The sole function of the fourth nerve is innervation of the superior oblique muscle, which is one of the six muscles of eye movement. Fourth nerve palsy or trochlear nerve palsy is a neurological defect resulting from dysfunction of the fourth cranial nerve. Double vision, also known as diplopia, may occur because of the inability of the eyes to maintain proper alignment.
Description
Trochlear nerve palsy has been described since the mid-1800s. Bielchowsky was first to describe it as the leading cause of vertical (two images appearing one on top of the other or at angles) double vision.
Injury to the fourth cranial nerve can stem from congenital or acquired causes with one or both nerves being affected. It is unclear whether the congenital variant of this disorder is due to developmental abnormalities of the nerve itself or nucleus, which is an area of the brain where the nerve begins and receives signals for proper functioning. In addition the muscle and its tendon may also display abnormal laxity and muscle fiber weakness. Most cases of acquired fourth nerve palsy results from dysfunction of the nerve itself, although cerebrovascular accidents (stroke) may directly injure the nucleus.
Demographics
Fourth nerve palsies have no predilection for males or females. It is difficult to accurately predict the occurrence of congenital palsies since some go unnoticed throughout a person's life. Acquired nerve palsies are more likely to occur in older patients with diabetes or vascular disease versus the general population.
Causes and symptoms
Causes of fourth nerve palsy can be broadly classified as congenital or acquired. Isolated congenital palsies may be heralded by head-tilting to the opposite side of the affected nerve in early childhood. In others a congenital palsy may go unnoticed because of a compensatory mechanism allowing for alignment of the eyes when focusing on an image.
Isolated acquired trochlear nerve palsies can be the result of numerous disorders. Most commonly an underlying cause cannot be found and this is known as an idiopathic palsy. Due to its long course within the brain, the fourth nerve is susceptible to injury following severe head trauma. Depending on the site of nerve compression during trauma one or both nerves may be affected. Aneurysms or brain tumors may directly compress or result in an increase of intracranial pressure (the pressure within the skull) resulting in nerve palsies.
Disorders such as myasthenia gravis, diabetes, meningitis, microvascular disease (atherosclerotic vascular disease) or any cause of increased intracranial pressure may result in trochlear nerve palsy. A congenital palsy that has gone undetected may manifest itself in adulthood when the compensatory mechanism for ocular alignment is lost. Additionally the removal of a cataract may restore clear vision to both eyes allowing the patient to become aware of their double vision.
A child with a congenital palsy may be found doing a head tilt by his or her parents or relatives. Children will very rarely complain of double vision.
Adults with a new onset fourth nerve palsy will note two images, one on top of the other or angled in position when both eyes are open. Covering of one eye, no matter which one is covered, will resolve their diplopia. Their double vision will worsen when looking down or away from the affected side. If both nerves are affected he or she may experience a horizontal diplopia (two images side by side) when looking downward. If a decompensated palsy is suspected, one should review old photographs to document a pre-existing head tilt to support the diagnosis.
Diagnosis
Diagnosing a fourth nerve palsy is for the most part a clinical diagnosis. Careful history taking and examination is the key to diagnosis. The Bielchowsky head-tilt test is one commonly used and reliable technique to diagnose isolated trochlear nerve palsies. Review of patient's old photographs can prove indispensable in diagnosing a decompensated palsy, obviating the need for additional testing.
Ophthalmologists, neuro-ophthalmologists, optometrists and neurologists are medical specialists who can evaluate and diagnose a patient with a fourth nerve palsy. Usually an optometrist or ophthalmologist will initially see a patient complaining of diplopia or displaying stigmata of trochlear nerve palsy. A referral will then likely be made to a neurologist or neuro-ophthalmologist for evaluation and workup.
Treatment
Since most fourth nerve palsies are idiopathic, treatment is conservative given the high rate of spontaneous resolution. Monitoring a patient for six months to one year for improvement can prove to be frustrating and disabling for the patient. A prism may resolve or greatly reduce a patient's diplopia during this period, allowing for return to normal daily activities, such as driving, shopping or reading.
Botulinum toxin used to weaken muscles that overact, causing ocular misalignment, in the presence of a trochlear nerve palsy has been disappointing thus far. Surgery aimed at weakening or strengthening one or more of the extraocular muscles has proven useful in many cases of persistent palsies. Indications for surgery include worsening diplopia, head-tilt resulting in neck pain and poor cosmetic appearance. Procedures performed include the Knapp, Plager or Harada-Ito techniques and are chosen based on the amount and type of ocular misalignment found on examination. These procedures weaken or strengthen extraocular muscles by relocating their attachments to the eye. Muscles may also be weakened by cutting across or removing a portion of the muscle.
Recovery and rehabilitation
A six-month to one-year waiting period is warranted to observe for spontaneous improvement. During this period the patient may benefit from prismatic lenses to eliminate or reduce their diplopia. Eye movement exercises have not proved useful for improving or expediting recovery.
Clinical trials
As of November, 2003 no clinical trials regarding trochlear nerve palsies were underway.
Prognosis
The prognosis for trochlear nerve palsies is dependent upon the underlying cause. Most cases of idiopathic or microvascular nerve palsies resolve within a several weeks to six-month time period without treatment. Traumatic nerve palsies may take up to one year to resolve, with less than half regaining any improvement. Palsies secondary to brain masses or aneurysms have the least likelihood of any recovery and may take up to one year to improve. If present, proper treatment of myasthenia gravis or other underlying systemic disease, excluding a cerebrovascular accident usually results in complete recovery in the vast majority of cases.
Special concerns
Patients afflicted with a fourth nerve palsy should refrain from driving unless an eye patch is used. In addition certain types of employment may warrant a medical leave or temporary change of duties.
BOOKS
Burde, Ronald M., Peter J. Savino, and Jonathan D. Trobe. Clinical Decisions in Neuro-Ophthalmology, 3rd ed. St. Louis: Mosby, 2002.
Liu, Grant T., Nicholas J. Volpe, and Steven L. Galetta. Neuro-Ophthalmology Diagnosis and Management, 1st ed. Philadelphia: W. B. Saunders Company, 2001.
Neuro-Ophthalmologic and Cranial Nerve Disorders; Section 14, Chapter 178. The Merck Manual of Diagnosis and Therapy, edited by Mark H. Beers and Robert Berkow. Whitehouse Station, NJ: Merck Research Laboratories, 1999.
Newman, Nancy J., ed. Ophthamology Clinics of North America, pp. 176-179. Philadelphia: W. B. Saunders Company, 2001.
PERIODICALS
Brazis, Paul W. "Palsies of the trochlear nerve: diagnosis and localization-recent concepts." Mayo Clinic Proceedings 68, no. 5 (May 1993): 501.
WEBSITES
Sheik, Zafar A., and Kelly A. Hutcheson. "Trochlear Nerve Palsy." eMedicine.com. <www.eMedicine.com>.
