Brainstem
Anatomy
The brainstem is the most primitive and caudal part of the brain. It is subdivided into three continuous portions, from rostral to caudal: the midbrain, the pons, and the medulla oblongata.
It has a complex internal structure. It is organized into 3 laminae, which extend into its entire length. From dorsal to ventral, these laminae are the tectum, the tegmentum, and the basis.
Most of the cranial nerve nuclei are located in the brainstem (with the mere exception of the olfactory (I), optic (II), and part of the accessory (XI) nerves). In addition to this, it is crossed by multiple tracts connecting the brain, the cerebellum, and the spinal cord.
Vascularization
The arterial blood supply of the brainstem derives fundamentally from the vertebrobasilar circulation. This is formed by:
- Large superficial vessels that run across the surface of the brainstem. These are the vertebral arteries (VAs), anterior spinal artery (ASA), posterior spinal arteries (PSAs), posterior inferior cerebellar arteries (PICAs), basilar artery (BA), anterior inferior cerebellar arteries (AICAs), and superior cerebellar arteries (SCA).
- Small penetrating arteries that arise from the superficial large vessels and carry the blood into the internal vascular territories of each part of the brainstem. These are the anteromedial, anterolateral, lateral, and posterior territories.
Ischemic strokes of the brainstem can result from stenosis or occlusions in both large vessels and small perforating branches. The most common etiologies are atherosclerosis, thromboembolism, lipohyalinotic microangiopathy, and dissection.
Clinical approach to brainstem stroke localization
Brainstem strokes produce location-dependent symptomatology, depending on both the affected brainstem portion (medulla, pons, or midbrain) and the internal location (ventral vs dorsal, medial vs lateral).
Clinically, neurologists try to determine the location of the stroke by identifying both the affected and the spared cranial nerves or fiber tracts.
Classic brainstem stroke syndromes are a heterogeneous group of syndromes that result from predominantly ischemic lesions in specific locations of the brainstem.
Medulla
The medulla oblongata is the lowest part of the brainstem, located between the pons and the spinal cord. It connects with the cerebellum through the inferior cerebellar peduncles.
It is subdivided into two portions:
- The ventral basis includes the pyramids and the olives.
- The dorsal tegmentum contains the cranial nerve nuclei as well as the white matter tracts.
The arterial irrigation of the medulla derives principally from the VAs and their branches, namely the ASA and the PICAs.
Medullary infarcts make up 7% of all ischemic brainstem strokes. Atherosclerosis of the VA and its branches is the most common cause. VA dissection is responsible for approximately 20%-30% of the cases.
The medulla is usually considered in two broad medial and lateral compartments.
Lateral
Lateral strokes are approximately five times more frequent than medial strokes.
They occur due to vascular occlusions in the VAs, the PICAs, or their perforating branches.
Wallenberg syndrome, due to an insult in the upper lateral medulla, is the classic stroke syndrome associated with this compartment.
Babinski-Nageotte
and Cestan-Chennais
are two less frequent variants of this syndrome.
Medial
They result from occlusions in the VAs, ASA, and their perforating branches.
Dejérine syndrome is the classic syndrome associated with medial strokes.
Incomplete strokes can show some clinical variations depending on the spared structures.
Hemimedullary strokes
They occur due to ipsilateral VA occlusions, proximal to the PICA and the anterior spinal artery.
This results in simultaneous infarction of both lateral and medial compartments, and the classic symptomatology is known as Reinhold syndrome.
Pons
The pons is the middle portion of the brainstem, located between the medulla and the midbrain. It has an extensive connection with the cerebellum via the middle cerebellar peduncles.
It is subdivided into two portions:
- The ventral basis pontis contains the pyramids, the pontine nuclei, and the pontine transverse fibers.
- The dorsal tegmentum contains the cranial nerve nuclei and the longitudinal white matter tracts.
The arterial blood supply originates essentially in the BA and its branches, namely the AICAs and the SCAs.
Isolated pontine infarcts represent 3% of all ischemic strokes. They mostly result from small perforating arteries microangiopathy or from hemodynamic effects due to basilar artery stenosis.
Due to anatomic and clinical similarities of the anteromedial and anterolateral zones, the pons is usually considered in two broad territories: the ventromedial and the lateral.
Ventromedial
It is the most commonly affected compartment.
Several classic stroke syndromes occur in this territory, which is irrigated by small perforating branches that originate in the BA.
- Ventral medial (Raymond)
- Ventral caudal (Millard-Gubler)
- Inferior medial (Foville)
- Upper ventral bilateral (Locked-in syndrome)
Lateral
Marie-Foix syndrome, resulting from the occlusion in small perforating branches from the BA and the AICAs, is the most characteristic syndrome in this territory.
Other lateral tegmental strokes will show their own location-dependent symptomatology.
Facial colliculus
The facial colliculus, which houses the abducens nucleus and the facial nerve fibers, is a unique feature located on the floor of the 4th ventricle in the pons tegmentum. Any lesion (mainly vascular or demyelinating) affecting this area can produce the so-called facial colliculus syndrome.
Midbrain
The midbrain is the upper part of the brainstem, located between the diencephalon and the pons. It connects with the cerebellum via the superior cerebellar peduncles.
It is subdivided into three portions, following the original laminae distribution:
- The ventral basis mesencephalic: it forms the cerebral peduncles, and contains the corticospinal and corticobulbar tracts.
- The tegmentum: lies between the peduncles and the aqueduct. Cranial nerve nuclei and various fiber tracts are located in this portion.
- The tectum: lies dorsal to the aqueduct, it is formed by the superior and inferior colliculi.
Arterial irrigation of the midbrain derives fundamentally from the SCAs and the PCAs. The internal carotid arteries provide little additional blood flow via the anterior choroidal arteries.
Isolated mesencephalic infarcts (due to small artery disease) account for only 0.9% of brainstem infarcts. More frequently, midbrain infarctions are secondary to large artery disease and accompany infarcts in the PCA territories (3%). Atherosclerotic artery-to-artery embolism is a more common cause than cardioembolism.
In clinical terms, the midbrain is usually considered in the ventromedial, tegmental, and posterior compartments.
Ventromedial
Ventromedial strokes result from the occlusion of small perforating arteries arising from the PCAs.
Weber or ventromedial syndrome, in paramedian ventral location, is one of the classic stroke syndromes in this compartment.
More laterally-located ventral strokes can present without oculomotor nerve palsy.
Classic stroke syndromes originating in the medial tegmentum are the Claude syndrome, in the dorsomedial aspect, and the Benedikt syndrome, also known as paramedian midbrain syndrome, slightly more peripherally located.
Other strokes will show variable symptomatology depending on the affected structures.
Lateral
Lateral tegmentum strokes also result from the occlusion of small perforating arteries arising from the PCAs.
Dorsal
Parinaud or dorsal midbrain syndrome is primarily caused by pineal gland masses that comprise superior colliculi in the tectum. Rarely, it can result from ischemic or demyelinating lesions. It is characterized by supranuclear paralysis of upward gaze, pupillary accommodation impairment, and eyelid retraction.