Clusters of conditions among US service members diagnosed with mild TBI from 2017 through 2019.Background
 Many US Military Service Members (SMs) newly diagnosed with mild Traumatic Brain Injury (mTBI) may exhibit a range of symptoms and comorbidities, making for a complex patient profile that challenges clinicians and healthcare administrators.  This study used clustering techniques to determine if conditions co-occurred as clusters among those newly injured with mTBI and up to one year post-injury.


 We measured the co-occurrence of 41 conditions among SMs diagnosed with mTBI within the acute phase, one or three months post-mTBI diagnosis, and chronic phase, one year post-mTBI diagnosis.  Conditions were identified from the literature, clinical subject matter experts, and mTBI care guidelines.  The presence of conditions were based on medical encounters recorded within the military health care data system.  Through a two-step approach, we identified clusters.  Principal component analysis (PCA) determined the optimal number of clusters, and hierarchical cluster analyses (HCA) identified the composition of clusters.  Further, we explored how the composition of these clusters changed over time.


 Of the 42,018 SMs with mTBI, 23,478 (55.9%) had at least one condition of interest one-month post-injury, 26,831 (63.9%) three months post-injury, and 29,860 (71.1%) one year post injury.  Across these three periods, six clusters were identified.  One cluster included vision, cognitive, ear, and sleep disorders that occurred one month, three months, and one year post-injury.  Another subgroup included psychological conditions such as anxiety, depression, PTSD, and other emotional symptoms that co-occurred in the acute and chronic phases post-injury.  Nausea and vomiting symptoms clustered with cervicogenic symptoms one month post-injury, but later shifted to other clusters.  Vestibular disorders clustered with sleep disorders and headache disorders one-month post-injury and included numbness and neuropathic pain one year post-injury.  Substance abuse symptoms, alcohol disorders, and suicidal attempt clustered one year post-injury in a fifth cluster.  Speech disorders co-occurred with headache disorders one month and one year post-injury to form a sixth cluster.


 Conclusion
 PCA and HCA identified six distinct subgroups among newly diagnosed mTBI patients during the acute and chronic phases post-injury.  These subgroups may help clinicians better understand the complex profile of SMs newly diagnosed with mTBI.
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