Ventrolateral partial dorsal root entry zone rhizotomy for occipital neuralgia

Section of Neurosurgery, Department of Surgery, University of Manitoba, Health Sciences Centre, Winnipeg, Manitoba Correspondence and reprints: Dr Anthony M Kaufmann, GB 137 Section of Neurosurgery, Department of Surgery, University of Manitoba, Health Sciences Centre, 820 Sherbrook Street, Winnipeg, Manitoba R3A 1R9. Telephone 204-787-7263, fax 204-783-7356, e-mail akaufmann@hsc.mb.ca S Rasskazoff, AM Kaufmann. Ventrolateral partial dorsal root entry zone rhizotomy for occipital neuralgia. Pain Res Manage 2005;10(1):43-45.

O ccipital neuralgia (ON) is a radiculopathic condition characterized by paroxysmal lancinating pains in the distribution of the greater or lesser occipital nerves (1).The disease course may be self-limited or progressive.Treatment algorithms usually begin with analgesics and nonsteroidal anti-inflammatories.An advanced pharmaceutical treatment typically includes oral administration of carbamazepine and gabapentin in an escalating fashion until pain control is achieved or intolerable side effects develop.Some patients, however, become disabled by unrelenting and severe ON.Diagnostic anesthetic blocks of the peripheral occipital nerves are useful to confirm the diagnosis and provide temporary pain relief.Repeated steroid injections may provide long-term pain control (2)(3)(4).For patients with intractable ON pain, a variety of ablative procedures are often considered.These include peripheral neurectomy, C2 ganglionectomy by surgery or radiofrequency and intradural rhizotomy (5)(6)(7)(8)(9).In selected cases caused by C1 to C2 arthrosis, foraminal decompression of the C2 nerve root and ganglion have been effective (4,(9)(10)(11), while occipital nerve stimulator implants have been sucessful in others (12).
Sindou (13) first reported a microsurgical approach to the dorsal root entry zone (DREZ).The goal of this technique is the specific destruction of the small, unmyelinated fibres that enter the substantia gelatinosa of the spinal gray matter dorsal horn.Microsurgical DREZ has been successfully applied for treatment of localized pain associated with cancer (14), neuropathic pain from brachial plexus avulsion, cauda equine and spinal cord lesions, peripheral nerve injury, herpes zoster (15) and amputation stump pain (16).Dubuisson (17) described an application of DREZ for the surgical treatment of ON involving sectioning of the ventrolateral aspect of each posterior rootlet of C1 to C3 at the root entry zone.He provided a neuroanatomically sensible rationale and reported a high success rate in this single case series.As a follow-up to Dubuisson's promising findings, we present a case of medically refractory ON treated by ventrolateral partial rhizotomy (pVL-DREZ) of the ipsilateral C1 to C3 cervical dorsal roots.A 48-month, long-term outcome is included.

CLINICAL PRESENTATION
A 46-year-old woman presented with a progressive history of severe right-sided ON over 14 months.Her past medical history was remarkable for a whiplash injury at 18 years of age and rightsided migraine headaches, without recent episode.The onset of ON pain followed a viral type illness with nausea, vomiting and 'plugged ears'.The ON pain radiated along the greater occipital nerve territory and was associated with periorbital and right temporal pain.The pain had become virtually constant and the superimposed lancinating attacks continued.The pain could be triggered by pressure along the great occipital nerve course.She denied any subjective numbness or dysesthesia of the scalp, although examination revealed slightly reduced pinprick sensation of the great occipital nerve distribution of the right side.There were no other neurological complaints or deficits.Investigations were normal, including cervical X-rays, computed tomography of the head and magnetic resonance imaging (MRI) of the head and neck.She had tried various medical therapies, including anticonvulsants (Epival [Abbott, Canada] and Tegretol [Novartis Pharmaceuticals, USA]) and topical drugs (capsaicin and ketamine), with little benefit.Local injections of anesthetic agents and steroids provided very short-term relief for only few hours.Her prescribed oral opiates required supplementation with frequent medically administered opioid injections.

INTERVENTION
The authors' experience with the treatment of ON was limited, with no prior performance of open surgical procedures for patients with ON.Several treatment options were considered for the patient, and pVL-DREZ of the right C1 to C3 dorsal roots, as described by Dubuisson (17), was elected.The procedure also offered the possibility of avoiding potential complications of complete deafferentation.The option of occipital nerve stimulation was not available at the time of the patient's presentation, nor were the long-term outcome results of pVL-DREZ.
Through a C1 laminectomy and C2 right laminotomy, the ipsilateral upper cervical dorsal roots were exposed.A 1 mm deep incision was made at a 45° angle to the sagittal midline at the ventrolateral aspect of each dorsal rootlet entry: 3 at C1, 4 at C2, and 3 at C3 (Figure 1).A single C2 dorsal rootlet was sectioned due to the close proximity of a small artery intimately associated with the ventrolateral aspect of the root entry point.

OUTCOME
Postoperatively, there were no further episodes of ON and only a partial, mild numbness in the greater occipital nerve distribution.There was some new disequilibrium that resolved over two weeks, and some intermittent subjective mild right arm numbness and fatigability, yet, no objective indications were evident.An MRI of the cervical spine identified a small area of edema or injury, which was thought to account for these symptoms (Figure 2).Her incisional pain and generalized postoperative headache were treated with modest and tapering doses of intravenous morphine and Tylenol No 3 with Codeine  Despite the fact that laboratory investigations and an MRI of her brain were normal, on postoperative day 6 the patient suffered a cluster of brief, generalized seizures.These seizures were attributed to withdrawal from high-dose opioids.The pain service prescribed a higher dose of opioids to be tapered over several weeks; in turn, anticonvulsants were not required.
At the time of her three month follow-up visit, the patient had not experienced any further seizures or symptoms of disequilibrium.She was free of ON pain and did not require analgesics.She reported mild subjective scalp numbness, yet still managed to identify light touch and hair movement sensations.Her intermittent arm symptoms had improved, although not completely, and she had resumed a full range of strenuous activities.A 48-month telephone follow-up revealed that she had remained free of ON pain and had not experienced any further arm symptoms or seizures.She no longer required pain medications.

DISCUSSION
Occipital nerves have a unique course, rendering them susceptible to compression or injury at the neural foramen or transmuscular course (18).Known causes of ON include acute or chronic trauma and entrapment or inflammation at any point along the course of the C2 or C3 nerves, although in the majority of cases, there are no identifiable causes (1).Medical treatment of ON includes trials of anticonvulsants, steroids, topical medications, and steroid or neurolytic injections of the occipital nerves or C2 root and ganglion.Common surgical treatment options for intractable ON include a variety of peripheral ablation procedures that have potential disadvantages, including the complete loss of scalp sensation, severe vertigo due to the loss of proprioceptive sensation of upper cervical facets and anesthesia dolorosa.
Dubuisson (17) described the treatment of 11 ON patients by partial posterior rhizotomy at the C1 to C3 level and applying the microsurgical technique of partial posterior rhizotomy first described by Sindou and colleagues (19).Organization of somatic sensory processing within the spinal segment allows for the relief of pain while preserving touch and proprioceptive sensation.Large-diameter axons that mediate touch and limb position enter dorsomedially to the more ventrolateral small-diameter nociceptive axons.Use of this anatomical nuance has allowed the development of selective partial posterior rhizotomy, dividing only the ventrolateral portion of each sensory rootlet at the dorsal root entry zone.Interruption of nociceptive fibre tracts of Lissauer is achieved by placing multiple lesions at the ventrolateral junction of the dorsal rootlet and the spinal cord (Figure 1).The pVL-DREZ is produced with a microsurgical blade advanced at a 45° angle from the sagittal plane to a depth of 1 mm.This provides an anatomically sensible approach for the treatment of ON by preserving proprioceptive sensory fibres coursing along the dorsal aspect of the nerve and minimizing risks of postoperative vertigo and scalp numbness.
Moreover, in Dubuisson's series of 11 patients (17), seven had excellent results (pain free with no medications) and three had good results (greater than 50% pain relief with medications) over follow-up periods ranging from three to 66 months.There are no other published reports of pVL-DREZ for ON treatment.The patient in our case report enjoyed complete and lasting pain relief following the pVL-DREZ over a four-year follow-up period (to date).The potential of cord injury was also highlighted by a transient subjective deficit and MRI-defined injury, although the low risk associated with DREZ procedures has been quite well-established (20).

CONCLUSION
In summary, a case report of medically intractable ON successfully treated using a pVL-DREZ at the C1 to C3 level is presented.The patient reported complete long-term pain relief with preservation of gross sensation and without permanent neurological complications.This procedure may be considered as an alternative to other neuroablative interventions or occipital nerve stimulation for severe and medically refractory ON.

Figure 1 )Figure 2 )
Figure 1) Cross-section of the spinal cord at the cervical level featuring the site of surgical ablation