Preliminary Ultrasonographic Optic Nerve Sheath Diameter Values in Clinically Healthy Adult Rahmani Sheep

Sheep reared in tropical and subtropical areas may develop neurological disorders associated with infectious, parasitic, metabolic, or nutritional diseases. Some of these disorders may be accompanied by increased intracranial pressure (ICP). Direct ICP measurement methods are invasive, and technically unavailable in clinical field settings. Ultrasonographic measurement of optic nerve sheath diameter (ONSD) has been reported as a noninvasive method of ICP indicator in humans and animal species. However, baseline ONSD values for clinically healthy sheep are not yet reported. Therefore, this pilot field study aimed to determine preliminary transpalpebral ultrasonographic ONSD values in clinically healthy adult Rahmani sheep.

Sixty clinically healthy adult Rahmani sheep, including 30 males and 30 nonpregnant females, were examined under field conditions. All animals underwent physical, neurological, hydration, external ocular, and direct ophthalmoscopic examination before inclusion. Animals with systemic illness, dehydration, ocular disease, or any clinical sign suggestive of neurological dysfunction or raised ICP were excluded. Bilateral transpalpebral ultrasonography was performed in conscious standing sheep without sedation using a 10-MHz linear probe. ONSD was measured at 3 mm posterior to the posterior retinal surface in the transverse plane. Because direct ICP measurement was not available and was not applicable under field conditions, ICP was not measured.

Transpalpebral ultrasonography was feasible in all examined sheep and was completed without sedation or adverse events. Mean ± SD ONSD was 4.65 ± 0.07 mm in the left eye and 4.70 ± 0.06 mm in the right eye, with no significant laterality difference (p = 0.34). No significant difference in animal-level mean ONSD was detected between males and nonpregnant females (p = 0.23). The preliminary animal-level ONSD interval was 4.55-4.77 mm. Inter- and intraobserver agreement was excellent (ICC = 0.93 and 0.94, respectively).

Transpalpebral ultrasonography allowed feasible and repeatable measurement of ONSD in conscious, standing adult Rahmani sheep under field conditions. This pilot study provides preliminary baseline ONSD values in clinically healthy Rahmani sheep without detectable clinical evidence of disease.

Because ICP was not directly measured, these values should not be interpreted as diagnostic thresholds for intracranial hypertension. They may serve as foundational data for future studies comparing healthy sheep with sheep affected by neurological disease and for studies validating ONSD against direct ICP measurement when feasible.