Hypertensive brainstem encephalopathy

Hypertensive brainstem encephalopathy (HBE) is a clinicoradiological diagnosis characterized by severe hypertension (SBP >200mmHg), vasogenic edema of the brainstem, and a variable presentation of acute-subacute neurological disturbances. One of the primary diagnostic features often seen is the apparent clinical-radiological dissociation between the relatively mild neurological symptoms and moderate to severe radiological changes seen in the brainstem. Whilst isolated brainstem involvement has been reported to be a rare presentation of HBE, it is more commonly seen in combination with posterior reversible encephalopathy syndrome (PRES) as an uncommon distribution location (13%) ^1,2.

Like other conditions on the spectrum contained within PRES, HBE is potentially reversible if treated appropriately, however can lead to permanent brain damage or death if the hypertension is not treated promptly ³.

The reversibility of HBE is best demonstrated with post treatment neuroimaging, acting as both a confirmatory test and a safeguard to any other serious differentials such as stroke or central pontine myelinolysis (CPM) in the hyponatremic patient ^4,5.

Terminology

The term hypertensive brainstem encephalopathy refers to isolated vasogenic edema of the brainstem in response to acute severe systemic hypertension, in the absence of pathological edema of other posterior structures.  Some extension to surrounding structures may be present provided it does not involve the parieto-occipital regions of the cerebrum, as it would then be termed PRES, of which HBE can be considered a subset of.

Clinical Presentation

Patients may present with a broad range of symptoms and sequelae typically categorized in 3 overlapping groups ^2-6:

• hypertensive sequelae
  • stroke
  • aortic dissection
  • myocardial infarction           
  • angina
  • pulmonary edema
  • congestive heart failure
  • renal failure  
•  neurological symptoms
  • headache
  • dizziness
  • vomiting
  • broad based gait
  • weakness
  • visual disturbances and retinopathy
  • behavioral change
  • altered consciousness
  • seizures
  • low mini mental state exam (MMSE) scoring
• combined
  • headache
  • visual disturbances and retinopathy
  • seizures

Pathology

HBE is thought to arise when the autoregulatory mechanisms involved in the maintenance of normal intracranial pressure (ICP) are overwhelmed due to severe hypertension, leading to endothelial dysfunction, brain hyperperfusion and brainstem vasogenic edema ^5,7.

It is hypothesized that this breakdown in autoregulation primarily affects the posterior circulation due to the relatively decreased sympathetic innervation of the posterior circulation ⁸.

Etiology 

• severe systemic hypertension ³
• essential hypertension
• secondary hypertension
  • pre-eclampsia / eclampsia
  • recreational drug toxicity or withdrawal
  • renal artery stenosis
  • pheochromocytoma
  • hyperaldosteronism
  • hyperthyroidism and hypothyroidism
  • idiopathic

Radiographic features

• CT
  • diffuse hypodensity of any part of the brainstem but primarily affecting the pons ³
  • ambient cistern volume reduction or obliteration suggestive of edema ³
  • may include:
    • apparent clinical-radiological dissociation where the radiological features are severe compared to provided clinical features
    • vascular edema of the posterior cerebrum as part of PRES or cerebellar white matter ³
• MRI 
  • increased FLAIR/T2 signal edema in the brainstem with resultant enlargement of the affected areas
  • lack of restricted diffusion commonly seen on DWI with infarction ²

History

HBE was described as a separate entity as early as 1999 ⁹, however brainstem pathology on neuroimaging due to hypertension, has been described since 1978 ¹⁰.

Differential Diagnosis

• PRES
  • more common diagnosis that may include the signs of HBE, but will usually have a parieto-occipital dominance
• osmotic demyelination syndromes ^2,511
  • usually presents with abnormal serum electrolyte levels and improves with reversal of electrolyte imbalances
• infectious brainstem encephalitis ^2,3,11
  • a lack of appropriate history, negative CSF culture and rapid clinical improvement with antihypertensive therapy often rules this out
• acute brainstem infarct ^2,3,5,11
  • in stroke, high DWI signal in the acute phase is typically seen, compared with no diffusion restriction seen in HBE
• neoplasia ^2-4
  • often ruled out with the absence of a mass-occupying lesion and rapid clinical improvement after antihypertensive treatment in HBE
• vasculitis ³
  • changes in the CNS are non-specific and rarely well localized
• hypertensive encephalopathy caused by SLE, cryoglobulinaemia, hemolytic uremic syndrome or cyclosporine/cisplatin/tacrolimus-related encephalopathy ²
• metabolic encephalitis ¹¹
  • usually occurs with a history of hypoxic brain damage, hyperglycemia, liver disease or other metabolic disorders