The effects of repeated low-level blast exposure on hearing in marines.

BACKGROUND: The study evaluates a group of Military Service Members specialized in blast explosive training called "Breachers" who are routinely exposed to multiple low-level blasts while teaching breaching at the U.S. Marine Corps in Quantico Virginia. The objective of this study was to determine if there are any acute or long-term auditory changes due to repeated low-level blast exposures used in training. The performance of the instructor group "Breachers" was compared to a control group, "Engineers". METHODS: A total of 11 Breachers and four engineers were evaluated in the study. The participants received comprehensive auditory tests, including pure-tone testing, speech-in-noise (SIN) measures, and central auditory behavioral and objective tests using early and late (P300) auditory evoked potentials over a period of 17 months. They also received shorter assessments immediately following the blast-exposure onsite at Quantico. RESULTS: No acute or longitudinal effects were identified. However, there were some interesting baseline effects found in both groups. Contrary to the expected, the onsite hearing thresholds and distortion product otoacoustic emissions were slightly better at a few frequencies immediately after blast-exposure than measurements obtained with the same equipment weeks to months after each blast-exposure. CONCLUSIONS: To date, the current study is the most comprehensive study that evaluates the long-term effects of blast-exposure on hearing. Despite extensive testing to assess changes, the findings of this study suggest that the levels of current exposures used in this military training environment do not seem to have an obvious deleterious effect on hearing.

The Vestibular Effects of Repeated Low-Level Blasts.

The objective of this study was to use a prospective cohort of United States Marine Corps (USMC) instructors to identify any acute or long-term vestibular dysfunction following repeated blast exposures during explosive breaching training. They were assessed in clinic and on location during training at the USMC Methods of Entry School, Quantico, VA. Subjects received comprehensive baseline vestibular assessments and these were repeated in order to identify longitudinal changes. They also received shorter assessments immediately following blast exposure in order to identify acute findings. The main outcome measures were the Neurobehavioral Symptom Inventory, vestibular Visual Analog Scale (VAS) of subjective vestibular function, videonystagmography (VNG), vestibular evoked myogenic potentials (VEMP), rotary chair (including the unilateral centrifugation test), computerized dynamic posturography, and computerized dynamic visual acuity. A total of 11 breachers and 4 engineers were followed for up to 17 months. No acute effects or longitudinal deteriorations were identified, but there were some interesting baseline group differences. Upbeat positional nystagmus was common, and correlated (p<0.005) with a history of mild traumatic brain injury (mTBI). Several instructors had abnormally short low-frequency phase leads on rotary chair testing. This study evaluated breaching instructors over a longer test period than any other study, and the results suggest that this population appears to be safe from a vestibular standpoint at the current exposure levels. Upbeat positional nystagmus correlated with a history of mTBI in this population, and this has not been described elsewhere. The data trends also suggest that this nystagmus could be an acute blast effect. However, the reasons for the abnormally short phase leads seen in rotary chair testing are unclear at this time. Further investigation seems warranted.

Flagellar basal apparatus and its utility in phylogenetic analyses of the porifera.

A broadly based comparative study was initiated to assess components of the flagellar basal apparatus as a character set in phylogenetic analyses of poriferans. The flagellated (monociliated) epidermal cells of sponge larvae were selected for study. Taken together, they create a field of locomotory cells analogous to a multiciliated surface. Larvae of six species in four orders of the Demospongiae were examined by transmission electron microscopy. Results are compared with findings taken from the literature on larvae of five additional species of demosponges and four species of calcareans. Data were assembled on six components of the basal apparatus: (1) basal body, (2) basal foot, (3) accessory centriole, (4) transverse cytoskeletal system, (5) longitudinal cytoskeletal system, and (6) association with Golgi body. Where evidence permits assessment, all have Type II basal bodies. Basal feet are diverse and are subdivided into three categories based on structural complexity. The most anatomically intricate (Type III) is found only in larvae of Mycale spp. Accessory centrioles are present or absent depending on the species, but their occurrence is without overall taxonomic pattern. When present, accessory centrioles are oriented perpendicularly to the long axis of the basal body, but as ascertained from relationship to the anterior-posterior axis of the larvae they are without consistent orientation with regard to the plane of effective beat of the flagellum. Transverse and longitudinal cytoskeletal systems are also diverse among larvae. The existence of cross-striated rootlets is convincingly established only in larvae of calcareans, and such rootlets are present in larvae of all four calcareans studied to date. Three apparently new rootlet structures are described: lateral arms of the transverse cytoskeletal system from larvae of Aplysilla sp. and Haliclona tubifera; laminar sheets of the longitudinal system from larvae of Aplysilla sp. and M. cecilia; and paraxial rootlet in larvae of H. tubifera. A robust similarity in structure of the basal appartus is observed among the three species of halichondrids reported here for the first time. In comparison with the flagellar basal apparatus found in adults, those of larvae are more complex and more diverse. Review of studies on adult sponges that include information on the basal apparatus reveals the absence of a longitudinal rootlet system in all cases. Additionally, there exists a high degree of concordance between properties of the basal apparatus in the one sclerosponge and the one hexactinellid studied to date. These basal apparatus are also the simplest in construction of those found in sponges. Conversely, the basal apparatus of demosponges are varied. Although consistent presentation of the basal apparatus is evident in certain taxa, any discernable systematic pattern in their overall configuration remains obscure. Finally, we conclude that the flagellar basal apparatus of sponges is more similar to that found in choanoflagellates than it is to that observed in eumetazoans. © 1995 Wiley-Liss, Inc.