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 Differential Diagnosis of Nervous System Disorders

= shift of normal brain from high to low pressure through rigid structures of skull ←⇑ intracranial pressure

Cause: mass effect by primary / metastatic tumor, trauma, infection (abscess) / inflammation, intracranial hemorrhage, subdural hematoma, ischemia / infarction, acute hydrocephalus, iatrogenic (after lumbar puncture / pneumocephalus following craniotomy)

Classification:

  1. SUPRATENTORIAL HERNIATION
    1. Uncal (transtentorial)
    2. Central
    3. Cingulate (subfalcine)
    4. Transcalvarial
    5. Tectal (posterior)
  2. INFRATENTORIAL HERNIATION
    1. Upward (upward cerebellar / upward transtentorial)
    2. Tonsillar (downward cerebellar)

Subfalcine / Cingulate Herniation (most common)  !!navigator!!

= contralateral shift of midline structures under falx cerebri

= herniation of cingulate gyrus across falx cerebri

Risk: compression of one of anterior cerebral arteries

May be associated with: transtentorial herniation

  • weakness / paresis of contralateral leg ← compression of parafalcine cortex
  • weakness ± sensory changes of contralateral leg ← infarction of paracentral lobule / superior frontal gyrus ← compression of ACA / pericallosal artery
  • somnolence ← raised intracranial pressure
  • early signs:
    • falx
      • shift of ipsilateral cingulate gyrus beneath falx
      • deviation of anterior falx with widened CSF space at contralateral side

      N.B.: posterior falx remains relatively undisplaced due to greater height + rigidity
    • cingulate gyrus
      • compression of contralateral cingulate gyrus
    • corpus callosum
      • depression of ipsilateral corpus callosum
      • depression / elevation of contralateral corpus callosum
    • ventricle
      • compression / effacement of ipsilateral ventricle with amputation of ipsilateral frontal horn
  • late signs:
    • displacement of lateral ventricle to opposite side
    • obstruction of foramen of Monro → contralateral dilatation of the lateral ventricle + subependymal edema
    • infarction of cingulate gyrus
    • compression of anterior cerebral artery → infarction of ACA territory

Assessment: degree of greatest displacement of septum pellucidum / falx measured in mm relative to a straight line drawn through anterior and posterior falcine attachments on axial image

Prognosis: good with shift of <5 mm; poor with shift >15 mm

Cx: traumatic aneurysm of ACA / pericallosal artery

Transtentorial (Central) Herniation  !!navigator!!

= herniation of brain up / down across tentorium cerebelli

Tentorium cerebelli = inelastic reflection of dura

Connected to: occipital bone posteriorly, petrous temporal bone laterally, clinoid processes anteriorly

Content: transverse sinus, straight sinus

Tentorial hiatus / incisura

Content: cerebral peduncles + brainstem

Alert: NO lumbar puncture with effacement of basal cisterns + displacement of 4th ventricle!

Descending Transtentorial Herniation

= downward herniation of brain toward posterior fossa

  • oculomotor nerve (cranial n. III) palsy:
    • ipsilateral dilated pupil (= mydriasis) due to uncal herniation → compression of parasympathetic fibers traveling on outside of CN III → unopposed sympathetic activity to iris sphincter m.
    • abnormal extraocular muscle function (except for superior oblique m., lateral rectus m., levator palpebrae superioris m.)
  • ipsilateral hemiparesis (on side of expanding lesion) (false localizing sign = Kernohan notch syndrome) due to severe lateral translation of midbrain against opposite tentorial edge → compression of opposite corticospinal tracts above decussation
  • permanent anterograde amnesia ← infarction of uncal / parahippocampal gyrus ← arterial compression
  • permanent visual field defect ← temporal / occipital lobe infarction ← compression of calcarine branch of PCA against tentorium

Location and degree of herniation:

  1. anterior / uncal herniation (see below)
  2. posterior: herniation of parahippocampal gyrus
  3. total: herniation of entire hippocampus
  • compression of ipsilateral cerebral peduncle
  • compression of contralateral cerebral peduncle → notching of midbrain (= Kernohan notch)
  • compression of aqueduct of Sylvius → early dilatation of temporal horn → obstructive hydrocephalus
  • widening of contralateral temporal horn
  • widening (obliteration) of ipsilateral (contralateral) basilar (ambient + quadrigeminal) cisterns

Cx:

  1. Occipital infarction ← compression of ipsilateral posterior cerebral artery against cerebral peduncle by uncus + parahippocampal gyrus
    • effacement / displacement of ipsilateral PCA
  2. Duret hemorrhage = hemorrhage in median / paramedian mesencephalon / tectum ← stretching of pontine perforators ← downward displacement of pons
  3. Respiratory arrest

Uncal / Anterior Transtentorial Herniation

= herniation of uncus (most medial part of temporal lobe) across tentorium cerebelli into suprasellar cistern

◊Most common subtype of transtentorial herniation caused by lesions in anterior half of brain

  • uncus displaced into suprasellar cistern → pressure on midbrain + brainstem
  • truncation of six-pointed star appearance of suprasellar cistern

Risk: (1) compression of midbrain (brainstem)

(3) Kernohan notch syndrome

Ascending Transtentorial / Cerebellar Herniation

= displacement of cerebellum through tentorial incisura superiorly = upward (superior vermian) displacement

Cause: slowly growing cerebellar / brainstem process, infarction

  • nausea & vomiting → obtundation → coma
  • compression + anterior displacement of 4th ventricle
  • occlusion of aqueduct → obstructive hydrocephalus
  • narrowing / effacement of ambient + quadrigeminal cistern
  • compression of pons against clivus
  • upward displacement of cerebellar vermis
  • superior displacement of tectum
  • “spinning top” appearance of midbrain due to bilateral compression on posterolateral aspect of midbrain
  • downward displacement of cerebellar tonsils

Cx:

  1. basilar artery compression ← displacement of midbrain / pons against clivus
  2. compression of vein of Galen / basal vein of Rosenthal → parenchymal congestion
  3. compression of posterior cerebral + superior cerebellar arteries ← superior displacement of cerebellum

Alar / Transalar / Retroalar / Sphenoid Herniation  !!navigator!!

= herniation of frontal lobe posteriorly across edge of sphenoid ridge

Associated with: transtentorial + subfalcine herniation

  • paucity of clinical symptomatology, clinically occult
  • posterior / descending: frontal lobe mass
    • frontal lobe displaced posteriorly
    • posterior displacement of sylvian fissure, temporal lobe + horizontal segment of MCA
  • anterior / ascending: temporal lobe / insula lesion
    • temporal lobe displaced anteriorly

Transforaminal / Tonsillar Herniation  !!navigator!!

= herniation of inferior mesial portions of cerebellum (= inferior tonsils) downward through foramen magnum

Commonly associated with: ascending (⅔) or descending (⅓) transtentorial herniation

  • neck pain, nystagmus, vomiting (in conscious patient)
  • Cushing response (= irregular respiration, bradycardia, hypertension) as warning sign in unconscious patient
  • decerebrate posturing

Risk: compression of medulla → respiratory arrest → cardiovascular collapse → coma → death

  • cerebellar tonsils at level of dens on axial images
  • cerebellar tonsils ≥5 mm below foramen magnum (= line connecting basion with opisthion) in adults; ≥7 mm in children on sagittal / coronal images
  • effacement of 4th ventricle / aqueduct → hydrocephalus of 3rd + lateral ventricles with transependymal CSF flow
  • ± concurrent upward displacement of vermis

Cx: compression of vulnerable PICA → cerebellar infarction

Alert: Known complication of lumbar puncture performed in context of elevated intracranial pressure!

Transcalvarial / External Herniation  !!navigator!!

= brain protrusion through fracture / surgical site of skull

Displacement of Vessels  !!navigator!!

  1. ARTERIAL SHIFT
    1. Pericallosal arteries
      1. Round shift = frontal lesion anterior to coronal suture
      2. Square shift = lesion behind foramen of Monro in lower half of hemisphere
      3. Distal shift = posterior to coronal suture in upper half of hemisphere
      4. Proximal shift = basifrontal lesion / anterior middle cranial fossa including anterior temporal lobe
    2. Sylvian triangle
      = branches of MCA within sylvian fissure on outer surface of insula form a loop upon reaching the upper margin of the insula; serves as angiographic landmark for localizing supratentorial masses
      Location of lesion:
      • anterior sylvian frontal region
      • suprasylvian posterior frontal + parietal
      • retrosylvian occipital, parietooccipital
      • infrasylvian temporal lobe + extracerebral region
      • intrasylvian usually due to meningioma
      • lateral sylvian frontal, frontotemporal, parietotemporal
      • central sylvian deep posterior frontal, basal ganglia
  2. CEREBRAL VEINS
    = indicate the midline of the posterior part of the forebrain showing the exact location of the roof of the 3rd ventricle

 Outline