BALACLAVA HELMET DISTRIBUTION & Dejerine's onion-peel sensory loss?

(A) Divisional pattern of sensory loss(B) segmental pattern of sensory loss(C) schematic diagram of the trigeminal system in the brainstem

The spinal trigeminal nucleus (STN) is the longest cranial nerve nucleus, extending caudally from the medulla to the upper cervical segment 4 of the spinal cord. 

Cytoarchitecturally, the STN is divided into three portions from rostral to caudal:
  • Pars Oralis (from the Pons to the Hypoglossal nucleus/inferior olive)
  • Pars Interpolaris (from the Hypoglossal nucleus/inferior olive to the obex)
  • Pars Caudalis (from the obex to C4)

The STN has somatotopic arrangement, the central area 
of the face is represented by the rostral portion of the STN, and the more outer areas of the face are represented by the caudal portion of the STN.
Therefore, lesions invading the subnucleus caudalis cause onion-skin pattern sensory impairment.

The onion skin distribution differs from the dermatome distribution of the peripheral branches of the fifth nerve. 

Lesions which destroy lower areas of the spinal trigeminal nucleus (but spare higher areas) preserve pain-temperature sensation in the nose (V1), upper lip (V2) and mouth (V3) and remove pain-temperature sensation from the forehead (V1), cheeks (V2) and chin (V3). 

Although analgesia in this distribution is "nonphysiologic" in the traditional sense (because it crosses several dermatomes), this analgesia is found in humans after surgical sectioning of the spinal tract of the trigeminal nucleus.

There are some cases of onion-skin pattern sensory impairment involved intraaxial lesions, and all the lesions were beyond the C4 spinal level, such as cerebral infarction (medulla), syringomyelia (cervicomedullary junction to T12), demyelination, and spinal cord injury caused by needle entry (medulla to C7).

Extramedullary lesions also affect the STN, such as atlanto-axial dislocation, retroodontoid mass, presenting onion-skin dysesthesia.

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