The brainstem has four main medial structures or tracts:    -Corticospinal tract: This is the motor pathway that enters at the level of the superior cerebellar peduncles and descends through the antero-ventral region of the pons and anterior pyramids of the bulb and reaches the medulla oblongata where most of it decussates before entering the lateral cords of the medulla. If it is affected at the pontine level, it will present ataxia and hypotonia, while at the bulbar level it will be associated with contralateral hemiplegia.     -The medial longitudinal longitudinal muscle: carries information from the mechanoreceptors and proprioception. These fibres reach the inferoposterior aspect of the medulla oblongata and their lesion affects the contralateral sensibility.     -Medial longitudinal fasciculus: It is located close to the periaqueductal grey matter. Clinically, its involvement manifests as ipsilateral internuclear ophthalmoplegia.     -Motor nerves and nuclei: Their involvement leads to ipsilateral loss of function of the involved nerve (nerves III and IV in the midbrain, VI in the pons and XII in the medulla oblongata).

As for the lateral structures:-Spinocerebellar pathway: It transmits proprioceptive information from the body to the cerebellum and originates in the spinal cord and reaches the ipsilateral hemicerebellum. -Spinotalamic pathway: this is a sensory pathway from the skin to the thalamus and decussates at the level of the spinal cord. Its fibres run through the anterolateral region of the brainstem and its alteration causes impairment of the reception of algesic, thermal, diffuse pressure and coarse touch stimuli.- Sensory pathway: Part of the nucleus of the 5th cranial nerve which has a long pathway running along the lateral pontine side to the medulla oblongata carrying thermal, firm pressure, proprioception and fine touch information. - Sympathetic pathway: Symptoms include ipsilateral Horner's syndrome (ptosis and miosis). The grey matter in the brainstem is found in clusters along the brainstem, mainly forming the cranial nerve nuclei, the pontine nuclei and the reticular formation. The nuclei of the various cranial nerves are distributed as follows:- Medulla oblongata: glossopharyngeal, vagus, spinal accessory, hypoglossal- Protuberance: Trigeminal, abducens, facial, auditory -Suprapontine: olfactory, optic, oculomotor and trochlear.

The clinical picture of brainstem stroke may help to identify the affected region: Ascending and descending tract involvement may present with weakness, loss of pain and temperature sensitivity, ataxia, Horner's syndrome, loss of proprioception and/or gaze palsy.  Integrative functions may also be affected leading to choreoathetosis, tremor, ataxia, central dysautonomia or even locked-in syndrome. Ataxia and gaze paresis may also occur.  If the nuclei and cranial nerves are damaged, specific clinical manifestations include weakness of the ocular and extraocular muscles, loss of facial sensitivity, dysphagia, dysarthria, dysphonia, vertigo or altered taste and hearing.In case of bulbar lesion we can find lateral medullary syndrome or Wallenberg syndrome which is the most common, and medial medullary syndrome or Dejerine syndrome whose typical triad is contralateral hemiparesis and loss of proprioception with ipsilateral tongue weakness. If the lesion occurs at the pontine level, ventral involvement, via paramedian branches, is most frequently affected and typically presents with contralateral hemiparesis in isolation or in association with other symptomatology.  Anteromedial infarcts may also present with contralateral ataxia as well as dysarthria, dysphagia and ipsilateral facial paralysis. If the involvement occurs in the anteromedial region of the midbrain, structures critical for vertical gaze will be injured, typically producing vertical gaze palsy, in fact involvement of the third cranial nerve is considered a localising sign of this territory. Anteromedial lesions do not usually present with hemiparesis, although hemiparesis may occur in the case of anterolateral or combined infarcts.Having schematically reviewed the basic anatomy and functions of the brainstem structures, it is essential to understand the blood supply to each of them in order to adequately address ischaemic pathology in this region.

The vascularisation of the brainstem depends mainly on the vertebrobasilar system including the basilar artery, vertebral artery, spinal artery, posteroinferior cerebellar artery (PICA), anteroinferior cerebellar artery (AICA), superior cerebellar artery and posterior cerebral artery as well as the posterior communicating and anterior choroidal artery. All these structures give multiple branches that will irrigate each of the brainstem structures and are schematically divided into four groups: anteromedial, anterolateral, lateral and posterior. 

The brainstem vasculature is divided into anatomical structures, which are further subdivided into the following arterial territories:Bulbar arterial groups:Both the anteromedial and anterolateral territories are supplied by the anterior spinal artery (branch of the vertebral artery) and the vertebral artery itself.Lateral territory mainly through the posteroinferior cerebellar artery.The arterial supply of the posterior territory depends on the posterior spinal artery (originating from the intracranial portion of the vertebral artery).      Arterial groups of the pons: The anteromedial territory is going to depend mainly on terminal branches of the ipsilateral vertebral artery as well as perforating branches of the basilar artery. The anterolateral territory receives its main supply from perforating vessels of the basilar artery.The lateral territory receives lateral pontine perforating branches of the basilar artery, which supply the superior cerebellar peduncle. It also receives supply from both the AICA and the superior cerebellar. Posterior territory that will depend mainly on the superior cerebellar artery. Mesencephalic arterial groups

The anteromedial territory is supplied by the posterior cerebral artery.Anterolateral territory is supplied by the posterior cerebral artery or branches of the anterior choroidal arteryLateral territory is supplied by the posteromedial and collicular choroidal arteries (both branches of the P1 segment of the posterior cerebral artery), and may receive branches from the posteroinferior cerebellar artery.Posterior territory via the posteromedial and collicular choroidal arteries as well as via the superior cerebellar artery. The basilar artery therefore runs along the ventral side of the pons, giving perforating branches (between three and five pairs) which in turn can be divided into: paramedian branches (which take care of the more medial region including the corticospinal tracts, medial longitudinal fascicle and the medial lemniscus) and short and long circumflex branches (which take care of the lateral structures). Occlusion of these arteries, most frequently the paramedian ones, will result in lacunar infarcts that do not cross the midline. However, at the mesencephalic level, infarcts can be found at the midline level as the involvement is not usually limited to a single branch.

The aetiology of truncal infarcts are similar to those of other types of stroke and share risk factors with those of the anterior circulation such as hypertension, hypercholesterolemia or cardiovascular disease. In spinal cord infarcts, three quarters are due to vertebral artery stenosis, about 25% to arterial dissection and the remainder to cardioembolic causes. However, the number of infarcts due to cardioembolic aetiology increases slightly in pontine infarcts, representing less than 10%, and, more strikingly, in mesencephalic infarcts where in some series they represent up to 45%. Haemorrhagic strokes of the brainstem are mainly associated with hypertension (90% of cases) and may also be related to anticoagulant therapy, arteriovenous malformations, occult vascular malformations (such as cavernomas or telangectasias) or amyloid angiopathy.