The role of optic nerve sheath diameter ultrasound in brain infection

Abstract

Brain infections cause significant morbidity and mortality worldwide, especially in resource-limited settings with high HIV co-infection rates. Raised intracranial pressure [ICP] may complicate brain infection and worsen neurological injury, yet invasive ICP monitoring is often unavailable. Optic nerve sheath diameter [ONSD] ultrasound may allow detection of raised ICP at the bedside; however, pathology in brain infection is different to traumatic brain injury, in which most studies have been performed. The use of ONSD ultrasound has been described in tuberculous meningitis, cryptococcal meningitis and cerebral malaria; however correlation with invasive ICP measurement has not been performed. Normal optic nerve sheath values are not yet established for most populations, and thresholds for clinical intervention cannot be assumed to match those used in non-infective brain pathology. ONSD ultrasound may be suitable for use in resource-limited settings by clinicians with limited ultrasound training. Standardisation of scanning technique, consensus on normal ONSD values, and action on abnormal results, are areas for future research. This scoping review examines the role of ONSD ultrasound in brain infection. We discuss pathophysiology, and describe the rationale, practicalities, and challenges of utilising ONSD ultrasound for brain infection monitoring and management. We discuss the existing evidence base for this technique, and identify knowledge gaps and future research priorities.

Keywords: AIDS, Acquired immunodeficiency syndrome; Brain infection; CSF, Cerebrospinal fluid; HIV, Human immunodeficiency virus; ICP, Intracranial pressure; IQR, Interquartile range; IRIS, Immune reconstitution inflammatory syndrome; LP, Lumbar puncture; MAP, Mean arterial pressure; Meningitis; ONSD, Optic nerve sheath diameter; Optic nerve sheath diameter; ROC, Receiver-operator characteristic; Raised intracranial pressure; SD, Standard deviation; TB meningitis, Tuberculous meningitis; TBI, Traumatic brain injury; Ultrasound.

Fig. 1

Anatomy of CSF flow relating to optic nerve sheath (Based on an image from the Intracranial Hypertension Research Foundation, with permission). CSF: Cerebrospinal fluid.

Fig. 2

Enlarged optic nerve sheath imaged by ultrasound. This first panel shows an ultrasound image of the optic nerve sheath, and surrounding structures. D1 denotes distance from the retina to the point of optic nerve sheath diameter measurement (3 mm). D2 denotes the measured optic nerve sheath diameter at 3 mm from the retina. In the second panel a corresponding image is shown labelling structures of the eye for direct comparison with the clinical image (first panel).