Table 2.

Outcomes and conclusions summarized for fourteen toxicology Pb ototoxicity studies

Study	Outcomes and conclusions	Other
In Vivo Studies
Gozdzik-Zolnierkiewicz, T. (1969)	• Temporal bone analysis was done with silver impregnation for 16 animals: Sensory cells in inner ear, spiral, and vestibular ganglion displayed no pathology. • Examine VIII nerve pathology was normal in five animals and lesions including demyelination and axonal degeneration were present in eleven. • Sudan black staining was carried out in 16 animals: four were normal; segmental demyelination and axonal degeneration observed in twelve.	• Eight animals died. • All remaining had systemic toxicity (weight loss and weakness) due to Pb. • Five surviving animals showed paralysis of limbs.
Wilpizeski, C. (1974)	• Pure tone detection thresholds trained shock avoidance showed hearing within normal ranges throughout the experiment (N = 2, treated for 10 and 21 weeks). • Temporal bone analyses and VIII nerve fiber study showed no damage to hair cells or demylenation (N = 3).	• Five died during treatment and two were sacrificed when near death. • Two developed arm and leg transitory paresis. • Severe weight loss was observed, however vomiting diarrhea and anorexia were not.
Takahashi, Y. (1984)	• Latency of N1 was significantly increased after day 2. • P2 latency was significantly increased over the control after day 3. • The amplitude between P1 and N1 increased on day 1, but was not significantly different on day 2 or 3. • After day 6, P1-N1 amplitude was again significantly different.	• Body weight decreased substantially due to Pb treatment initially. • At day 8, treatment weights were no longer significantly different from controls. • Hematocrit significantly decreased due to Pb treatment at day 6.
Yamamura, K. (1984)	• No changes to pseudo threshold or maximum output voltages were observed between control and all three experimental groups. • Action potential latency of N1 was significantly longer in the highest exposure group than the control. • Highest exposure conditions required a greater sound intensity (dB) to illicit similar action potential maximum output voltage regressions in comparison to control.	• All experimental animals lost weight on average. • Three died in 10 mg group • Four died in 15 mg group • Fourteen died in 20 mg group
Yamamura K. (1987)	• No changes in cochlear microphonics, either the maximum output voltage or the pseudothreshold. • Input-output function of action potential was different in highest exposure – output voltage was reduced especially below 20 dB (indicating VIII nerve axonal impairment).	• One died in 10 mg group • Five died in 15 mg group • Eight died in 20 mg group
Yamamura K. (1989)	• High-dose Pb exposure caused dysfunction of the VIII nerve. • Whole nerve action potentials elevated across treatment lengths; control was lowest, with all treatments compared to control 2 wks was 10 dB higher, 4 wks was 20 dB higher, and 5 wks was 25 dB higher.	• Three died in 2 wk group • Twelve died in 4 wk group • Twelve died in 5 wk group
Hotta S. (1996)	• Pb treatment alone did not cause cochlear electrophysiological changes. • Potassium ion concentration in the scala media was not altered. • Pb + noise groups displayed a significant decrease in AP output voltage from both control and Pb groups. • Pb + noise CM output and intensity were significantly lowered due to Pb and noise. • Pb + noise EP latency was significantly longer than controls and the combined exposure groups showed a lowered mean absolute value of negative potential.	• 6 died in 4-week group not exposed to noise. • 14 died in 4-week group exposed to noise. • No animals showed hind limb paralysis.
Nagymajtényi, L. (1996)	• Electrocorticogram from auditory centers showed decreases in amplitudes that were not significant, however displayed trends with dose and time. • Increases in mean frequencies also trended with dose and time, but were only significantly different from control in the 320 mg/kg for 12 weeks group. • Decreases in auditory electrocorticography – index again trended with dose and treatment time and were significantly different from controls only at the 320 mg/kg for 12 weeks group.	• Relative weights of organs from treated rats did not differ significantly from controls.
Tuncel, U. (2002).	• Acute hearing loss within two hours of exposure. • TEPb had a higher degree of toxicity to cochlea, though Pb content is equal to the PbAc solution. • Compound action potential thresholds were elevated in the Pb acetate group over controls 5–10 dB and significant from tested frequencies 4–40 kHz TEPb exposure significant at 20 and 24 kHz. • Cochlear microphonics isopotential curves were not significantly different.	• Animals were euthanized 120 minutes after baseline recordings. • No changes in cochlear microphonics or compound action potential at single injection doses of 20 mg/kg Pb acetate and 17.1 mg/kg TEPb. • Results at doses in the study were not seen after 60 minutes of treatment.
Fazakas, Z. (2005).	• Analysis of total electrocorticography in the auditory centers showed decreases in delta activity for the high Pb dose and Pb+Hg+Alcohol group. These changes were not shown as significant. • Changes in auditory cortical evoked potential latency and durations following acoustic stimulation of 1 Hz 40 dB was not pronounced (and not shown).	• Weight gain in the high Pb group was significantly lowered. • Liver/brain weight was significantly lower in the Pb-high group, Pb-high+Hg, and Pb+Hg+alcohol. Lung/brain weight was also significantly lower in the Pb-high group.
Liu, S. (2011).	• Significantly increased latencies for all ABR waves I-V were observed after Pb treatment, especially in males. • Amplitudes, especially for waves I and II were reduced, and to a greater degree in males.	• No animal loss or measures of systemic toxicity were reported. • Latencies of waves I-V in the Pb+Cu group were significantly lower than the Pb group.
Carlson, K. (2018)	• No cochlear outer hair cell loss was observed due to Pb. • Pb exposure did not cause significant changes in ABR or DPOAEs • No significant changes in ABR peak and latency were observed due to Pb treatment. • Potentiation due to Pb and noise exposures were not observed in ABR results or outer hair cell counts. • Pb and Cd together did not alter auditory results observed from the highest Pb treatment alone.	• One animal died due to unrelated causes (urologic syndrome). • Mild lesions in the kidney were observed in 91% of 3 mM Pb: karyomegaly in the S3 tubular epithelium.
Jamesdaniel, S. (2018)	• Pb treatment induced shifts of 8–12 dB (this was significant at the click, 4, 16, 24, and 32 kHz). • Pb treatment significantly downregulated oxidative stress genes Sod1, Prdx4, and Idh1 in cochlear RNA • Pb and noise treated animals had threshold shifts of 10–25 dB significantly higher than shifts due to noise exposure alone at the click stimulus, 4 and 32 kHz.	• Normal weight gain was not altered.
In vitro Study
Liang, G.-H. (2004).	• Potassium current was reduced over time; this reduction was dose dependent. • Outward voltage-gated potassium relative current increased with increasing doses of Pb. • After washing Pb, these changes were not reversed. • Changes are small and are not indicative of causing hearing loss.	• Cells selected were middle to apical areas of the cochlea; cells sensitive to mid- and low-frequency sounds.