John F. Howe

Mechanosensitivity of dorsal root ganglia and chronically injured axons: A physiological basis for the radicular pain of nerve root compression

&NA; The radicular pain of sciatica was ascribed by Mixter and Barr to compression of the spinal root by a herniated intervertebral disc. It was assumed that root compression produced prolonged firing in the injured sensory fibers and led to pain perceived in the peripheral distribution of those fibers. This concept has been challenged on the basis that acute peripheral nerve compression neuropathies are usually painless. Furthermore, animal experiments have rarely shown more than several seconds of repetitive firing in acutely compressed nerves or nerve roots. It has been suggested that “radicular pain” is actually pain referred to the extremity through activation of deep spinal and paraspinal nociceptors. Our experiments on cat lumbar dorsal roots and rabbit sural nerves have confirmed that acute compression of the root or nerve does not produce more than several seconds of repetitive firing. However, long periods of repetitive firing (5–25 min) follow minimal acute compression of the normal dorsal root ganglion. Chronic injury of dorsal roots or sural nerve produces a marked increase in mechanical sensitivity; several minutes of repetitive firing may follow acute compression of such chronically injured sites. Such prolonged responses could be evoked repeatedly in a population of both rapidly and slowly conducting fibers. Since mechanical compression of either the dorsal root ganglion or of chronically injured roots can induce prolonged repetitive firing in sensory axons, we conclude that radicular pain is due to activity in the fibers appropriate to the area of perceived pain.

A neurophysiological theory for the pain mechanism of tic douloureux.

&NA; In attempting to understand the mechanism of pain production in tic douloureux, one must account for the myelination pathology seen in the primary afferent fibers, the cases where the trigger is in a different division than the pain, the frequent lack of a fixed neurologic deficit, the effective trigger stimuli corresponding to large caliber axons which would not seem to involve the small axons usually associated with pain production, and similar puzzling features of the disease. We present a theory which satisfactorily predicts, or is consistent with, most known features of tic; it is based upon two mechanistic assumptions, both of which have strong experimental foundations in the literature. The first is the trigeminal dorsal root reflex, and the second is the creation of extra action potentials at sites of altered myelination.

Comparison of percutaneous radiofrequency gangliolysis and microvascular decompression for the surgical management of tic douloureux

Forty-two patients with tic douloureux underwent posterior fossa craniectomy and microvascular decompression (MVD) or partial rhizotomy of the trigeminal nerve and were followed an average of 25 months after operation. Thirty-six patients were found to have anatomical distortions of the nerve by an artery, vein, bony prominence, or a combination of factors, and 30 patients (83%) of this groups hav remained pain-free postoperatively. Six patients had no discernible pathological condition at the time of operation and underwent partial trigeminal rhizotomy. No patient underwent repeated MVD or rhizotomy, although 4 patients whose pain recurred after MVD underwent rhizotomy at a second operation. Eight of the 10 patients treated by rhizotomy are currently pain-free. The overall success rate of the entire group is 90%; 2% experienced a complication, and there was 1 perioperative death. Seventy-eight patients with tic douloureux who underwent 92 percutaneous radiofrequency trigeminal gangliolysis (PRTG) procedures were evaluated on average of 56 months postoperatively. Sixty-eight per cent of these patients when evaluated 1 year postoperatively were pain-free. However, only 35% of the PRTG procedures resulted in continued pain relief 5 years after operation. Twelve of the 78 patients (15%) required repeat gangliolysis because of recurrent tic pain. Considering all 78 patients treated with 92 PRTG procedures, 64% were pain-free at follow-up examination. PRTG was associated wtih an 8% risk of complications, which included anesthesia dolorosa, corneal anesthesia with keratitis, and significant facial paresthesias. Both PRTG and MVD have advantages. MVD should be considered because: (a) it attacks what is believed to be the primary etiology of tic douloureux, (b) the trigeminal nerve is preserved, (c) postoperative pain relief dose not depend upon the production of sensory deficit, and (d) it may have a greater potential for producing long-lasting pain relief. However, PRTG has other advantages: (a) it avoids the risks of craniectomy, (b) it is repeated easily if tic pain recurs, (c) morbidity is minimal and there is essentially no risk of mortality, and (d) it is much less expensive.

Can Neuralgias Arise from Minor Demyelination? Spontaneous Firing, Mechanosensitivity, and Afterdischarge from Conducting Axons

Abstract Mammalian peripheral axons respond to local disruption of their myelin sheath with membrane changes which support continuous conduction of the impulse through the affected region. We report here that sites of demyelination may become foci of spontaneous impulse initiation. Such sites may also generate ectopic discharges upon slow mechanical distortion. Finally, conduction of an impulse train through a demyelinated region may set off an ectopic afterdischarge that may last many seconds. Rhythmic ectopic firing in dysmyelinated but conducting axons is very similar to that observed in regenerating axons and nerve-end neuromas. Although the latter have long been recognized as sources of pathophysiologic sensations, this is the first indication that neuralgias could arise following minor dysmyelination in peripheral nerves without substantial conduction deficits.

Chlorpromazine treatment for neurogenic pulmonary edema

A patient with neurogenic pulmonary edema was successfully treated with the alpha-blocking agent, chlorpromazine. A pathophysiologic basis for this drugs efficacy is discussed.

Intraoperative Ultrasound Detection of Metastatic Tumors in the Central Cortex

Real-time sector ultrasonography precisely located metastatic tumors of the central motor cortex during craniotomy in two patients. In these two cases, the only surface abnormality was swelling of several overlying gyri. Intraoperative ultrasonography precisely located the 1.5- and 2.5-cm-diameter tumors to a position below a specific gyrus, enabling the surgeon to excise the tumors through small, accurately placed cortical windows. The precise location minimized exploratory probing and the size of the cortical incision required to identify and remove the tumors. This technique will have general application in similar situations when the cortical surface gives no indication of underlying tumor location. By charting the best trajectory for the surgical approach, this technique may replace a variety of stereotactic and computed tomography-guided techniques for biopsy of deep brain tumors.

Phantom limb pain — a re-afferentation syndrome

Abstract The physiological mechanism of the deafferentation pain of brachial plexus avulsion is unknown. The recent neurophysiological documentation of dorsal horn neuroplasticity following primary afferent destruction predict a variety of anomalous sensory phenomena. The case presented here demonstrates that these predicted sensory phenomena do occur following brachial plexus avulsion and they can be eliminated by dorsal horn coagulation. Based on the consistency of the neurophysiological and clinical observations, a hypothesis involving dorsal horn neuroplasticity following deafferentation is presented to explain phantom limb pain. In brief, the hypothesis states that deafferented high-threshold second-order dorsal horn neurons become ‘re-afferented’ by low-threshold primary afferents, either mono- or polysynaptically from adjacent and distant dermatomes.

The time course of functional alterations in degenerating dorsal column afferents to lateral cuneate nucleus.

Abstract Unilateral dorsal funiculotomies at the C5 level were done in 13 adult cats. At 24-hr intervals from 24 to 120 hr after funiculotomy the function of the dorsal column afferents to the lateral cuneate nucleus was studied. Synaptic function was compromised by 24 hr and essentially absent by 48 hr. Cortically induced primary afferent depolarization in the dorsal column terminals in lateral cuneate nucleus was present at 24 hr but could not be detected at 48 hr or later. Axonal conduction persisted for 48 hr but was not detected at 72 hr. These results are compared to the development of deafferentation hyperactivity in lateral cuneate nucleus neurons and to the sequence of fine structural alterations in the lateral cuneate nucleus following dorsal rhizotomy.

Physiological evaluation of the effect of fascicular ligation on neuromas in the rat

Microfascicular double ligation and interligature heat sealing has been clinically successful in relieving neuroma pain. In rats, we found that this technique had no effect on the abnormal activities (such as mechanosensitivity and spontaneous activity) of single myelinated axons from neuromas or on their conduction velocity distribution. Assuming that the clinical success of this method is not fortuitous, we challenge the prevailing theory of the relationship between these abnormal unit activities and pain.

How to recycle a used brain surgeon.

THE NEUROSURGICAL APPRENTICESHIP is arduous, and the rigor of the discipline combined with the demands of any type of neurosurgical practice is, if not exhausting, at least wearying. By the age of 60 or so, operating all-day and taking call all night is no longer as compelling as it was earlier. However, preparing to retire, we might discover that having become a board-certified neurosurgeon has excluded many other options. Accustomed to working, complete retirement is not appealing to some. Maybe there is more to learn, more delight to be found, more care to be rendered.