Localisation of Brainstem lesions : Rule of Four

Ugh got destroyed by another brainstem lesion question on Qbank? Having hard time with neurology?
then you are at right place..

a simplified method for answering the question ‘Where is the lesion?’  

Below I have summarized RULE OF 4. 

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‘ (lateral) beginning with S
3. There are 4 cranial nerves below the pons, 4 in the pons and 4 above the pons.
4. The 4 motor nuclei that are in the mid line 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).
   

 The 4 Medial structures and the associated deficits are:

1. Motor pathway (or corticospinal tract): CL weakness of the arm and leg 
2. Medial Lemniscus: CL loss of vibration and proprioception in the arm and leg 
3. MLF: IPSI inter-nuclear ophthalmoplegia (failure of adduction of the IPSI eye +nystagmus in the opposite eye as it looks laterally)
4. Motor nucleus and nerve: IPSI loss of the cranial nerve that is affected (3, 4, 6 or 12).

The 4 ’Side’ (lateral) structures and the associated deficits are:

1. Spinocerebellar pathway: IPSI ataxia of the arm and leg 
2. Spinothalamic pathway: CL alteration of pain and temperature affecting the arm, leg and rarely the trunk
3. Sensory nucleus of the 5th cranial nerve: IPSI alteration of pain and temperature on the face in the distribution 
4. Sympathetic pathway: IPSI Homer’s syndrome, that is partial ptosis and a small pupil (miosis)

The 4 cranial nerves in the medulla are CN9-12:

1. Glossopharyngeal (CN9): IPSI loss of pharyngeal sensation
2. Vagus (CN10): IPSI palatal weakness
3. Spinal accessory (CN11): IPSI weakness of the trapezius and stemocleidomastoid muscles
4. Hypoglossal (CN12): IPSI weakness of the tongue.

The 4 cranial nerves in the pons are CN5-8:

1. Trigeminal (CN5):IPSI alteration of pain, temperature and light touch on the face back as far as the anterior two-thirds of the scalp and sparing the angle of the jaw.
2. Abducent (CN6): IPSI weakness of abduction (lateral movement) of the eye (lateral rectus).
3. Facial (CN7): IPSI facial weakness.
4. Auditory (CN8): IPSI deafness. The 6th cranial nerve is the motor nerve in the medial pons.

•  The 7th is a motor nerve but it also carries pathways of taste, and using the rule of 4 it does not divide equally in to 12 and thus it is not a motor nerve that is in the midline.
• The vestibular portion of the 8th nerve is not included in order to keep the concept simple and to avoid confusion. Nausea and vomiting and vertigo are often more common with involvement of the vestibular connections in the lateral medulla. 

 The 4 cranial nerves above the pons are CN1-4: Olfactory and Optic not in midbrain

1. Oculomotor (CN3): impaired adduction, supradduction and infradduction of the IPSI eye with or without a dilated pupil. The eye is turned out and slightly down.
2. Trochlear (CN4): eye unable to look down when the eye is looking in towards the nose (superior oblique). The 3rd and 4th cranial nerves are the motor nerves in the midbrain.

Thus a medial brainstem syndrome will consist of the 4 M’s and the relevant motor cranial nerves, and a lateral brainstem syndrome will consist of the 4 S’s and either
the 9-11th cranial nerve if the lesion is in the medulla, or the 5th, 7th and 8th cranial nerve if the lesion is in the pons.

Handy tip: If there are signs of both a lateral and a medial (paramedian) brainstem syndrome, then one needs to consider a basilar artery problem, possibly an occlusion.

Gates, P. The rule of 4 of the brainstem: a simplified method for understanding brainstem anatomy and brainstem vascular syndromes for the non-neurologist. Internal Medicine Journal 2005; 35: 263-266