Clinical and Experimental Optometry 81.5 September–October 1998 206 Fourth nerve palsy Yeung and Lam OPTOMETRY C L I N I C A L A N D E X P E R I M E N T A L The fourth cranial nerve (Trochlear) is the longest and most slender of all cra- nial nerves controlling the extraocular muscles. 1 The fourth nerve nucleus con- tains approximately 3,400 fibres, inner- vates the contralateral superior oblique muscle and is the only cranial nerve to exit at the dorsal aspect of the brainstem and has complete decussation. 2,3 The fourth cranial nerve is vulnerable to a variety of insults, especially trauma, because of its long intracranial course and its anatomic relation to the anterior med- ullary vellum and the tip of the temporal lobe. 4,5 Fourth nerve palsy may be unilat- eral or bilateral and it may be congenital, traumatic or due to vascular lesions. 1 Trauma is the most frequent cause of fourth nerve palsy 6 and it is frequently bi- lateral. 1 The superior oblique is the major intorter of the eyeball and its subsidiary actions include infraduction and abduc- tion. Therefore, a patient with fourth nerve palsy usually has an abnormal head posture, where the head is tilted away from the affected side, to compensate for the affected eye, which is in an extorted posi- tion. 6 CASE REPORT Ms L, a secondary-school teacher aged 45 years, visited the Optometry Clinic at the Hong Kong Polytechnic University. She complained of blurred near vision with her current reading glasses (RE: +4.25 D and LE: +3.25 D), especially under dim light. She had a pair of progressive glasses (RE: +1.25 D and LE: +1.75 D with +1.75 D addition) with which she obtained good distant vision but her near vision was blurred. She seldom used the progressive glasses and her vision was unaided when she attended the clinic. An obvious right hypotropia with right head tilt was observed (Figure 1). Her ocular health history was uncertain. She remembered the diagnosis of tropia at the age of 11. The reason for her eye exami- nation at that age was a poor depth per- ception which had been recognised by her parents while she was sweeping the floor. She started wearing spectacles after that examination but did not keep any record of her previous prescriptions. She had no complaint of double vision. CASE REPORT Fourth nerve palsy with monovision Pauline HN Yeung BSc (Hons) Andrew KC Lam PD (Optom) MPhil FAAO Department of Optometry and Radiography Hong Kong Polytechnic University Accepted for publication: 28 April 1998 Key words: fourth nerve palsy, head tilt, Parks-Bielschowsky three-step test, vertical tropia The fourth cranial nerve is the most vulnerable cranial nerve. Its damage gives rise to a vertical ocular deviation and usually the patient comes with a right head-tilt. The Parks-Bielschowsky three-step test is a simple and useful test for identifying the paretic extraocular muscle, giving rise to such a vertical ocular deviation. A case of long- standing fourth nerve palsy in a 45-year-old woman is reported. Monovision spectacles were prescribed to give the patient satisfactory distant and near vision. (Clin Exp Optom 1998; 81: 5: 206–209) Figure 1. The habitual head posture of the patient demonstrating a right head tilt and a right hypotropia