Obex

The obex is a distinct anatomical landmark in the human brainstem, marking the point at which the fourth ventricle narrows inferiorly to become the central canal of the spinal cord. It is located within the medulla oblongata and plays an important role in cerebrospinal fluid dynamics, neuroanatomical orientation, and clinical diagnosis.
The obex is routinely referenced in anatomical, neurological, and neuropathological studies due to its fixed position relative to surrounding brainstem structures and its relevance in several disease processes.

Anatomical location and structure

The obex is situated on the dorsal surface of the medulla oblongata at the caudal end of the fourth ventricle. At this point, the ventricular cavity tapers and continues as the narrow central canal that runs through the spinal cord. Anatomically, the obex represents the inferior apex of the rhomboid fossa.
In standard anatomical studies, the obex is typically found approximately 10–12 millimetres above the level of the foramen magnum, the large opening at the base of the skull through which the brainstem transitions into the spinal cord. Its position makes it a useful internal reference point when assessing craniovertebral relationships.
In individuals with a low cerebellar tonsillar position, such as those with Chiari malformations, the obex may be located at or below the plane of the foramen magnum. This downward displacement reflects altered brainstem and hindbrain anatomy and has diagnostic relevance.

Relationship to cerebrospinal fluid flow

The obex forms part of the pathway for cerebrospinal fluid (CSF) circulation. CSF flows from the fourth ventricle through the ventricular system and into the central canal of the spinal cord at the level of the obex. Although the majority of CSF exits the ventricular system via the median and lateral apertures into the subarachnoid space, the patency of the obex remains important for normal CSF dynamics.
Obstruction at this level can disrupt normal CSF flow and contribute to ventricular enlargement.

Neuroanatomical significance

The obex is located within the medulla oblongata, a region responsible for vital autonomic functions, including respiratory and cardiovascular regulation. It lies near important ascending and descending neural pathways.
The sensory decussation, where certain ascending sensory fibres cross from one side of the central nervous system to the other, occurs in close proximity to the obex. This contributes to the contralateral representation of sensory information in the brain and underscores the obex’s importance as a structural landmark within the brainstem.
Because of its consistent position, the obex is frequently used in neuroanatomical descriptions to demarcate the caudal extent of the fourth ventricle.

Clinical significance

Lesions involving the obex can have serious clinical consequences. Due to its role in ventricular narrowing, pathological processes at this site may lead to obstructive hydrocephalus, a condition characterised by impaired CSF drainage and increased intracranial pressure.
The most common tumour associated with the obex is subependymoma, a typically benign, slow-growing neoplasm arising from the ventricular lining. Although often asymptomatic, subependymomas at the obex can cause symptoms by obstructing CSF flow or compressing adjacent brainstem structures.
Other tumours have also been reported at this location, including hemangioblastomas, which are vascular tumours that may be associated with genetic conditions such as von Hippel–Lindau disease.
Because of the obex’s proximity to vital autonomic centres, surgical intervention in this area requires particular caution.

Role in prion disease diagnosis

The obex holds special importance in neuropathology, particularly in the diagnosis of prion diseases. Immunohistochemistry (IHC) performed on tissue samples from the obex is used to detect abnormal prion proteins.
In veterinary and wildlife pathology, positive IHC findings in the obex are considered the gold standard for diagnosing transmissible spongiform encephalopathies, such as chronic wasting disease in deer. The region is selected because prion accumulation is consistently detectable there during disease progression.
This diagnostic application highlights the obex’s significance beyond human anatomy, extending into comparative and veterinary neuroscience.

Diagnostic and imaging relevance

In neuroimaging, especially magnetic resonance imaging (MRI), the position of the obex is used as a reference point for assessing hindbrain abnormalities, brainstem displacement, and craniovertebral junction disorders. Its relationship to the foramen magnum is particularly relevant in evaluating congenital and acquired conditions affecting the posterior cranial fossa.
Radiological identification of the obex assists clinicians in correlating imaging findings with neurological symptoms and planning appropriate management strategies.