Patient #2

Differential Diagnosis and Discussion:

Case 2: The differential diagnosis of vertical diplopia includes:

• Myasthenia gravis

• Thyroid related orbital disease

• Orbital disorders such as tumor, inflammation, infection, and trauma related problems such as a blow-out fracture of the orbital floor

• Paralysis of the Third Cranial Nerve

• Paralysis of the Fourth Cranial Nerve

• Brown's Syndrome

• Skew Deviation

Disorders affecting the Superior Oblique muscle:

• Fourth Cranial Nerve Paralysis

• Brown's Syndrome

Usually the patient history and clinical examination can distinguish between these problems. Myasthenia gravis typically is characterized by patient fatigue, intermittent ptosis, and a variable ocular motility pattern. Thyroid related orbitopathy usually presents with other findings as discussed in Patient #1 above, and the motility disturbance is usually characterized by restriction of upgaze or lateral gaze. Third Cranial Nerve paralysis results in paralysis of ocular adduction, elevation, and depression, often with ptosis and pupillary dilation. (See Case#1 for and example of this.) Brown's Syndrome, and Paralysis of the Fourth Cranial Nerve are discussed below.

The Superior Oblique muscle is one of six extra-ocular muscles, and is exclusively innervated by the Fourth Cranial Nerve. The muscle originates from the posterior orbit and travels along the superomedial wall of the orbit to the trochlea. The tendon of the muscle passes through this pulley-like structure at the superior orbital rim and then courses back toward the globe itself. The muscle inserts upon the postero-supero quadrant of the eye.

The Superior Oblique muscle is involve with depression of the eye especially when it is turned inward (adducted). Otherwise, it is involved with the torsional rotation of the eye, that it, with keeping the eye oriented straight up and down when the head tilts from side to side.

Paralysis of the Superior Oblique muscle leads to a reduced ability to depress the eye when turned in (such as with reading), and thus a vertical diplopia. It can also lead to a torsional diplopia, with objects appearing tilted. Both of these symptoms are very gaze dependent, with vertical diplopia generally being greater with adduction of the eye, and torsional diplopia being greater with abduction of the eye.

Brown's Syndrome is usually a congenital condition where the superior oblique tendon is short and tethered, and movements of the tendon through the trochlea are restricted. This leads to limitation of elevation of the eye due to restriction of the muscle itself. Thus, the eye tends to be held somewhat in downgaze, and the patient may develop a chin-up head position.

Course of the Fourth Cranial Nerve:

The Fourth Cranial Nerve is the only cranial nerve to exit from the dorsal aspect of the brainstem, and it does so at the level of the inferior colliculi. It has the longest unprotected intracranial course of any cranial nerve, making it susceptible to injury from trauma. The nerve passes between the superior cerebellar and posterior cerebral arteries, but more laterally than the Third Cranial nerve, making it less susceptible to aneurysmal compression. It courses through the subarachnoid space into the cavernous sinus, and finally enters the orbit through the superior orbital fissure to innervate the superior oblique muscle.

Some potential causes of an acquired, isolated Fourth Cranial Nerve Paralysis include:

• Trauma
  This is a common cause, and sometimes fairly incidental, minor head trauma is capable of causing damage to the nerve.

• Microvascular Ischemia
  Similar to Third and Sixth Nerve palsies, ischemia along the course of the nerve can lead to a temporary paralysis. This is common in patients with a history of vascular disease, hypertension, diabetes, and age over 50 years.

• Aneurysm or Neoplasm
  These are fairly rare causes of a Fourth Cranial Nerve paralysis.
  
  

Another frequent cause of a seemingly new Fourth Nerve Paralysis is a decompensated congenital paralysis. Later in adulthood, the sensory and motor adaptations acquired during childhood development to compensate for a congenital paralysis of the Fourth Cranial Nerve may fail. Examination of old photographs of the patient may reveal a tilted head position (discussed below), and the patient will less often complain of torsional diplopia than with a newly acquired paralysis.

Clinical Diagnosis and Evaluation of a Fourth Cranial Nerve Palsy:

A patient with a Fourth Cranial Nerve palsy usually complains symptomatically of vertical diplopia, especially in downgaze (such as with reading through bifocals). There may be complaints of torsional diplopia. When ocular motility is examined, often poor depression of the eye is evident while the eye is adducted. There may also be exaggerated elevation of the eye during adduction, due to the unopposed action of the inferior oblique muscle. Furthermore, if the head is tilted toward the side of the affected muscle, the vertical diplopia will worsen. Thus, patients with a Fourth Cranial Nerve Palsy often adopt a head-tilt away from the affected side, and a face-turn away from the affected side (here the eye will be place in abduction), as this minimizes diplopia. For more discussion on evaluating ocular motility problems, go to Eye Motility, and Evaluation of Ocular Vertical Misalignments.

Evaluation of a Fourth Cranial Nerve Palsy:

In a patient with a history of head trauma, or in a patient with potentially microvascular disease (or age over 50), observation may be a reasonable course of action. If the paralysis does not spontaneously resolve within several months, neuroimaging may be required. It may be reasonable to perform a sed rate, as temporal arteritis can rarely present as an isolated cranial nerve palsy.

In a younger patient, head MRI and a laboratory evaluation to rule-out undiagnosed vascular causes is warranted. Neurological referral may be necessary for cases of demyelinating disease, and for undiagnosed cases.

In some patients, herpes zoster ophthalmicus may cause an isolated Fourth Cranial Nerve Palsy, due to inflammation of the nerve sheath.

More extensive evaluation is required for cases of multiple cranial nerve palsies, to rule out cavernous sinus or orbital disease.