Operative procedures on the second neuron.
a) Anterolateral chordotomy (Spiller& Frazier, 1912, Adson,1956) is useful when we wish to delimit the reception of pain over a larger, less well-defined, area, i.e. where rhizotomy is inadequate. It has the advantage that pain perception and temperature are eliminated, but not touch or deep sensitivity. Visceral pain is also eliminated. The insensitivity produced starts two segments below the level of section. Sometimes this interval is twice as long, since the fibres of the spinothalamic tract do not always cross over in the anterior commissure at the same level. If we wish to achieve complete elimination of pain below the level of operation, the anterolateral bundle may be severed bilaterally. This is not done at the same level on both sides: at least one spinal segment is left between the two operative sites. There are various opinions on the most advanageous level for this operation: it is clear from experience that the decisive factor is the site of the disturbance and its upper boundary. This sometimes involves the superior thoracic vertebrae (T 2—T 3) or even the cervical cord (C 2).
This operation necessitates opening the dura. The ligamentum denticulata is located alongside, this being the anchor of the cord. Sometimes it is advantageous to section neighbouring sensory roots in addition, in order to free the cord. If we cut bundles, or only one bundle, and we do not wish to operate on the contralateral side, the cord is rotated on its axis by 70°. Then a sharp narrow scalpel is inserted anterior to the insertion of the Hgamenta denticulata[to a depth of 4—5 mm (this distance is precisely marked on the scalpel blade), and the scalpel is passed anterior to the anterior root, so that a quarter of the spinal cord is severed (Plate IV, Fig. 25).
The description of this operation shows that this procedure is not as precise as we might wish, from the anatomical and surgical points of view. It is clear that the depth measurement of 4—5 mm is a variable, since the cord has different dimensions at different levels: its diameter in the thoracic section is 12 + 1 mm, at С 2 it is 12.5 mm, and at С 5, it is 15+1 mm. I have already stated above my doubts as to whether this will become a selective operation. We are able with a knife to section a quadrant of the cord, in order to get, posteriorly, the sacral fibres and, anteriorly, the superior thoracic or cervical fibres.
Even if thus far nothing untoward has happened it cannot be denied that we have no guarantee of optical control over the entire procedure. We work here by numbers, by rough orientation and by guesswork: everything is not as precise as we would wish for our operation. Despite the uncertainty which I have felt at each attempt I have never overstepped the boundaries which we have set ourselves.