Occipital Nerve Stimulation

Anatomy and Physiology

The nerves that innervate the occipital area includes the greater (a branch of C2 spinal root), lesser (a branch of C2 and C3 spinal root), and least occipital nerve (a medial branch of the posterior division of C3). The greater occipital is the largest pure sensory nerve and provides sensory innervation to the skin of the posterior skull, occipital, and temporal area. It is closely associated with the occipital artery at the exit site. The lesser occipital nerve is a cutaneous branch that provides sensory innervation to the skin of the upper and back part of the auricle. The least occipital nerve lies medial to greater occipital nerve and supplies C2-C3 zygapophyseal joint, part of semispinalis capitis, and a small area below the occiput.[7] While the precise mechanism of action of ONS is still unclear, there has been a proposal that the continuous stimulation of the occipital nerves may produce diverse functional and metabolic changes in many brain regions, including pons, midbrain, and periaqueductal gray.[8]

Indications

ONS is indicated in primary (without an underlying organic cause) and secondary (caused by an underlying condition, such as neck injuries, infections, or severe high blood pressure) intractable headache disorders including migraine and non-migrainous chronic headaches, and neuropathic pain in the occipital region.[9] Chronic headache is commonly defined as a headache on 15 or more days every month for longer than three months.[10] Patients with refractory headaches usually have failed results with conventional drugs either because of lack of effect, intolerable side effects, or contraindication to using.[10]

Indications for occipital nerve stimulation include: 

1. Occipital neuralgia is a neuropathic pain condition featuring severe sharp, shooting, or electric shock pain involving the occipital or suboccipital region, probably due to long-term compression of the C2 nerve. It is the third most common headache syndrome following migraine and tension-type headaches.[6]
2. Chronic migraine. It is the third most prevalent ailment in the world, affecting approximately one billion people worldwide.
3. Cluster headache: An excruciating attack of pain in 1 side of the head, often felt around the eye.
4. Cervicogenic headache: A chronic headache that arises from the atlantooccipital and upper cervical joints.
5. Hemicrania continua: It is a primary chronic daily headache disorder.

Recently, this approach has been successful in treating a case of chemotherapy-related headaches.[11]

Contraindications

The contraindication of ONS are as follows[12]:

1. If there is no improvement in the quality of life after a trial period, this procedure is not advised.
2. If there is any concern about opioid misuse, then that may disqualify the patient from getting implant despite improvement with the trial block.
3. Local infection
4. Bleeding abnormalities
5. Arnold-Chiari malformation
6. An implanted electrical device (e.g., pacemaker, spinal cord stimulator) that may interfere with this therapy
7. Pregnancy

Equipment

The equipment required for ONS include[13]:

• Electrodes and leads: There are two types of leads; percutaneous (thin and cylindrical), and surgical or paddle (broad and flat). The percutaneous leads can be inserted using a needle, while paddle leads require a surgical method. The electric field around the percutaneous lead is circumferential; it gets directed towards one side in the case of paddle leads, which helps to stimulate even at low amperage, thereby prolonging battery life.
• Anchors: These help to attach leads and extensions to connective tissue.
• Extensions: These cables come in various lengths and are used to connect leads to the power source.
• A power source (battery or internal pulse generator, IPG): It is usually implanted in a subcutaneous pocket away from the lead. The battery life depends on the amplitude and number of hours of use the device has in a day. Usually, a lithium battery lasts for 3 to 5 years, and rechargeable batteries require replacement after 7 to 9 years.

Preparation

Preprocedural planning. A trial is necessary before permanent ONS implantation. In the trial period, leads are placed under the skin and connected to an external battery. The leads get inserted into the trailing cable. Stimulation is provided temporarily using a radiofrequency transmitter to different electrode combinations; this helps the patient to report the location, intensity, and sensation of stimulation. The trial period lasts for 4 to 7 days. During this period, the patient must maintain a headache diary. ONS is considered successful if there is more than fifty percent improvement in pain, or there is a decrease in the number of headache days or the patient reports improvement in the quality of life. The patients with successful block qualify for permanent implantation.[14] After the procedure, a complete healing time lasts for approximately six weeks. During that period, the patient is to avoid rigorous activity.

Local anesthesia vs. general anesthesia. Weiner and Reed performed the placement of electrodes under local anesthesia. The advantage with local anesthesia is that it allows the patient to assess the site of paresthesia and thus verify electrode placement. This procedure can also take place under sedation with propofol (i.e., procedural sedation), and the staff can wake the patient to check areas of paresthesia intra-operatively. Recently, physicians have also been performing the procedure under general anesthesia in the prone position, which decreases the risk of lead migration.[15]

Positioning. The procedure is possible in a lateral or prone position, depending on the entry point of incision.

Technique or Treatment

ONS is a minimally invasive technique in which a stimulator is implanted either unilaterally or bilaterally at the level of occipito-cervical junction. The stimulation parameters, like pulse width, amplitude, and frequency, can be adjusted to create paresthesia in the distribution of the occipital nerve. The procedure is performable under a lateral or medial approach.

Lateral approach. Weiner and Reed originally described the lateral approach in which the clinician makes an incision on the mastoid process, and the catheter was then subcutaneously advanced towards midline at the level of C1 with the help of fluoroscopy.[2]

Medial approach. A 2 cm midline incision is made at the level of C1, and then leads are inserted laterally from the midpoint.

The advantages of the medial approach include[16]:  

1. There is more subcutaneous fat in midline than laterally, which gives enough space to make a large subcutaneous pocket for placement of leads and adequate fixation to minimize dislocation.
2. It is a more acceptable approach for patients who use spectacles and have complained of pain at the incision site.
3. The bilateral electrodes are implantable through a single incision.

Implant placement. The different positioning sites of the IPG include the upper buttock, abdomen, infrascapular region, and infraclavicular regions.[13] The advantage of placement in the upper buttock is that both electrode and generator can be placed in a prone position, whereas lateral or supine position is necessary for the other sites. It takes place under general anesthesia.

Programming. Specific personnel appointed by the manufacturer do the device programming and maintenance. 

A skull X-ray is taken after procedure completion to document the final lead position; this is also useful in the future to assess complications associated with lead migration or fracture.

Complications

The procedure-related complications are hardware-related and biological.

Hardware-related complications

Lead migration is by far the most frequent complication reported with ONS. In a retrospective analysis of fifteen patients treated for a medically refractory headache, Schwedt et al. reported the rate as high as 60 to 100%.[17] Moreover, this was also the most frequently reported adverse event (12 of 51 patients) in the ONSTIM randomized controlled trial on cephalgia.[18] The various studies on occipital neuralgia report an 8 to 10%  incidence of lead migration.[19][20][2] Of note, the occurrence of the complications is reducible by decreasing mechanical stress on the leads by avoiding excessive neck movement. Moreover, these complications are usually related to severe trauma, such as a major car accident. Other methods described in the literature to prevent lead migration include the use of silicone glue, soft neck collars, and the use of a midline approach.[13] The other hardware-related complications include lead fracture and malfunction and repositioning of the generator.[17][18][2][9]

Biological complications

Patients treated with implanted neuromodulation devices often report pain related to the site of device components.[21] Saper et al. reported two cases of pain, one over the IPG site and other burning pain over the lead site. The other complications reported in retrospective studies include muscle spasms, wound dehiscence, infection (up to 4%), and skin erosion.[18][22] Infection can be prevented by prophylactic antibiotics, maintaining asepsis throughout the procedure, and trimming of occipital region hair using clippers.

Some other complications associated with ONS include device malfunction, compliance issues, ineffective stimulation, and device shock.[21]

Clinical Significance

Chronic headache affects 3 to 5% of the world population and ranks third highest cause of years lost due to disability.[23] Recent advances in the medical management of headache disorders have led to the effective treatment of the majority of patients. However, a small percentage of these patients do not respond to conventional treatment options. While ONS could be an interesting option in the treatment of this subset of patients, there is only limited scientific evidence regarding its effectiveness as the studies conducted so far have enrolled a small number of patients and are uncontrolled investigations. Furthermore, there is little data on the long-term results. A multicentric, controlled (ONS compared to the optimal medical management), and randomized study organized by the Nantes University Hospital is ongoing. The estimated study completion date is the end of 2022, and its results should clarify many doubts about the actual utility of the technique.

Enhancing Healthcare Team Outcomes

A systemic review on chronic migraine demonstrated that the short term effect of ONS might be modest but still clinically significant in patients with refractory headache.[24] While some patients apparently experienced significant improvement, the data is limited. At the same time, it correlates with the procedure and device-related complications. Currently, ONS seems a valuable option for patients who have failed all conventional treatment, but further studies are needed to evaluate the safety and efficacy. Due to the lack of conclusive data, currently, the use of ONS should be based on individual preference and treatment response.