Indications and Clinical Considerations
Trochlear injection is useful in the diagnosis and treatment of primary trochlear headache. As with most headache syndromes, the exact cause of the pain of primary trochlear headache is unknown, and whether the trochlear nerve plays a role in pathogenesis of this uncommon source of head and face pain is the subject of ongoing debate.
In patients with primary trochlear headache the presenting symptom is unilateral periorbital pain radiating from the trochlear area with associated headache. The pain of primary trochlear headache is exacerbated by supraduction of the affected eye, although no limitation of range of motion of the superior oblique should be noted. The pain of this uncommon headache syndrome is often worse at night; initially the pain is characterized by remissions and exacerbations, but without treatment it can become chronic. As the name implies, primary trochlear headache is a diagnosis of exclusion, because it occurs in the absence of primary orbital, retro-orbital, or ocular pathology.
Often confused with acute ocular diseases such as glaucoma or herpes zoster of the first division of the trigeminal nerve or Charlin syndrome, pathology of the orbit and the retro-orbital region must be ruled out before the diagnosis of primary trochlear headache can be made. Inflammatory and autoimmune conditions involving the trochlear nerve anywhere along its path, such as multiple sclerosis, cranial neuritis, and Tolosa-Hunt syndrome, as well as compromise of the trochlear nerve by tumor, abscess, or vascular abnormality must be carefully ruled out before the diagnosis of primary trochlear headache can be considered ( Figures 3-1 and 3-2 ). The diagnosis of primary trochlear headache is then confirmed by injection of the trochlear region with local anesthetic and antiinflammatory steroid. Primary trochlear headache will uniformly respond to this injection.
Clinically Relevant Anatomy
The trochlear nerve (cranial nerve IV) is composed of somatic general efferent motor fibers. It innervates the superior oblique extraocular muscle of the contralateral orbit ( Figure 3-3 ). Contraction of the superior oblique extraocular muscle intorts (rotates inward), depresses, and abducts the globe. The superior oblique extraocular muscle works in concert with the five other extraocular muscles to allow the eye to perform its essential functions of tracking and fixation on objects.