Postcraniotomy Pain

Definition

Acute postoperative pain is defined as the pain occurring within first 24–48 h after a craniotomy.

In a pilot study, De Benedittis et al. found that 60% of postcraniotomy patients suffered from moderate to severe postoperative pain. In two-thirds of the patients, the intensity of pain was moderate to severe. Pain was mainly seen in the first 48 h after surgery, with subtemporal and suboccipital craniotomies associated with the highest incidence of postoperative pain. Despite the advances in the treatment of acute pain, a gold standard for analgesic therapy in this subgroup of patients is still lacking.

Anatomical and Physiological Basis of Pain Following Craniotomy

The calvarium encloses and protects the brain. The facial skeleton forms the lower part of the skull and articulates with the mandible. The scalp consists of five layers: skin, subcutaneous tissue, epicranium, subaponeurotic areolar tissue, and the pericranium.

The interior of the cranium is lined with a fibrous membrane, the endocranium, which is the outer zone of the dura mater. It becomes continuous with the periosteum on the outer surface of the skull, the pericranium. The brain is enveloped by three layers of meninges: the dura mater, the arachnoid, and the pia mater.

Innervation of the scalp and the dura is mainly arising from:

• 1.

  The trigeminal nerve, and its three principal divisions (mandibular, maxillary, and ophthalmic) and their branches;

• 2.

  The upper three cervical nerves and the cervical sympathetic trunk;

• 3.

  Minor branches from the vagus, hypoglossus, facial, and glossopharyngeal nerves.

The anterior scalp region is innervated by the supraorbital and supratrochlear nerves (branches of the frontal nerve). The temporal scalp region is supplied by the zygomaticotemporal, temporomandibular, and auriculotemporal nerves (branches of the trigeminal nerve; Figure 1 ). The occiput and scalp regions receive their sensory innervations from the greater auricular and the greater and lesser occipital nerves (originating from the cervical plexus). The dura mater is innervated by nerves that accompany the meningeal arteries.

Surgical approaches to the skull are mainly supratentorial and infratentorial. Supratentorial craniotomies are mainly frontal, frontotemporal, temporal, and pterional.

Pathogenesis of Postcraniotomy Pain

Postcraniotomy pain is superficial in character, suggesting a somatic rather than a visceral origin. The pain mainly originates from the pericranial muscles and soft tissues. The suboccipital and subtemporal craniotomies have the highest incidence of pain due to nociceptive pain from surgical incision and reflection of major muscles as temporal, splenium capitis, and cervicis. The skull can be drilled and opened without discomfort to the patient (no sensory innervation). Pain is not thought to arise from the dissection of the brain tissue itself.

Compared to the traditional theory of pain perception, where it was believed that pain is directly transmitted from somatic receptors to the brain, clinical and experimental evidence shows that noxious stimuli may sensitize the central neural structures involved in pain perception. Experimental evidence shows the development of sensitization, wind-up, or expansion of receptive fields of central nervous system neurons.