Cranial Neuropathies › Cranial Nerves 5, 7, 9, 10, & 12

Onionskin Facial Sensory Loss in Syringomyelia

Notes

Onionskin Facial Sensory Loss in Syringomyelia

Onionskin Facial Sensory Loss in Syringomyelia

Key concepts: central pathways and central somatotopic maps of the central sensory nuclei, and facial sensory disturbances in syringomyelia and other medullary and high cervical central cord syndromes.

Key Central Trigeminal Nerve Anatomy

  • Principal sensory nucleus lies lateral to the motor trigeminal nucleus within the mid-pons. It receives sensory fibers.
  • Spinal trigeminal nucleus spans from the mid-pons to the upper cervical spinal cord (its termination is variably listed as anywhere from C2 to C4).
    • Trigeminal sensory afferents enter the pons and descend the spinal trigeminal tract and then synapse in the adjacent spinal trigeminal nucleus.

The caudal end of the spinal trigeminal nucleus is continuous with the substantia gelatinosa, which lies within the dorsal horn of the spinal cord, and the spinal trigeminal tract is continuous with the posterolateral fasciculus (aka Lissauer's tract), which lies along the dorsal edge of the dorsal horn of the spinal cord.

The trigeminothalamic tracts

  • The central sensory afferents of the trigeminal nerve relay to the cortex through two different trigeminothalamic pathways to the ventroposterior medial nucleus of the thalamus: Anterior & Posterior.
  • The anterolateral portion of the principal sensory nucleus projects via the anterior trigeminothalamic tract to the contralateral thalamus.
  • The posteromedial portion of the principal sensory nucleus projects via the posterior trigeminothalamic tract to the ipsilateral thalamus.
  • The spinal trigeminal nucleus sends its' fibers via the anterior trigeminothalamic tract to the contralateral thalamus.
  • Small fiber sensory modalities project to the spinal trigeminal tract and nucleus.
  • Large fiber sensory modalities project to the principal sensory nucleus.

These leads us to create analogies between the principal sensory nucleus and the gracile and cuneate nuclei and the spinal trigeminal nucleus and the spinothalamic tract; however, postsynaptic interconnections make this an imperfect relationship that falls apart when we try to implement it, clinically.

The somatotopic organization of the principal sensory nucleus:

  • The divisions organize as divisions 1, 2, then 3, from anterior to posterior: the ophthalmic is anterior, maxillary is intermediate, and the mandibular division is posterior.

The spinal trigeminal nucleus:

Divide it into three different cytoarchitectural regions as follows:

  • Pars oralis is the superior-most subnucleus: it spans from the pons to the mid-medulla.
  • Pars interpolaris is the middle subnucleus: it lies in the mid-medulla.
  • Pars caudalis is the inferior-most subnucleus: it spans from the lower medulla to the upper cervical spinal cord. Again, its inferior extent is variably listed as anywhere from C2 to C4.

The onionskin somatotopic map of pars caudalis: the central sensory processing map for pain/temperature information in the face.

This map will help us understand facial sensory deficits in syringomyelia and other medullary and high cervical central cord syndromes.

  • The lips and perioral area constitute the outermost layer of the onion — they lie within the most superior area of pars caudalis; next innermost layer, moving inferiorly: the nose, eyes, and outer oral areas; then, the cheeks and forehead; then, the vertical band just in front of the ears; lastly, lies the partial spinal trigeminal sensory coverage of the external ear (from cranial nerves 7, 9, and 10).
  • We can actually best understand this map, if we draw a stretched out head (imagine pulling a rubber mask off of your face) to fit them into the long, columnar spinal trigeminal nucleus.
  • The superior features of the face (eg, the eyes) lie anterior and the inferior features (eg, the jaw) lie posterior; the most superior portion of the pars caudalis subnucleus receives the lips and perioral area and the most inferior component receives the outer ears.
  • Neurosyphilis was previously a common cause of onionskin loss, but since the advent of syphilis treatment in the US, now, the most common cause of onionskin loss is central cord syndrome.

Pay attention that our somatotopic discussion of the spinal trigeminal nucleus refers to the pars caudalis subnucleus, only; therefore, the most superior area is the inferior medulla and the most inferior area is the upper cervical spinal cord. Clinically, when we discuss the spinal trigeminal nucleus, we really only referring to the pars caudalis subnucleus.