Trochlear Nerve Palsy

Etiology

The etiology of trochlear nerve palsy may be idiopathic, trauma, vascular, or others. Most idiopathic trochlear nerve palsy cases are believed to be congenital, with presentation occurring in adult life. Trochlear nerve palsy resulting from moderate frontal head trauma is a frequent cause of bilateral trochlear nerve palsy. Dosunmu et al studied the causes of fourth nerve palsy and found that the most common causes were presumed congenital (49%), hypertension (18%), and trauma (18%). Other causes seen included hypertension with diabetes (5%), post-neurosurgery (3%), intracranial neoplasms (1%), diabetes (1%), and post-Herpes Zoster ophthalmicus (1%).  No cause could be determined for 4% of cases.[2] In a study by Bagheri et al,  presumed congenital was found to be the etiology of fourth nerve palsy in 76.7% of cases, and trauma accounted for 16.4 % of the cases.[3] Similarly, Ellis et al found the superior oblique muscle paresis to be congenital in 83 out of 108 patients (76.8%).[4] Rare causes of trochlear nerve palsy that have been reported include pseudotumor cerebri, meningioma, Lyme disease, and Guillain-Barre Syndrome.[1]

Epidemiology

The annual incidence of trochlear nerve palsy is 5.73 per 100,000 per year.[2] Several studies have observed that this entity is much more common in the male gender.[5] This may be due to a higher incidence of head trauma in males.[6] It has also been seen that most cases of fourth nerve palsy present in the fourth decade of life. The likely cause for this delayed presentation is the decompensation of the control of deviation over time. The most common presentation of post-head injury cases was in the third decade of life.

Pathophysiology

Classically, in most cases of presumed congenital trochlear nerve palsy, the exact pathophysiology is poorly understood. Studies using high-definition magnetic resonance imaging (MRI) have been done, and these are suggestive of 2 different pathogenic mechanisms in patients with congenital palsy of the superior oblique. The trochlear nerve was absent in most patients, and the superior oblique muscle was atrophied. This entity should be considered as a congenital cranial dysinnervation syndrome. In the second group of patients, the trochlear nerve was found to be normal, but the superior oblique tendon was presumed abnormal.[7] Cases of acquired post-traumatic palsy of the trochlear nerve have been seen to be associated with mild to moderate degrees of closed-head injury.[8] In a study by Khawan, only 2 out of 27 patients had a loss of consciousness lasting more than 30 minutes.[9] Post-mortem examination suggests the site of injury to be at the exit of the nerve from the brainstem in the dorsal midbrain.[10] Another injury that has been reported is a parenchymal contusion at the dorsal midbrain.[11] Burger, in his study on 13 patients, found that frontolateral blows were associated with unilateral trochlear nerve palsy, while mid-frontal blows had resulted in bilateral palsy of the trochlear nerve.[12]

History and Physical

Patients with trochlear nerve palsy complain of double vision vertically (vertical diplopia) or the images being tilted or rotated (torsional diplopia). The diplopia is binocular and may worsen or improve in different gazes. In the case of a right superior oblique involvement, the vertical diplopia is maximum when the right eye is directed downwards and inwards.[13] The use of red-green colored glasses helps to document the diplopia more accurately. The main actions of the superior oblique muscle are intorsion, depression, and abduction of the eye. Palsy of the fourth cranial nerve results in the impairment of these actions. The clinical examination may show the following:

• Abnormal head posture (torticollis): To compensate for the diplopia, the patient may adopt a head tilt and face turn to the opposite side.
• Facial asymmetry: Midfacial hypoplasia may be seen in congenital cases of paresis.
• Strabismus: Hypertropia, excyclotorsion, and, in some cases, esotropia can be present in the affected eye.

Maddox Rod test helps to determine ocular cyclotorsion. When the lines of a Maddox rod are oriented vertically, a horizontal line is seen by normal eyes. In the case of trochlear nerve palsy, the horizontal line is perceived as tilted downwards nasally because of the excyclotorsion.[13]

• Parks 3-step test: This test identifies the paretic muscle in cases of vertical diplopia.
  • Step 1: Identification of the hypertrophic eye in the primary position.
  • Step 2: Identifying the hypertrophic eye in the right and left gaze.
  • Step 3: This is also known as the Bielschowsky head tilt test. The head is tilted to each side to determine which side the hypertropia worsens.

In a case of trochlear nerve palsy, the hypertropia of the paretic eye increases on the opposite gaze and on tilting the head to the same side. The Parks 3-step test, however, is not infallible and can lead to an erroneous diagnosis of cyclovertical muscle palsy in cases with skew deviation, prior strabismus surgery, blowout fracture of the orbit, bilateral trochlear nerve palsy, and myasthenia gravis.[14]

• In some long-standing cases, the hypertropia may be equal in different positions of gaze. This is known as the spread of comitance.[15]

In the case of trochlear nerve palsy, this spread of comitance is characterized by:

1. Contracture of the ipsilateral antagonist (Ipsilateral inferior oblique)
2. Overaction of the contralateral yoke muscle (contralateral inferior rectus)
3. Secondary inhibitional palsy of the contralateral antagonist (contralateral superior rectus)

• Bilateral superior oblique palsy should be suspected in cases with alternating hypertropia in gazes and tilts, objective torsion more than 10 degrees, V- pattern esotropia, bilateral fundus torsion, or a chin-down head posture. A case of bilateral superior oblique paresis may be misdiagnosed as unilateral paresis; a condition referred to as masked bilateral superior oblique paresis.[4]

• Assessment of torsion:

  Subjective method: Torsion can be subjectively assessed using the double Maddox rod test. Subjective torsion is usually very little or absent in congenital trochlear nerve palsy, while acquired palsy has a measurable degree of torsion. Excyclotorsion of more than 10 degrees is seen in cases of bilateral palsy of the superior oblique.

  Objective method: Objectively, torsion is measured using an indirect ophthalmoscope or fundus photography.

• Knapp Classification:
  • Class 1: Maximum hypertropia is in the opposite upfield (Overaction of the antagonist inferior oblique)
  • Class 2: Maximum hypertropia is in the opposite downgaze (Underaction of the paretic superior oblique)
  • Class 3: Maximum hypertropia in the entire field to the opposite side
  • Class 4: Maximum hypertropia in the entire field to the opposite side and across the lower field
  • Class 5: Maximum hypertropia is across the lower field.
  • Class 6: Bilateral superior oblique paresis
  • Class 7: It is caused by direct trauma to the superior oblique resulting in paresis and restriction of relaxation (Brown syndrome). This is also known as ‘Canine tooth syndrome.’[16] 

Evaluation

An extensive neurological evaluation is not needed in most patients with isolated fourth nerve palsy. The majority of cases are congenital, and documentation of long-standing head tilt and facial asymmetry helps to rule out an acquired etiology. In cases of acquired palsy, the absence of association with head trauma or the presence of any associated neurological findings mandates neuroimaging (magnetic resonance imaging). Rarely, conditions like myasthenia gravis may mimic trochlear nerve palsy. Suspected cases should be investigated for anti-acetylcholine receptor antibody, computed tomography of the chest to look for thymic mass, and nerve conduction studies.

Treatment / Management

Non-Surgical: Diplopia can be managed with the use of prism glasses.[17] Injection of Botulinum toxin into the overacting inferior oblique muscle has been tried in cases of acute traumatic trochlear nerve palsy while awaiting spontaneous recovery. Studies have shown that this results in a better binocular status of the patients in the acute setting.[18](B2)

Surgical: Surgical correction is required in many cases, and the choice of the procedure depends on the pattern and severity of deviation. The most commonly performed procedure is inferior oblique weakening with additional vertical and horizontal rectus surgery if needed. The general principles that guide the management are mentioned below:

1. Intraoperatively, forced duction testing of the obliques is done. This helps in the identification of a lax tendon, often seen in cases with congenital superior oblique palsy. Superior oblique tuck is a procedure performed in such cases.[19]
2. For cases with hypertropia (less than 15 prism dioptres) and inferior oblique overaction, weakening procedures of the inferior oblique muscle are done. They include myectomy, recession, and anterior transposition.[20]
3. If there is no overaction of the inferior oblique, the surgical options include the recession of the ipsilateral superior rectus muscle or the contralateral inferior rectus muscle.
4. For deviations of more than 15 prism dioptres, inferior oblique weakening procedures can be combined with a recession of ipsilateral superior rectus or contralateral inferior rectus, or both.[21]
5. Large degrees of excyclotorsion seen in bilateral cases cannot be sufficiently corrected with inferior oblique weakening. The Harada-Ito procedure has been described for such cases and involves the splitting of the superior oblique and the advancement of the anterior half of the tendon. The advantage of this procedure is that the vertically acting posterior fibers can be left undisturbed, and only the anterior fibers that are responsible for torsion are operated upon. The Harada-Ito procedure has been classically described for objective torsion more than 10 degrees but being performed for lesser degrees of torsion by some surgeons.[22]
(B2)

Differential Diagnosis

1. Skew Deviation: The perception of the earth-vertical (gravity) is mediated by the vestibulo-ocular pathway. Any lesion along this pathway, or in the cerebellum can lead to an erroneous estimation of gravity, and the body may perceive the head to be tilted, even when it is not. This results in a compensatory head tilt to the opposite side. In patients with skew deviation, the higher eye is elevated and incyclotorted, while the lower eye is depressed and excyclotorted (a consequence of the ocular tilt reaction). The underlying lesion can be due to ischemia, trauma, multiple sclerosis, abscess, or neoplasia. Patients can be frequently asymptomatic or complain of vertical diplopia. The vertical diplopia in skew deviation does not alleviate with head tilt (unlike trochlear nerve palsy). Associated neurological features depend on the site of the lesion. The 3-step test may fallaciously point towards a fourth nerve palsy. The Supine-Upright test is a very specific test that helps in differentiating between the 2 clinical entities. The vertical misalignment between the 2 eyes decreases by more than 50% on changing the patient's position from upright to supine in a skew deviation. This occurs due to a change in the position of the utricle and a reduction of the utriculo-ocular reflex.[23][24]

2. Thyroid-associated ophthalmopathy (TAO): Restriction of the inferior rectus may mimic a fourth nerve palsy in the other eye. Presence of proptosis, lid retraction, lagophthalmos heps in clinical differentiation. Radiologically, enlargement of the muscle belly with sparing of the tendon is seen in TAO.

3. Primary overaction of the Inferior Oblique muscle: These patients do not have primary position vertical deviation, subjective torsion, and the Bielchowsky head tilt test is negative.

4. Myasthenia gravis: It can mimic cyclovertical muscle palsies, and suspected cases should be investigated accordingly. Clinically, an edrophonium test has been described.

5. Blow-out fracture of the orbit: Orbital blow-out fracture with entrapment of the inferior rectus muscle can result in a misleading 3-step test. Infraorbital hypoaesthesia, enophthalmos, and findings on imaging help in arriving at the diagnosis.

6. Congenital torticollis: Patients with congenital palsy may have a head tilt and face turn from early childhood that may lead to a torticollis-like picture. Orthopedician may be the first person whom parents meet thinking it to be a musculoskeletal disorder. Thus both ophthalmologists and orthopedists must be aware of either entity as management is entirely different.

Prognosis

Spontaneous recovery in cases of trochlear nerve palsy, as seen by Park et al, occurs in 82.6% of cases, with as many as 52.2% of patients showing complete recovery.[25] Complete or partial spontaneous recovery has been noted in 45% of the cases of acquired fourth nerve palsy by Phuljhele et al.[26] For cases that need surgical intervention, the success rate is usually good.[21][22]

Complications

The complications that can occur with trochlear nerve palsy include: 

1. Overcorrection: Surgical intervention in cases of trochlear nerve palsy may occasionally result in an overcorrection, and the hypertropia may be converted into hypotropia. The resulting diplopia may be incapacitating for the patient.
2. Iatrogenic Brown syndrome: Deficiency of elevation in adduction can occur due to excessive tucking of the superior oblique muscle.
3. The manifestation of trochlear palsy: In cases of bilateral masked palsy of the fourth nerve, the paresis in the other eye may manifest after surgery of 1 eye if initially misdiagnosed as a unilateral case.