SENSORY EXAM: ANATOMY
Clinically, there are 2 major somatosensory pathways that are examined. The first is the spinothalamic (ST) part of the anterolateral system and the second is the dorsal column-medial lemniscus (DCML) system. The principle sensory modalities for the ST system are pain and temperature. The principle sensory modalities for DCML system are vibratory, position sense and discriminatory or integrative sensation.
The anatomical pathways for the 2 major sensory systems is as follows:
ST- the axons from the 1st order neuron located in the dorsal root ganglion enter the dorsal root entry zone and within several segments synapse with 2nd order neurons in the dorsal horn. Axons from the 2nd order neuron cross immediately via the ventral white commissure to the anterolateral quadrant of the spinal cord then ascend as the spinothalamic tract to the ventral posterior lateral nucleus (VPL) of the thalamus. The axons of the 3rd order neurons project to the postcentral gyrus or somatosensory cortex (there are also projections to the insular and anterior cingulate cortex but we are mainly focusing on the primary somatosensory cortex).
Dorsal Column-Medial Lemniscus
The axons from the 1st order neurons located in the dorsal root ganglion enter the dorsal root entry zone and then ascend in the dorsal columns on the same side of the cord until they reach the 2nd order neurons in the medulla. Axons from the 2nd order neurons cross at the level of the medulla and then travel near the midline in the medial lemniscus. By the time the medial lemniscus reaches the rostral pons it has moved laterally and at this point it is in close proximity to the spinothalamic tract as it ascends to the VPL of the thalamus. The 3rd order neuron projects to the primary somatosensory cortex in the postcentral gyrus.
The trigeminal system is the somatosensory system for the face, which is clinically tested in the cranial nerve exam. For the trigeminal system it is important to remember that the descending tract of the trigeminal nerve, which serves pain and temperature, descends to the level of the upper cervical spinal cord and then axons from the 2nd order neurons cross over to the opposite side and ascend to the ventral posterior medial (VPM) nucleus of the thalamus.
Level of Crossing
The following are important anatomical points to remember that have significant power in localizing lesions:
The level of crossing of the axons of the 2nd order neurons is immediate for the ST system and not until the medulla for the DCML system.
Location of Tracts
The ST tract is lateral in the cord and lower brainstem while the DCML system is dorsal and medial in the cord and medial in the lower brainstem. It is not until the rostral pons that the 2 tracts are anatomically close to each other.
The descending trigeminal tract is ipsilateral to its origin and axons from the 2nd order neurons cross at the lower medulla-upper cervical spinal cord level.
The above anatomical points translate into the following clinical findings:
Spinal cord and lower brainstem lesions can result in sensory dissociation, which means one sensory system is affected without the other one.
Crossed or alternating findings. For example one side of the face is affected and the opposite side of the body for brainstem lesions. In the spinal cord, lesions can cause DCML system findings on one side of the body and ST findings on the opposite side.
The ST is examined by testing:
The DCML is examined by testing:
Discriminative sensation (must have intact DCML plus intact parietal cortex):
Double simultaneous Stimulation
Light touch is represented in both the ST and DCML system so it is OK for sensory screening but not specific for either system.
The sensory exam is perhaps the most subjective of the entire neurological exam so patient response can be difficult to interpret or at times be misleading.
A sensory level is valuable in determining if there is spinal cord disease. Pain (sharp) is used to determine a sensory level. The sensory level on examination is usually 1-2 spinal cord segments below the actual spinal cord lesion.
A sensory deficit from a spinal nerve lesion will be in a dermatome distribution.
A sensory deficit from a peripheral nerve lesion will be in the distribution of that peripheral nerve.
A sensory deficit from a polyneuropathy will have a stocking and glove distribution because the longest axons are the most affected.