The rule of 4 of the brainstem: a simplified method for understanding brainstem anatomy and brainstem vascular syndromes for the non-neurologist.

The rule of 4 is a simple method developed to help ‘students of neurology’ to remember the anatomy of the brainstem and thus the features of the various brainstem vascular syndromes. As medical students, we are taught detailed anatomy of the brainstem containing a bewildering number of structures with curious names such as superior colliculi, inferior olives, various cranial nerve nuclei and the median longitudinal fasciculus. In reality when we do a neurological examination we test for only a few of these structures. The rule of 4 recognizes this and only describes the parts of the brainstem that we actually examine when doing a neurological examination. The blood supply of the brainstem is such that there are paramedian branches and long circumferential branches (the anterior inferior cerebellar artery (AICA), the posterior inferior cerebellar artery (PICA) and the superior cerebellar artery (SCA). Occlusion of the paramedian branches results in medial (or paramedian) brainstem syndromes and occlusion of the circumferential branches results in lateral brainstem syndromes. Occasionally lateral brainstem syndromes are seen in unilateral vertebral occlusion. This paper describes a simple technique to aid in the understanding of brainstem vascular syndromes. Any attempt to over simplify things runs the risk of upsetting those who like detail and I apologise in advance to the anatomists among us, but for more than 15 years this simple concept has helped numerous students and residents understand, often for the first time, brainstem anatomy and the associated clinical syndromes that result. In the rule of 4 there are 4 rules: 1 There are 4 structures in the ‘midline’ beginning with M . 2 There are 4 structures to the side beginning with S . 3 There are 4 cranial nerves in the medulla, 4 in the pons and 4 above the pons (2 in the midbrain). 4 The 4 motor nuclei that are in the midline are those that divide equally into 12 except for 1 and 2, that is 3, 4, 6 and 12 (5, 7, 9 and 11 are in the lateral brainstem). If you can remember these rules and know how to examine the nervous system, in particular the cranial nerves, then you will be able to diagnose brainstem vascular syndromes with ease. Figure 1 shows a cross-section of the brainstem, in this case at the level of the medulla, but the concept of 4 lateral and 4 medial structures also applies to the pons, only the 4 medial structures relate to midbrain vascular syndromes. The 4 medial structures and the associated deficit are: 1 The M otor pathway (or corticospinal tract): contra lateral weakness of the arm and leg. 2 The M edial Lemniscus: contra lateral loss of vibration and proprioception in the arm and leg. 3 The M edial longitudinal fasciculus: ipsilateral internuclear ophthalmoplegia (failure of adduction of the ipsilateral eye towards the nose and nystagmus in the opposite eye as it looks laterally). 4 The M otor nucleus and nerve: ipsilateral loss of the cranial nerve that is affected (3, 4, 6 or 12). The 4 lateral structures and the associated deficit are: 1 The S pinocerebellar pathways: ipsilateral ataxia of the arm and leg. 2 The S pinothalamic pathway: contra lateral alteration of pain and temperature affecting the arm, leg and rarely the trunk. 3 The S ensory nucleus of the 5th: ipsilateral alteration of pain and temperature on the face in the distribution of the 5th cranial nerve (this nucleus is a long vertical structure that extends in the lateral aspect of the pons down into the medulla). 4 The S ympathetic pathway: ipsilateral Horner’s syndrome, that is partial ptosis and a small pupil (miosis). Correspondence to: Associate Professor Peter Gates, Director of Neuroscience, The Geelong Hospital, Barwon Health, Geelong, Vic. 3220, Australia. Email: [email protected]