Clinical Vignette

A 58-year-old car salesman presented to his physician noting recent onset of a subtle numbness on his left cheek first apparent while shaving or washing his face. He pointed to a “dead area about the size of a dime” over his left zygoma. This patient had a minor pea-sized skin lesion removed from that same area of his face 2 years earlier and the wound healed uneventfully. He otherwise enjoyed perfect health. Twenty months later, he first noted the numbness, describing it as a minor sense of something crawling under his skin. He first mentioned this symptom during a routine checkup with his internist, who could not find any abnormalities such as skin induration, tenderness, or lymphadenopathy. A referral to his previous surgeon also failed to demonstrate any abnormalities; his examination was “totally benign.” For completeness, the surgeon had the tissue pathology reviewed by a pathologist, who reassured that total removal of the tumor was achieved and tumor margins were “clean.”

The area of numbness was described as “dead skin.” These symptoms had an insidious onset. When the patient’s symptoms persisted and became more well defined, he was referred to a neurologist. A very small area of subjective sensory loss, the size of a dime, was noted; the remainder of his neurologic examination was normal. MRI scan of the face and skull base was normal. The patient was advised to return for follow-up examinations every 6 months. Although the small degree of sensory loss persisted, no new clinical or MRI findings were defined during the next 2.5 years. The patient did not return for his 3-year follow-up despite his neurologist advising him to do so. Almost 3.5 years after the onset of his facial numbness, the area of sensory loss enlarged and the patient’s wife felt that his left eye was more prominently seen within the orbit. Repeat neurologic exam confirmed the presence of a larger “quarter-sized” sensory loss. MRI now demonstrated a large infraorbital mass. Biopsy demonstrated metastatic squamous cell skin cancer.

Comment: Invasion of trigeminal nerve perineural spaces is a rare but well appreciated complication of various facial skin malignancies. As this case dramatically illustrates, these can have an insidious onset, with no more than subtle sensory loss at the onset. Excellent but nondiagnostic imaging studies can lead to a false sense of security for the patient and inexperienced clinician alike. Eventually the tumor may metastasize along a peripheral cutaneous nerve per se, in this case the maxillary nerve (second division of the trigeminal), to the base of the brain, where the tumor penetrated the foramen rotundum. This centripetal spread along the trigeminal nerve and its gasserian ganglion is rare. Any epithelial squamous cell carcinoma (SCC) has this discrete potential for a perineural spread with potential central nervous system dissemination. The indolent progression of a cranial nerve palsy in any patient with history of a resected cutaneous SCC of the head and neck must raise clinical suspicion of perineural spread, even in the absence of positive findings on detailed and repeated MRI imaging.

Anatomy

The trigeminal cranial nerve (CN-V) has three major divisions: ophthalmic, maxillary, and mandibular (Figs. 6-1 and 6-2). CN-V is the major sensory nerve of the face, mouth, and nasal cavity. It also supplies motor and proprioceptive fibers to the muscles of mastication. The trigeminal nerve sensory and motor roots are derived from a large sensory and smaller motor nucleus within the pons. These roots converge to emerge at the midlateral pons, then continue toward the trigeminal (Gasserian) ganglion at the base of the middle cranial fossa. General sensory fibers are derived from their cell bodies of origin within this ganglion. Distal to the trigeminal ganglion, three sensory divisions respectively exit the skull through the superior orbital fissure, foramen rotundum, and foramen ovale. The motor component passes through the trigeminal ganglion into the mandibular division.

Figure 6-1 Trigeminal Nerve (V): Schema.

Figure 6-2 Ophthalmic (V1), Maxillary (V2), and Mandibular (V3) Nerves.

CN-V Nuclei

The sensory nucleus is the largest of the trigeminal pontine complex. This begins rostrally within the midbrain, and extends caudally through the pons and medulla into the second segment of the cervical spinal cord (Fig. 6-3). It is subdivided into three portions: (1) the spinal tract nucleus primarily dedicated to pain and temperature fibers; (2) the principal sensory nucleus, the pontine trigeminal portion, which primarily receives tactile stimuli and, therefore, principally subserves light touch; (3) the mesencephalic sensory nucleus, which contains cell bodies of sensory fibers carrying proprioceptive information from the masticatory muscles. Lastly, there is a motor nucleus located within the pons medial to the large sensory nucleus.

Figure 6-3 Cranial Nerve Nuclei in Brainstem: Schema.

Principal Sensory Component

This component of CN-V conveys general sensation from the facial skin and scalp to the top of the head, tragus of the ear, and anterior wall of the external auditory meatus (Fig. 6-4). Also, it provides general sensation from the mouth, including the tongue and teeth, nasal and paranasal sinuses, and meninges lining the anterior and middle cranial fossae.

Figure 6-4 Trigeminal Sensory Components.

Trigeminal (Gasserian, Semilunar) Ganglion

The cell bodies of almost all sensory CN-V fibers are located within the trigeminal ganglion (Fig. 6-5). This is contained within a skull-based depression, the Meckel cave, that is located in the floor of the middle cranial fossa. Central processes of neuronal cell bodies constitute the large sensory root that enters the pons and projects into the pontine trigeminal main sensory nucleus and spinal tract nucleus. The peripheral processes of this nerve divide into the three sensory divisions that exit the skull through the superior orbital fissure, the foramen rotundum, and the foramen ovale.

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