14d) showing robust responses. Repeated TBI, especially with longer recovery intervals between injuries, results in more severe and prolonged facial allodynia, neurological deficits, and CNS neuroinflammation than a single mild TBI. These findings illuminate SIS pathophysiology and underscore the critical need for protection post-concussion.